Inpainting: Pigments

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The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.

Compiler: Catherine A. Metzger

Pigment Overview

Characteristics to Match

Hue

Hue refers to what is most often thought of as color, i.e., the apple is reddish-orange. It must be remembered that the same pigment may have a different hue when applied transparently versus opaquely. Hue is affected by the choice of binder. The same pigment may appear to be a slightly different hue in different binders. In addition, the hue of a pigment may change subtly depending on how fully it is coated and bound by a binder.

Intensity, Saturation, Chroma

These terms describe how pure or intense a color is, unrelated to hue or how light or dark the color is. Generally, a pigment appears more saturated when applied transparently than when applied opaquely. The intensity of a pigment’s color is affected by the choice of binder and how fully the pigment is bound. Leanly bound pigments tend to appear less intense than those that are more fully bound. Lean paint films generally will become more saturated if varnished or coated with additional medium. Burnishing a paint film, especially a leanly bound one, may permanently increase its color saturation.

Value

Value refers to how light or dark a color is irrespective of its hue and saturation. The value of a pigment is affected by the choice of binder and how well it is bound. A pigment is more transparent, and therefore, darker when there is minimal difference between the refractive index of the pigment compared to that of the binder. Leanly bound pigments tend to be lighter in value than those that are fully bound because light is scattered by the textured paint surface. Leanly bound paint films will generally become darker if “varnished” with additional medium. Burnishing a paint film, especially a leanly bound one, will often permanently darken it.

Particle Size

The particle size of a pigment may affect the appearance of the resulting paint film. Sometimes the optical quality provided by coarse natural pigments cannot be matched by finely ground modern replacements. In addition, a paint containing exceedingly fine modern pigments may not match well if used to inpaint a loss in a film containing very coarse pigments such as azurite. This optical disparity may be diminished if the painting is given a substantial coat of varnish but would be heightened if left uncoated.

Pigment Qualities

Refractive Index

The refractive index (RI) describes the relative amount that light is refracted or bent when moving from a vacuum into a given material. When light moves between materials possessing the same RI, it is not slowed and its direction remains constant. The two materials would be virtually transparent in each other. The closer the RI of a pigment is to its binding media, the more transparent the pigment will appear. The refractive indices of the various binders used in inpainting are covered in other chapters in this volume and volumes in the catalog. Comprehensive lists of pigments and their refractive indices can be found in Gettens and Stout , Eastaugh , and the Artists’ Pigments handbooks issued by the National Gallery of Art, Washington, D.C.. Individuals interested in specific RI numbers should consult those volumes.

To reduce variables when describing and comparing pigments in the sections below, the author utilized a single binder—a 50/50 v/v mixture of PVAc AYAA and PVAc AYAC in a 80/20 v/v mixture of ethanol and diacetone alcohol. The resin mixture was diluted with the solvents as required. The exact percentage of resin in solution was not standardized. The RI of the PVAc mixture is 1.4665 at 20ÅãC. This should be kept in mind when the author’s observations differ from your own.

Compatibility

It is important that the conservator choose pigments that are compatible with each other and with the binders and varnishes used for the inpainting and restoration of a painting. There are historical pigments that cannot be safely mixed together, especially when an aqueous medium is used. In historical practice, painters were warned, among other restrictions, not to mix pigments that contained lead with those that contained sulfur. Modern pigments do not suffer these limitations. The conservator may need to resort to archaic pigments to avoid metamerism (e.g., azurite) or for specific optical qualities (e.g., lead white). The conservator using archaic pigments should consult Gettens and Stout, Harley, Wehlte, and the aforementioned Pigment Handbooks issued by the National Gallery of Art for compatibility issues.

Some pigments are unstable in alkaline and/or acidic environments. The pigments discussed in this chapter are stable in the common inpainting mediums with the exception of Prussian blue, which may be too sensitive to high pH to be used in an acrylic dispersion binder. Conservators using media with a pH outside of the norm should consult the appropriate literature.

Undertone and Mass Tone

Many pigments exhibit two different colors depending on whether they are applied as a transparent or an opaque film. The hue perceived when the color is applied transparently is known as the undertone. The top tone or mass tone is the color of the pigment when viewed opaquely. The undertone is generally warmer and more saturated than the mass or top tone.

Lightfastness

Lightfastness describes the relative ability of the pigment to withstand ultraviolet and visible light without changing color or fading. Pigments that are quickly affected by light are called fugitive. Almost all of the pigments discussed below are sufficiently lightfast to be used for inpainting. The few remaining pigments have been retained because they are either very commonly used despite their deficiencies or have unique qualities that make them valuable, such as alizarin crimson. (For an in-depth discussion of lightfastness, see Church 1901 , Doerner 1934 , Gettens and Stout 1966, Harley 2001, Wehlte 1967, and the Pigment Handbooks published by the National Gallery.)

Specific Gravity

It is sometimes important to think about the relative density of a given quantity of pigment compared to those with which it is mixed. Conservators could encounter problems if they were to mix very dense pigments with less dense ones, especially if the paint is highly dilute. The less dense pigments may float to the surface of the paint before the medium has set. This could greatly distort the final appearance of the color mixture.

Size, Shape, and Uniformity

Size, shape, and uniformity/heterogeneity of pigment particles can greatly affect the optical and physical properties of the resulting paint. It is often advantageous for conservators to emulate the textural/tactile quality of the paint they are attempting to match, especially when the painting is leanly bound or unvarnished or when trying to match a historic coarsely ground mineral color.

Splintery pigments, those that lack uniformity of particle size and shape, and natural earths that contain colorless components tend to open up a paint film. This allows light to pass through the film and bounce back at the viewer, creating a more vibrant color effect. Mixtures of pigments that possess these qualities are often brighter and cleaner than those created using homogeneous, finely divided pigments. The author first heard of this phenomenon from the paintings conservator Jim Bernstein.¹

Tinting Strength

Tinting strength refers to the power of a given pigment to impart its color to a mixture. This is usually measured by mixing a given percentage of the pigment with white and comparing the strength of the tint to that of other pigments mixed with the same white. Pigments with a high tinting strength impart more color than those of lower tinting strength.

Covering Power

Covering power refers to the relative degree of opacity a pigment confers to a paint layer of a given thickness and bound in a given medium compared to other pigments.

Toxicity

Many pigments are composed of heavy metal salts that can be toxic when inhaled or absorbed through the skin. Nontoxic pigments should be substituted for toxic ones when the circumstances allow. Gottsegen has included the hazards associated with each pigment in his pigment table. When it is deemed necessary to use more dangerous pigments, conservators should limit their exposure by always wearing gloves and a mask. (See also Church 1901, Doerner 1934, Gettens and Stout 1966, and Wehlte 1967.)

Pigments Arranged by Color

The author made paintouts of most of the pigments described below. Kremer Pigments produced almost all of the pigments tested, although some of them had been repackaged by Sinopia. The mid-value azurite and vivianite were procured from Natural Pigments. The remaining pigments were obtained from the collections of the author and that of the Winterthur/University of Delaware Program in Art Conservation. The author used x-ray fluorescence (XRF) to confirm the authenticity of a few samples that were not definitively labeled.

Whites

Titanium white (titanium dioxide, rutile) PW 6. Titanium white is very opaque and has the most covering power of any of the white pigments. It is a very cold white and can quickly gray out color mixtures and tints. Titanium white and tints made from it are quite cool and bluish when applied in a translucent manner over a darker color. All colors mixed with titanium white gray quickly. Grays mixed from it are noticeably cold.

Zinc white (zinc oxide) PW 4. Zinc white is a semi-opaque white with average tinting strength. It is thought to whiten in resin colors. Stephen Pichetto used zinc white for retouching, and his inpainting significantly whitened. Some conservators suggest that zinc white should not be used for inpainting.

Lead white (basic lead carbonate; flake white, cremnitz white, ceruse) PW 1. Lead white is a semi-opaque pigment with moderate tinting strength. Its hue is a warm white. Colors tinted with lead white retain far more of their innate hue than those tinted with titanium white. Colors mixed with lead white are generally warmer than those of the same value mixed with titanium white. Scumbles tinted with lead white are less blue than those containing titanium white. Lead white is a very toxic pigment that should only be used if it is considered absolutely necessary and with appropriate safety gear, including gloves and dust mask. The author suggests that if conservators must use lead white, they should pre-disperse a substantial amount of the pigment into the appropriate medium following proper safety practices. This would eliminate the need to handle the dry pigment on a routine basis.

Titanium buff (natural titanium dioxide) PW 6:3. Titanium buff is opaque and possesses a creamy yellowish hue. The warmth of titanium buff can be used to create very warm grays and tints.

Lithopone (zinc sulfide-barium sulfate mixture) PW 5. Lithopone is semi-opaque, more than zinc white but less than lead white. The author did not test this pigment.

Other whites and inert pigments: Barium sulfate (natural mineral and synthetic: barium sulfate; permanent white, blanc fixe: base for lakes) PW 21. The pigment is relatively transparent but much less so than the calcium pigments. It could be used to contribute some turbidity to a color mixture while only slightly raising its value.

Calcium carbonate (natural calcium carbonate: whiting, lime white; synthetic calcium carbonate: precipitated chalk, oyster shell white, Gofun shirayuki, Japanese pearl white) PW 18. Calcium carbonate is a very transparent pigment with very little covering power or tinting strength in resin-bound colors. It is much more transparent than titanium, lead, and barium white. It is slightly more transparent than kaolin. Calcium carbonate is less transparent than the silicate fillers mentioned in the appropriate section below. The pigment can be used to adjust the transparency of any of the pigments mentioned or as a matting agent without greatly altering hue or saturation.

Calcium sulfate (natural gypsum, precipitated calcium sulfate) PW 25. Transparent. The pigment has little covering power or tinting strength and resembles calcium carbonate in its chromatic qualities.

Kaolin (china clay, hydrous aluminum silicate) PW 19. Kaolin is transparent but more opaque than the calcium whites and possesses a low tinting strength. The pigment tested was warmer in hue when bound in PVAc than the other pigments in this section. Kaolin could be used in a similar manner as calcium carbonate but the resulting mixture would be lighter and warmer.

Glass platelets, 15 microns. Glass platelets are a very transparent white/filler. They, along with glass beads, are probably the best choice for matting a color without changing its value or hue. By adding the platelets along with additional binder, the conservator can make a color more transparent and cut its tinting strength without changing its hue or saturation. In this manner, titanium white can be made more transparent and modern organic colors can be tamed to more closely resemble the traditional lakes. Jim Bernstein has performed extensive experiments with the addition of colorless inert materials to inpainting media.

Glass beads 0–50 microns. These are another matting agent that has little effect on hue or value. They can be used in a manner similar to glass platelets.

Fumed silica works well as a matting agent. Extremely fine varieties less than 1 micron may impart a grayish cast to the paint to which it has been added.

Blacks

Bone black (“ivory” black from burnt bones) PBk 9. Bone black is a semi-transparent black with good tinting strength. It will gray and muddy all mixtures containing any white. It can be added to pure colors to darken them or to those containing white to gray and cut saturation when this effect is required. Bone black makes a cold gray with titanium white and a warmer gray with lead white.

Mars black (manufactured iron oxide black) PBk 11. Mars black comes in a few variations. All are opaque, have very high covering power, and most are warm blacks. They are less saturated and lighter in color than bone black especially when leanly bound. The exact hue of grays made of it depends upon the variety used. Mars black quickly dirties paint mixtures.

Lamp black, PBk 10. Lamp black is a very fine, dense, greasy pigment. It is a quite saturated bluish-black when fully bound. Grays made with it and titanium white are very blue, while those made with it and lead white are much less so.

Vine black (ground charcoal) PBk 8. Vine black is composed of splinters and chunky particles. It is a less saturated and colder black than bone especially when leanly bound. The pigment exhibits a pronounced granularity especially when diluted or mixed with another finer pigment. Vine black does not impart a lot of color. It is useful for darkening a color without making it muddy.

Spinel black, PBk 26. Opaque. The pigment is very dense and possesses high tinting strength and covering power. Spinel black was created to be the blackest black and is evenly nonreflective across the visible spectrum. It could be useful for creating deep, rich blacks and is the deepest of the black pigments even when leanly bound.

Manganese black (manganese dioxide) Pbk 14. Opaque. The pigment is very heavy, with high covering power and tinting strength. Manganese black is quite gray especially when leanly bound.

Black dyes. The use of dyes for inpainting is discussed in section V of this volume.

Blues

Ultramarine blue light (complex silicate of sodium and aluminum with sulfur) PB 29. The pigment is very transparent, a lighter and less violet version of ultramarine. It is probably a better choice when attempting to match true lapis ultramarine. Tints with titanium white appeared far grayer than those tested with lead white.

Ultramarine blue deep, reddish (complex silicate of sodium and aluminum with sulfur) PB 29. The pigment is a very transparent, clean, and reddish-blue of moderate to good tinting strength. Its hue cannot be mixed from other pigments.

Cobalt blue, standard (oxides of cobalt and aluminum) PB 28. Cobalt blue is a moderately transparent blue of low to moderate tinting strength. Its hue is a mid-tone blue that is difficult to mix from other pigments. The pigment is much lighter in hue when leanly bound. The hue does not gray as much in tints with white as do similar mixtures with most other blues.

Cerulean blue (oxides of cobalt and tin) PB 35. Cerulean blue is a semi-transparent pigment with low tinting strength, and its hue is a slightly greenish blue. The color of leanly bound cerulean is much lighter than when it is fully bound. The pigment grays in mixtures with white. Cerulean blue is valuable for its unique color despite its low tinting strength. Cerulean may be useful for matching the finer, greener varieties of azurite.

Prussian blue (ferric ammonium ferrocyanide) PB 27. The pigment is extremely transparent and possesses a very high tinting strength. Prussian is a very saturated greenish-blue that appears almost black in mass tone. Leanly bound, the pigment can exhibit some bronzing. It is greener than phthalocyanine blue, green shade. Prussian blue is cooler and grayer when mixed with titanium white than when mixed with lead. Mixtures in either titanium or lead white produce a color that is grayer than the same mixtures made with phthalocyanine blue. Prussian blue is indispensable for 18th-century paintings and can usually be substituted for phthalocyanine blue when working on traditional paintings. The pigment can be added to bone black to produce deep, inky, and cold blacks.

Phthalocyanine blue, green shade (copper phthalocyanine) PB 15. The pigment is extremely transparent, high in tinting strength, and saturated. Its tinting strength can make it hard to control. The pigment exhibits bronzing when leanly bound. It is much lighter in mass tone when leanly bound than is Prussian blue. Very clean greens can be mixed from the pigment and saturated yellows. Tints with white are cleaner than those with Prussian.

Phthalocyanine blue, red shade (copper phthalocyanine) PB 15.6. This pigment is very similar to PB 15 except that its hue is less green. It exhibits the same bronzing when full strength and leanly bound.

Cobalt turquoise (cobalt, chromium, aluminum, oxide spinel) PB 36. The pigment is made in a range of shades from bright blue-turquoise, often called cobalt blue greenish, to deep blue green turquoise. The author only tested the latter variety. Cobalt turquoise, dark is semi-transparent with a very low tinting strength. Its hue is a clean but substantially greener version of cerulean blue. It is grayed when mixed with both lead and titanium white.

Indathrene blue, indigo shade (organic) PB 60. The pigment is very transparent and possesses a good tinting strength. The hue is a deep but unsaturated blue. Tints with white are substantially grayed but appear more reddish than when unmixed. While the pigment purports to resemble natural indigo, one would need to add some black to the paint to approximate the historic color.

Manganese blue (barium sulfate permanganate) PB 33. The pigment is an extremely transparent, saturated greenish blue with a very low tinting strength. It is a very useful color but is no longer being manufactured. Its hue is quite difficult to mix from other pigments but can be approximated using phthalocyanine blue green shade, phthalocyanine green or viridian, and a transparent extender. It can cause metamerism when used to inpaint sky and therefore is generally not a good choice for skies containing ultramarine or azurite

Historic blues: Azurite deep, coarse grind (natural copper carbonate) PB 30. Coarsely ground azurite is a translucent pigment of weak tinting strength that possesses a deep mid-blue hue. It is very difficult to manage and so granular as to make it almost impossible to create a workable paint without the addition of another fine textured pigment. It is impossible to make a coherent, evenly covering paintout of this pigment alone in a resin binder. The result resembles a wash of blue sand. The gross particle size of coarsely ground azurite is so alien to modern pigments that the azurite will often sink in mixtures, especially when well diluted in a solvent. This pigment may be useful when inpainting losses in an area of azurite of a similar color to avoid metamerism or when the coarse texture of the original pigment plays an important role in the final appearance of the painting.

Azurite fine, PB 30. The pigment is semi-transparent, less so than the coarse grind above and more so than the extra fine grind below. It possesses a weak tinting strength and a relatively clean hue between that of cerulean and Prussian blue. Azurite fine made a granular but satisfactory paint when ground into PVAc. The granularity became more pronounced as the paint was diluted.

Azurite, extra fine grind, PB 30. The pigment is semi-transparent with less than average tinting strength. The hue of finely ground azurite shifts drastically towards a dull greenblue as compared to the varieties described above. The pigment can be made into a satisfactory paint. The color and other optical properties of the pigment can be well utilized when inpainting the distant vistas seen in Flemish and Renaissance backgrounds.

Browns and Neutrals

Burnt umber (iron oxide with some manganese dioxide) PBr 7. The pigment is a deep, rich, relatively transparent brown with average tinting strength. The hue becomes very dull in mixtures containing white.

Raw umber, Cyprus (iron oxide with some manganese dioxide) PBr 7. The color is semitransparent with a moderate bias towards green. The pigment is easily diluted to transparency. Raw umber appears much lighter in value when leanly bound. The pigment is useful and necessary for inpainting many pre-modernist paintings. Raw umber can be added to color mixtures and glaze layers to dirty them. One conservator uses raw umber to shade mixtures rather than black to avoid the desaturation associated with adding black to color mixtures (Roth-Wells, personal communication, 2008). Grays are often best mixed from raw umber and white rather than black and white unless a very cold gray is required.

Raw umber greenish, dark (iron oxide with some manganese dioxide) PBr 7. The pigment available from Kremer Pigments is semi-transparent, slightly gritty, and exhibits a very greenish-brown hue. The color could be very useful for inpainting losses to the background on 19th-century American portraits.

Mars Browns (synthesized iron oxide) PBr 6. Mars browns are made in a few colors. All are very opaque and dense. They make a good covering paint but may be quite dull compared to the natural earths.

Davy’s gray (gray green slate) PBk 19. The pigment is a warm, slightly greenish gray with very low tinting strength. It can be used to shade a color mixture without overly cutting chroma or appearing too heavy. It is probably best used to dirty a color or as a superimposed layer to emulate patina.

Rottenstone. Rottenstone may be used in a manner similar to Davy’s gray as a transparent greenish-gray “dirt” color. The pigment can be used to dirty up other colors or, even better, as a surface effect to emulate an incompletely cleaned surface.

Greens

Chromium oxide green, PG 17. The pigment is completely opaque, even over black, and exhibits a very high tinting strength. The color of chromium oxide is a dull, warm green. The hue, saturation, and value remain constant no matter how leanly it is bound. The color shifts slightly towards a cooler green with great dilution.

Phthalocyanine green, blue shade (chlorinated copper phthalocyanine) PG 7. The pigment is an extremely high tinting strength, clean, and transparent bluish-green. Its color can be far too intense and garish for traditional paintings; however, almost any green can be mixed from it. Its color cannot be mixed from other pigments. Use phthalocyanine blue, phthalocyanine green, yellow shade, or the transparent yellows to adjust its temperature while retaining the saturation of the mixture. Less saturated pigments or complementary colors can be used to adjust the intensity of mixtures made with phthalocyanine green, blue shade. Very transparent browns can be mixed from the pigment when added to mixtures of transparent saturated reds and yellows. The tinting strength is so powerful that it quickly overpowers mixtures containing it.

Phthalocyanine green, yellow shade (chlorinated copper phthalocyanine) PG 36. The pigment is very transparent with an extreme tinting strength. Its hue is a middle-green, which is retained with dilution. The pigment is very dark when full strength and leanly bound. Tints with white are cooler. The pigment would be helpful for mixing very strong warm greens. It would probably be a good starting point when attempting to match true emerald green (copper aceto-arsenite).

Viridian (hydrated chromium oxide) PG 18. The pigment is transparent with average tinting strength. Its color is cold, minty green, which shifts slightly bluer with dilution and even more so with white. Mixtures with viridian become muddy much more quickly than those with phthalocyanine green. The pigment is useful to inpaint cracks within skies when mixed with white, often with additions of burnt sienna. The light passing through a turbid medium effect makes the color appear blue.

Cobalt green light (Rinmann’s green, complex of cobalt and zinc) PG 19. The pigment is semi-opaque with a low tinting strength. Its hue is a subtle blue-green. The pigment can be granular.

Cobalt green (cobalt green-blue oxide, oxides of chromium, cobalt, and aluminum) PG 26. The pigment is semi-opaque with low to medium tinting strength. Two shades of the pigment are produced: a blue green and a grass green.

Cobalt bottle green (oxides of titanium and cobalt) PG 50. The pigment is relatively transparent, with low to medium tinting strength. Its hue is a grayish cold green that does not brighten up on dilution. Mixtures with white are even more gray and cold. The sample tested was slightly gritty.

Green earth or terre verte PG 23. Green earth is found in a number of hues ranging from a warm green-gray to a moderately clean, subtle green. All are rather dull, transparent, and earthy greens with minimal tinting strength in resin colors. Green earth is probably more useful in water media than resin colors. The pigment is often quite gritty.

Bohemian green earth, extra fine. PG 23. This grade of green earth is an especially clean green variety. It may be useful despite its low tinting strength in resin bound colors.

Oranges

Cadmium orange (cadmium sulfoselenide; some variants are co-precipitated with barium sulfate or as a filler) PO 20. The pigment is relatively opaque with good tinting strength. Cadmium orange is made in hues ranging from a warm scarlet to a deep yellow. They are bright as single pigments but dull somewhat in mixtures and with white. The color and qualities of cadmium orange can make it useful for scumbling over dark spots or fly specks where the orange hue can offset the turbid medium effect.

Titanium orange (mixed phase system of titanium, chrome, and tin) PBr 24. The pigment is an opaque dull orange with low to medium tinting strength. Titanium orange can be used in a manner similar to cadmium orange to cover dark spots.

Irgazine orange (DPP: diketo-pyrrolo-pyrrole) PO 73. DPP colors are relatively opaque organic colors that resemble the qualities of the cadmium colors of the same hue. DPP pigments lack the toxicity of the cadmiums, which they have replaced in the coating industry.

Irgazine orange, transparent PO 73.The pigment is extremely transparent with good tinting strength. The mass tone is warm-red, but the pigment becomes more orange with dilution.

Isoindolor orange, PO 61.The pigment is a very transparent, relatively pure orange pigment. The hue shifts towards warm yellow with dilution. It can be used as a replacement for the fugitive and unavailable alizarin orange, although it may require a touch of a transparent red to achieve the exact hue.

Orange earth (natural iron oxide) PR 102. Orange earth is a relatively opaque pigment with strong tinting strength. The hue is a very warm orange-red-brown. The color becomes surprisingly saturated with dilution. The hue is grayed somewhat when mixed with white but the mixture is very like pink flesh.

Red

Cadmium red light (cadmium sulfoselenide; some variants are co-precipitated with barium sulfate or as a filler) PR 108. The pigment is a warm, scarlet, and opaque red, which is a good substitute for genuine vermilion (mercury sulfide). Cadmium red light is useful for making and modifying opaque oranges. It exhibits good tinting strength and coverage but dulls in mixtures and with white.

Cadmium red medium (cadmium sulfoselenide; some variants are co-precipitated with barium sulfate or as a filler) PR 108. The pigment is an opaque, cherry-red body color with good tinting strength and coverage. Cadmium red medium dulls in mixtures and with white. The pigment can be used to make dull opaque purples.

Cadmium red deep, extra deep (cadmium sulfoselenide; some variants are co-precipitated with barium sulfate or as a filler) PR 108. The pigment is a cold, dull, opaque, and deep red color with good tinting strength and covering power. Cadmium red deep is dull in mixtures and with white. It can be used to make dull, opaque purples.

Irgazine reds (DPP: diketo-pyrrolo-pyrrole) PR 254, PR 255, PR 264. DPP colors are relatively opaque organic colors, which resemble the qualities of the cadmium colors of the same hue. DPP pigments lack the toxicity of the cadmiums, which they have replaced in the coating industry.

Alizarin crimson (synthesized organic lake) PR 83. Alizarin crimson is very transparent, darker, warmer, and slightly browner in hue than quinacridone red. The pigment’s mass tone is much darker when leanly bound. Alizarin retains its warmth with dilution. The pigment is a lake of synthesized alizarin, the primary coloring component of rose madder, which it replaced. The conservator could add an extender such as glass platelets to more closely emulate the earlier pigment. Alizarin exhibits less than satisfactory lightfastness and should probably be replaced with quinacridone red or another lightfast transparent cool red whenever possible.

Irgazine transparent scarlet. The pigment is a very transparent color of good tinting strength that approximates the hue of cadmium red light. It remains brighter and more saturated in mixtures and with white than does the cadmium pigment.

Quinacridone red (organic, linear quinacridone) PR 192. The pigment has high tinting strength, moderate transparency, and is a very intense, transparent, and clean cold red. Quinacridone red appears quite rose when diluted. The sample tested was granular and difficult to disperse into PVAc. The pigment makes clean violets with dioxazine purple, ultramarine blue, or quinacridone violet and surprisingly clean oranges with the transparent Quinacridone red may be a good place to start when attempting to emulate alizarin crimson, rose madder, and the cochineal lakes. The conservator would probably need to add substantial amounts of extender and an additional neutral color such as burnt umber to help offset quinacridone’s intense chroma and to provide the warmer hue associated with the historic colors.

Other modern organic red pigments:

There are a vast number of organic red pigments available. Almost all are bright, transparent pigments but they do vary greatly in lightfastness. The author has listed only those used or tested by him or suggested by other conservators.

Iron oxide reds: Iron oxide pigments come in a wide variety of hues and color purity. Natural earths will vary in hue and opacity from batch to batch. Conservators should experiment with a number of iron oxide pigments to determine their favorites. The author has only listed the pigments he has experience with.

Venetian red (natural iron oxide) PR 101. Venetian red is an opaque pigment with strong tinting strength. Manufacturers often substitute the mars colors for the natural earths in their products. The hue is a mid-reddish-brown. The color becomes much more saturated with dilution. Tints with white are a warm pink.

Burnt sienna (natural iron oxide) PB r7. Burnt sienna is a transparent, rich, reddishorange- brown with moderate tinting strength. It is darker in mass tone than most iron oxides. The hue is quite rich when used alone as a glaze but dulls quickly with white.

Transparent red oxide (iron oxide) PR 101. The pigment is a more transparent, cleaner, deeper, and colder version of burnt sienna. It is great for natural looking but intense glazing colors. The sample tested was quite gritty.

Mars red (synthesized iron oxide) PR 101, 102. Mars reds are produced in a number of shades from orange-red to violet-red. All are very opaque and dense. The color of the mars reds tends to be cleaner than their natural counterparts at least in mass tone. Natural earths can sometimes make a brighter paint as they are more transparent and are a mix of particle sizes.

Indian red (synthetic iron oxide) PR 101. Indian red is an extremely opaque pigment with strong tinting strength. The hue is a cold red-brown, which warms slightly with dilution. The hue is grayed when mixed with white.

Violets

Cobalt violet light (cobalt(II)-phosphate) PV 49. The pigment is semi-transparent and possesses an extremely weak tinting strength. The color of cobalt violet light is a pale subtle pinkish violet. The pigment is of minimal use in resin-bound colors except as a glaze.

Cobalt violet dark (cobalt(II)-phosphate) PV 14. The pigment is more transparent and possesses a better tinting strength than the light variety. The hue of cobalt violet dark is a deep reddish violet.

Manganese violet (manganese ammonium pyrophosphate) PV 16. The pigment is transparent, with a relatively weak tinting strength, and is not an especially pure violet. The mass tone is much cooler and lighter when leanly bound. Manganese violet is quickly muddied when mixed with titanium white but substantially less so when mixed with lead white.

Dioxazine purple, red shade (organic, carbazol dioxazine) PV 23 RS. The red shade, discussed here, is much more lightfast than the blue shade. The pigment is an extremely intense, powerful, and transparent violet with a high tinting strength. It appears almost black in mass tone. The hue becomes warmer as the paint is diluted. Dioxazine purple can be useful for making transparent blacks. The pigment may have a bronze sheen if it is leanly bound and not intermixed with other pigments. Mixtures with titanium white appear quite gray-purple but are more saturated in lead white.

Mars violet (iron oxide) PR 101. Mars violet is manufactured in a few variations, from cold red-brown to dark brown-purple. The pigments exhibit extreme opacity and tinting strength. The paint is far lighter in mass tone when leanly bound. All of the mars violets have a hue in the brownish range and gray to a great extent when mixed with titanium white. This is less pronounced when mixed with lead white.

Quinacridone violet (organic, linear quinacridone) PV 19. The pigment is extremely transparent, possesses a pronounced tinting strength, and is a very saturated red-violet. Quinacridone violet becomes bluer as it is diluted. Its color temperature can be further adjusted with quinacridone red to warm or with dioxazine purple or ultramarine blue to cool. The addition of an extender and a brown pigment would be required when attempting to emulate the less saturated red and violet lakes used in the past.

Yellows

Cadmium yellow medium and deep (cadmium sulfide; some variants are co-precipitated with barium sulfate or as a filler) PY 37. The pigments come in a number of hues, but all are opaque, warm yellows. The pigments exhibit good tinting strength and coverage but are less opaque than cadmium orange and red. The color of the pigment is somewhat dulled in mixtures and with white. Cadmium yellow medium and deep are useful for making and modifying opaque oranges.

Cadmium yellow light or cadmium lemon (cadmium zinc sulfide; some variants are co-precipitated with barium sulfate or as a filler) PY 35. The pigments are opaque cold yellows with good color saturation. Tints with white retain the color’s brightness and saturation. The pigment is useful for making and modifying opaque greens with blues and greens. Some believe that the lighter cadmium yellows, those containing zinc, may be prone to lightening or the chalking seen with zinc white.

Irgazine yellow, PY 110. The pigment is relatively opaque, possesses high tinting strength, and resembles cadmium yellow medium. The hue is quite orange in mass tone but shifts towards yellow with dilution.

Nickel titanium yellow (nickel titanate, titanium yellow, oxides of nickel, antimony, and titanium) PY 53. The pigment is a moderately opaque, pale, greenish yellow. The color is only slightly dulled when mixed with white but greens made from it are not especially bright. The pigment is probably a good starting point to emulate lead-tin yellow and Naples yellow light. Nickel titanium yellow closely resembles the obsolete barium yellow. It could also be used to modify other yellows or greens, like brightening up an ochre.

Cobalt yellow (cobalt potassium nitrate) PY 40. The pigment is a transparent middle yellow that maintains its saturation in mixtures with white. The color of the mass tone is less intense than when diluted.

Iron oxide yellows: Iron oxide pigments are available in different hues and color purity. Natural earths will vary in hue and opacity from batch to batch. The conservator should experiment with a number of iron oxide pigments to determine which are suited for the task at hand. The descriptions below only include those pigments that the author painted out.

Yellow ochre (natural iron oxide) PY 42. The pigment exhibits moderate opacity and tinting strength. Its hue is a slightly dull yellow. It becomes more intense the more thinly it is applied.

Raw sienna (natural iron oxide) PBr 7. The pigment is a darker, cleaner, and more transparent version of yellow ochre. Raw sienna mixed with bone black can make a good transparent “dirt” color. Raw sienna mixed with white has been used to inpaint the cracks in skies to create a greenish blue, exploiting the effect of light traveling through a turbid medium.

Mars yellow (synthesized iron oxide) PY 42. The pigment is moderately opaque. Mars yellow can be made in a few different shades. These are all purer and are cleaner in hue than the natural products. Despite this, the color effect using the mars colors is sometimes duller than that made from natural earths that have a mixture of particle sizes and include colorless components.

Transparent yellow oxide (iron oxide) PY 42. The pigment is a more transparent, cleaner version of raw sienna. It is great for natural looking but intense glazing colors. The sample tested was quite gritty.

Quinacridone gold (organic; linear quinacridone) PO 48. The pigment is extremely transparent, more so than nickel azo yellow. Its mass tone is dull orange-brown-yellow but becomes a strong yellow-gold with dilution. Its color is substantially dulled in mixtures containing white. This pigment is very useful for warming up paint mixtures and providing the yellow associated with degraded natural resin varnish.

Quinacridone brown-gold (organic; linear quinacridone) PO 49. The pigment is very transparent. Its mass tone is brownish-yellow, and it becomes more yellow when diluted. Mixtures with white resemble full strength raw sienna.

Nickel azo yellow, artificial Indian yellow (nickel complex azo) PY 150. The pigment is quite transparent. Its color is a dull greenish yellow in mass tone but exhibits a moderately saturated and slightly acidic yellow when diluted. The color dulls slightly when mixed with white. This is more pronounced with titanium white than with lead white. The sample tested was quite gritty and difficult to make into a paint. This quality diminished when other pigments were added to the mixture. Nickel azo yellow is a very useful color when a transparent warm yellow is required.

Quindo (Azo) green-gold (nickel azo) PG 10. The pigment is quite transparent. It exhibits a dull sap green hue in mass tone but brightens up to a saturated, acidic, and greenish yellow when diluted. The pigment’s saturation is greatly diminished when mixed with white. The sample tested was slightly gritty.

Other modern organic yellows: A wide variety of organic yellow pigments is available. Like the organic reds, these tend to be bright transparent pigments with varying lightfastness.

Special Effect Pigments

Bronze powders quickly oxidize and change color even when well bound. Therefore, there is little to recommend their use. Mica iridescent colors are a superior substitute when one needs to emulate a metallic finish with pigments.

Mica iridescent pigments are created by coating mica flakes with titanium dioxide and/or iron oxides which create a realistic metallic effect but do not oxidize. Iridescent pigments are available in a variety of metallic and pearlescent shades. They can be useful for inpainting losses to metallic finishes in place of bronze powders.

Fluorescent pigments are inherently fugitive to light. Their only use to the conservator is for inpainting losses in areas of fluorescent paint.

There are a number of specialty pigments such as duotone, iridescent, and interference pigments created for the coating industry. Conservators would have little use for these unless they are called to inpaint losses in a contemporary work that utilized such a pigment.

Useful Pigment Mixtures

Historic Strategies and Mixtures

The following are strategies and mixtures used by notable conservators.

Helmut Ruhemann

In his book, The Cleaning of Paintings, Helmut Ruhemann provided a list of the pigments he used for inpainting. Within this chapter he included a discussion of modern pigments and pigment mixtures that he suggested for emulating archaic, unstable colors. This information is summarized in the following table:

Modern Pigment	Can Substitute For
Titanium white	lead white
Prussian blue	Prussian blue and to a lesser extent, azurite and blue verditer
Opaque oxide of chromium	less stable mixtures of verdigris with lead white or yellow lead oxide
Zinc White	lead white when titanium white would be too opaque
Cadmium yellows & oranges	Yellow lead oxide, Lead tin oxide, red lead, orpiment, realgar, chrome yellows and oranges, Naples yellow, and other obsolete and impermanent colors in this hue range
Cadmium red	vermillion
Mars colors	natural earths
Ultramarine artificial	Natural lapis lazuli ultramarine
Cerulean blue	azurite
Davy’s grey	inpainting where the use of black and white would be too heavy
Indian yellow (coal tar) & Hansa yellow	vegetable dye lakes, gamboges, saffron, and the unavailable genuine Indian yellow
Viridian	verdigris, malachite, green copper resinate, and emerald (Paris) green
Cobalt blue	azurite and smalt
Cobalt green	malachite and other copper greens
Alizarin crimson	natural madder lake
Nickel titanium yellow	pale cadmium yellows
Quinacridone red	red lake, despite the fact that it is not as deep

Ruhemann includes the following pigment mixture:

Alizarin orange mixed with black makes a useful transparent brown for the emulation of bituminous brown glazes and brown that have become more transparent with time.

Bettina Jessell

Bettina Jessell recorded the inpainting strategies and pigment mixtures that she learned and adapted from the conservator Helmut Ruhemann. Those that relate to pigment choices are summarized below:

• Conservators should choose their pigment palette in an attempt to follow that of the original painter.

• Pigments chosen to inpaint a ground layer should probably be more opaque than those for retouching the paint layer proper.

• Jessell points out that the oil binder in aged paintings has darkened and become brown. This has an effect on the color of the paint film. She therefore based “…all colors (even black and white) on a mixture of red and green, covering or transparent according to the layer being inpainted.” She then added whichever pigments seem to match those of the painter’s, again transparent or covering as required.

• The heightened transparency and saturation of aged paint films can be replicated by the addition of very small amounts of highly transparent lake colors and bone black.

Color mixtures:

• Deep blacks: ivory black with a little Prussian blue.

• Caucasian skin tones.

• Medium skin tones: light red (or cadmium red or burnt sienna), viridian, Naples yellow, and titanium white.

• Shadows: add ultramarine blue to medium tone.

• Skies: viridian (not blue), burnt sienna, and titanium white. This mixture takes the turbid medium effect and the darkening of the oil into effect.

• White garments (shirts, ruffs, etc.): titanium white, viridian, burnt sienna, and often some ivory black.

• Very dark hot glaze: bone black and alizarin orange.

• Fiery red glazes on early Italian and Northern painting: alizarin crimson, alizarin orange, and bone black.

• Hot glazes: Indian yellow and burnt sienna.

• Brilliant, light colors: First underpaint in pure white, then glaze with transparent color.

• Semi-transparent monochromatic brown underpainting: burnt sienna, raw sienna with a little black.

• Fly-specks, or small stains, should be given a covering coat of underpaint and then glazed with the design layer color in a transparent medium.

Joyce Plesters

Joyce Plesters used the following recipe and mixture of pigments to imitate copper resinate greens (Court, personal communication, 2008):

Transparent green:
7 parts by wt.	Indian yellow (Winsor and Newton, synthetic)
2.5 parts by wt.	Monastral (phthalocyanine) green (I.C.I. grade GS)
Opaque green:
10.5 parts by wt.	Yellow ochre (Ralph, Nye, & Biddle)
1.5 parts by wt.	Cadmium yellow pale (Winsor and Newton)
1 part by wt.	Monastral (phthalocyanine) green I.C.I. grade GS)

Mix the dry pigments together on a ground glass sheet with a palette knife. Then add a few ml. of ethanol to give a fairly wet paste. Grind very well with a muller. Allow to dry out thoroughly in a warm place, preferably overnight, then scrape from the glass plate and muller. Wet and rub out again a small sample and if it is not completely homogeneous and streaks of any one pigment are visible, repeat the wet grinding. Pass the thoroughly dried pigment through a fine sieve and pack into dry jars.

Tom Carter

In 1980, Tom Carter at the National Portrait Gallery, Washington, D.C. mixed varying proportions of bone black, alizarin crimson, and Indian yellow on the palette to create an incredible range of saturated transparent browns such as are commonly encountered in old oil paintings.

John Brealey

John Brealey suggested the following pigment mixtures (Stoner, personal communication, 2008):

Transparent discolored oil medium color: mixtures of burnt sienna and viridian.

Transparent dirt color: raw sienna and bone black.

Opaque dirt color: red earth and chromium oxide.

Brealey used a translucent mixture of raw sienna and white to inpaint dark cracks in a sky. The turbid medium effect would make the inpainting appear greenish blue.

Turbid Medium Effect

When a lighter color is applied translucently over a darker color, the resulting hue appears far more “blue” because of the “effect of light passing through a turbid medium.” The conservator can compensate for this bluing by adding more of the complementary color, orange, to the mixture, thereby neutralizing the blue. As mentioned earlier, cadmium orange and titanium orange have been suggested for this purpose.

Another strategy is to take the shift toward blue into account when mixing a color. This is the principle behind Ruhemann’s and Brealey’s use of yellow tints to imitate blue-green.

One conservator mentioned the use of genuine Naples yellow (lead antimonate) for tints rather than white to diminish the bluing effect associated with applying a lighter color over a darker one (Roth-Wells, personal communication, 2008).

Matching Aged Surfaces and Patination

Increased Transparency

The refractive index of oil paint increases over time, coming closer to the refractive indices of most pigments. This results in the oil paint appearing to become more transparent over time. This can create a paint layer that is deeper and more intense than when it was originally painted depending on the ground color below A newly applied paint containing the same pigments would appear less saturated than the original. Conservators often need to use more intense, saturated pigments to inpaint losses in old paintings. Using very transparent pigments and applying them in a manner more transparent than the original paint usually achieves this effect. The problem has diminished in recent years with the ready availability of stable, very saturated, and transparent pigments in almost any hue required. Very transparent, inert fillers such as glass platelets, glass beads, and fumed silica can be added to a paint to make it more transparent without altering its hue. The following are a few suggestions for specific issues:

• To get darker and richer blacks, highly saturated and deep colors like Prussian blue, phthalocyanine green, or dioxazine purple can be added to bone black. Other conservators substitute non-historic blacks such as spinel black to achieve the necessary depth and richness. Some even add alcohol-soluble black dye to their PVAc medium to solve the problem.

• The conservator may require rich browns not possible by mixing standard earth colors. A range of very intense dark browns can be mixed from a few deep, very transparent, and saturated colors. Some use a mixture of ivory black or Prussian blue, alizarin crimson, and synthetic Indian yellow (nickel azo yellow). The author has experimented with mixtures of quinacridone red, Prussian blue or phthalocyanine green, and nickel azo yellow to create glowing browns. Dioxazine purple, phthalocyanine green, and nickel azo yellow can also be mixed to create very useful intense browns. One variant resembled a bituminous glaze.

Darkened Media

The darkening of the oil binder over time can change the color of a paint layer. To replicate this effect in their inpainting, Ruhemann, Jessell, and Brealey added brownish pigment mixtures to all of their colors to impart the necessary color shift.

Patination

Often a paint layer acquires a yellowish patina from remnants of degraded varnish and grime that it was considered prudent to leave on the surface. Some address this by adding brownish-yellow pigments to paint mixtures. Raw sienna is a common ingredient in such mixtures. Additions of quinacridone gold and quinacridone brown-gold are useful for creating a paint that approximates the color of very yellowed natural resin varnish.

It is often necessary to approach this problem in stages. A common method is to inpaint the loss the way the conservator believed that it originally looked. This is generally slightly lighter and slightly colder or grayer. The patination layer can then be applied on top of this layer. The conservator can tailor the patina, adding raw sienna, nickel azo yellow, or quinacridone gold to suggest remaining varnish or add a layer that resembles dirt, using such recipes as listed above from Ruhemann, Jessell, and Brealey. Davy’s grey or rottenstone may be well-suited for emulating accumulated grime.

Pigment Timeline and Primary Elements for Characterization Using X-Ray Fluorescence (XRF)

Pigment	Primary Element(s)	Introduction
White
Chalk	(Ca, S)	Ancient
Gypsum	(Ca, S)	Ancient
Lead White	(Pb)	Ancient
Barium sulfate, artificial	(Ba, S)	1782
Zinc white	(Zn)	1780s
Lithopone	(Zn, Ba, S)	1850
Titanium white	(Ti)	1916
Black
Bone black	(Ca, P)	Ancient
Ivory black	(Ca, P)	Ancient
Mars black	(Fe)	Mid-1800s
Brown
Brown ochre	(Fe & pos. Al, Si, Ca)	Ancient
Umber	(Fe & pos. Al, Si, Ca)	Ancient
Sienna	(Fe & pos. Al, Si, Ca)	Ancient
Van Dyke brown	(Organic, Fe, Al, etc. pos.)	1500s
Mars colors	(Fe)	Mid-1800s
Blue
Lapis lazuli ultramarine	(Al, S, Si, Na, & pos. Ca)	Ancient
Egyptian blue	(Cu, Si)	Ancient
Azurite	(Cu)	Ancient
Viviante	(Fe, P)	Medieval
Synthetic copper blues	(Cu)	Medieval
Smalt	(Co, Si, K)	1400-1500s
Blue verditer	(Cu)	1600s in the West
Indigo	(Organic)	Medieval
Prussian blue	(Fe)	1704
Cobalt blue	(Co, Al)	1802
Synthetic ultramarine	(Na, S, Si, Al)	1827-1830
Cerulean blue	(Co, Sn)	1860
Manganese blue	(Mn, Ba)	1935
Pthalocyanine blue	(Cu, Cl)	1935
Cobalt turquoise light and dark	(Co, pos. Li, Ti, Zn, Cr)	1973
Green
Green earth	(Fe, Si, Al, K & Mg)	Ancient
Malachite	(Cu)	Ancient
Verdigris	(Cu)	Ancient
Synthetic copper greens	(Cu)	Medival
Scheele’s green	(Cu, As)	1775
Cobalt green, Rinman’s green PG 19	(Cu, Zn)	1780 (introduced in 1830s)
Ultramarine green	(Na, S, Si, Al)	after 1827
Chromium oxide green	(Cr)	1840s
Viridian or hydrated chromium Oxide	(Cr)	1840s
Cobalt Green-Blue oxide PG 26	(Co, Cr, Al)	Late 1800s
Pthalocyanine green	(Cu, Cl)	1938
Cobalt green PG 50	(Co, Ni, Ti)	1960
Yellow
Yellow ochre	(Fe & pos. Al)	Ancient
Raw sienna	(Fe & pos. Al)	Ancient
Orpiment	(As, S)	Ancient
Yellow lead	(Pb)	Ancient
Naples yellow	(Pb, Sb, & pos. Zn in 19th c)	Ancient, 1600s-still avail.
Lead tin yellow	(Pb, Sn)	1300-1750
Patent yellow, Turner’s yellow	(Pb, Cl)	1781
Strontium chromate yellow	(Sr, Cr)	1807
Barium chromate yellow	(Ba, Cr)	1807
Indian yellow	(Mg)	1800s-1908
Chrome yellow	(Pb, Cr)	1809
Cadmium yellow	(Cd, S, & possible Zn & Ba)	1846
Zinc yellow	(Zn, Cr)	1847-1850
Mars yellow	(Fe)	Mid-1800s
Cobalt yellow or aureolin	(Co)	1861
Nickel titanium yellow	(Ni, Ti, Sb)	1960s
Red
Red ocher	(Fe & pos. Al, Si)	Ancient
Burnt sienna	(Fe & pos. Al, Si)	Ancient
Burnt umber	(Fe & pos. Al, Si)	Ancient
Realgar	(As, S)	Ancient
Cinnabar	(Hg, S)	Ancient
Red lead	(Pb)	Ancient
Vermillion	(Hg, S)	Medieval
Red lakes based on plants and bugs	(possible Al and/or Ca)	Medieval
Cochineal lakes	(Possible Al and/or Ca)	1500s
Chrome red	(Pb, Cr)	1809
Pure scarlet, iodine scarlet	(Hg, I)	1814
Mars red	(Fe)	Mid-1800s
Ultramarine red	(Na, S, Si, Al)	after 1827
Alizarin crimson, synthesized madder	(Al)	1868
Cadmium red	(Cd, Se, S, & possible Ba)	1910
Violet
Murex or imperial purple	(organic)	Ancient
Red lakes based on plants and bugs	(Possible Al and/or Ca)	Medieval
Cochineal lakes	(possible Al and/or Ca)	1500s
Ultramarine violet	(Na, S, Si, Al)	after 1827
Mars violet	(Fe)	Mid-1800s
Cobalt violet	(Co, early versions had As)	1859
Manganese violet	(Mn)	1868
Alizarin violet	(Al)	1868
Dioxazine violet	(organic)	1929

Brian Baade

Submitted November 2008

Opaque and Transparent Pigments

Introduction

A working knowledge of the relative transparency or opacity of various pigments will allow a conservator to exploit this property when inpainting. Opacity has been defined as “the degree of obstruction to the transmission of visible light”. Opacity is thus a relative term, and few paint films are either absolutely opaque or absolutely transparent. Hiding power, a term often used synonymously with opacity, is actually a measure of opacity that can be expressed numerically. Hiding thickness is the thickness of a specific paint formulation required to achieve opacity (Eastaugh, personal communication, 2005).

The opacity of a paint film is determined to a large extent by the relationship of the refractive indices of the pigment(s) and the binder (or whatever surrounds the pigment, including air pockets). In general, the closer the refractive indices are, the more transparent the paint. Exact refractive indices (η) of many pigments are listed elsewhere. The refractive index of polymerized linseed oil is 1.48 (fresh) – 1.57 (aged). Walnut oil and poppy oil are listed as η =1.480 and η =1.477 respectively. The refractive indices of many natural and synthetic resins used as binders in inpainting can be found in Volume 1 of the Painting Conservation Catalog. Most of these refractive indices fall within the range given above for linseed oil. Microcrystalline wax with a molecular weight of 600 has η =1.45. It is difficult to find refractive indices for water-soluble media. Casein has η =1.53 and dried egg yolk η =1.525.

The other factors affecting paint opacity are complex, but include pigment particle size, shape and regularity, dispersion and flocculation, pigment-volume concentration, smoothness of the surface, and thickness of the paint layer. Both adulterants and natural impurities can affect opacity. Thomas Brill has written an excellent and in-depth explanation of how refractive indices and other factors affect paint opacity. Because of these factors, even the same type of pigment from different sources can vary in opacity. The properties of a pigment from a natural source will vary from the properties of the synthetic analogue. In addition, some relatively opaque pigments can be used in glazes due to their high tinting strength, since only a small concentration of pigment is necessary.

In general, historic pigments except lead pigments are more or less transparent, modern organic pigments are transparent, and most modern inorganic (metal-based) pigments are opaque (see Website. However, there also seem to be exceptions to these rules.

Needless to say, any chart of pigment opacity can only give an indication of the opacity of an inpainting mixture, which will be affected by all the factors described above. In addition, it happens occasionally that sources disagree on a certain pigment’s opacity. In the list that follows, opacity is generally understood to be that in oil or resin-type media. Pigments with “shifting” characteristics, i.e., those that are more opaque when used in some water-based media, are marked. Finally, specialist pigments suppliers offer such a wide variety of pigments that it is impossible to include them all on the list.

Conservators are encouraged to consult these companies and their catalogs when seeking solutions to specific problems or to find the characteristics of modern pigments. Please note that pigment permanence is only summarily indicated and needs to be checked separately, as does toxicity.

Chart of Pigment Opacity

Paint Opacity

Whites +
Barium white (Natural barium sulfate. Barites, permanent white)	transparent -
Blanc fixe (Synthetic barium sulfate)	transparent - but less than natural barium sulfate
Chalk (Natural calcium carbonate. Whiting, lime white. Synthetic calcium carbonate: precipitated chalk)	transparent -
Calcite (Natural calcium carbonate. Oyster shell white, Gofun shirayuki, Japanese pearl white)	transparent -
Lead white (Basic lead carbonate. Flake white, Cremnitz white, ceruse)	opaque
Lithopone (Zinc sulfide-barium sulfate mixture)	opaque
Titanium white (Titanium dioxide. Rutile)	very opaque
Zinc white (Zinc oxide. Chinese white, permanent white)	transparent -
Yellows
Barium yellow (Barium chromate. Lemon yellow, permanent yellow)	semi-opaque
Bismuth yellow (Bismuth-vanadium-molybdate)	opaque
Cadmium yellows and oranges (Cadmium zinc sulfide. Adjectives: lemon, opaque, pale, middle, deep, orange)	opaque, slightly less than chrome yellow. Paler shades slightly more opaque due to smaller particle size
Cadmium yellow (or orange) lithopone (Cadmium zinc sulfide-barium sulfate mixture)	fairly opaque
Chrome yellows and orange (Pure lead chromate. Adjectives: lemon, middle, deep, orange)	fairly opaque
Chrome yellow (Lead chromate and lead sulfate)	opaque, but less than pure lead chromate
Cobalt yellow (Potassium cobaltnitrite. Aureolin)	transparent
Diarylide yellow (Diazo)	transparent
Gamboge (A tree gum resin)	transparent
Hansa yellow and orange (Product of aniline derivatives + acetoacetanilide.)	transparent, decreases with increasing proportions of extenders
Indian yellow, true (Magnesium salt of euxanthic acid)	translucent
Indian yellow, synthetic (Nickel azo)	transparent
Intense yellow (Zirconium-praseodymium-silicate)	transparent
Lead-tin yellow (Lead-tin oxide, types I and II)	opaque
Mars yellow (Hydrated iron oxide)	semi-opaque, but the most translucent of the yellow iron oxides
Massicot (Yellow monoxide of lead. Litharge differs slightly;	opaque
Naples yellow (Lead antimoniate. Antimony yellow)	opaque
Nickel-titanium yellow (Nickel titanate. Sun yellow)	translucent
Orpiment (Yellow sulfide of arsenic. Arsenic yellow. Synthetic: King’s yellow)	opaque
Quinacridone gold (Quinacridone)	transparent
Realgar (Orange-red sulfide of arsenic. Arsenic orange)	opaque -
Sienna, raw (Iron and aluminum oxides plus hydrous silicates)	transparent, especially Italian sources
Strontium yellow (Strontium chromate. Strontian yellow, lemon yellow)	semi-opaque
Yellow ocher (Hydrated iron oxide with silica and clay.)	opaque - , clay-rich types most opaque; calcium carbonate-rich types less opaque
Zinc yellow (Zinc chromate. Zinc chrome, citron yellow, primrose yellow)	semi-opaque
Reds
Alizarin crimson (Synthetic organic lake. Alizarin madder, rose madder)	transparent
Cadmium red (Cadmium sulfo-selenide. Selenium red. Adjectives: scarlet, light, medium, deep)	opaque
Cadmium red lithopone (Cadmium sulfo-selenide and barium sulphate)	opaque
Carmine lake * (Natural organic lake. Crimson lake, Florentine lake)	transparent -
Chrome red (Basic lead chromate. Chrome orange, American vermillion)	opaque
Irgazine red (Synthetic organic)	opaque
Iron oxide reds, natural earths (Iron oxide 10–95% + silica and clay. Red ochre, caput mortuum, sinopia, Spanish red, Pozzuoli red)	opaque, but high tinting glazes
Iron oxide reds, synthetic (Mostly pure iron oxide: Red oxide, Indian red, English red, Turkey red, iron oxide + extenders: Mars red, Venetian red, Pompeian red.)	opaque, but high tinting strength. Mars red makes satisfactory glazes.
Madder lake * (Natural organic lake. Rose madder, crimson madder)	transparent
Naphthol red * (some) (Monoazo)	translucent
Ocher, burnt (Yellow ocher heated to remove the water of crystallization. Light red)	semi-opaque
Permanent red (Precipitated organic pigment. Helio fast red, scarlet lake)	semi-opaque
Quinacridone reds and magenta (Quinacridone.)	transparent
Red lead * (Lead tetroxide. Minium, Saturn red, orange lead)	opaque
Vermilion (Mercuric sulfide. Natural European mercuric sulfide: cinnabar.)	opaque
Chrome red (Basic lead chromate. Chrome orange, American vermillion)	opaque
Browns
Bitumen * (Natural hydrocarbon mixture. Asphaltum)	transparent
Brown ochers (Natural admixture of ocher and manganese oxide)	opaque
Green earth, burnt (Iron-III- silicate and alumina. Burnt terre verde, Veronese brown)	translucent
Quinacridone burnt orange (Quinacridone quinone)	transparent
Sienna, burnt (Calcined iron oxide. Pompeii red, Italian earth)	transparent
Umber, raw (Hydrous iron oxide and manganese dioxide with aluminum silicate Cyprus umber)	translucent
Umber, burnt (Calcined iron oxide and manganese dioxide with aluminum silicate)	translucent, but a few dark varieties are fairly transparent
Van Dyke brown * (Bituminous lignite or brown coal. Cassel earth, Cologne earth, Rubens’ brown)	transparent
Greens
Blue-green oxide (Cobalt chromium compound)	translucent
Chrome green (Lead chromate + ferric ferrocyanide. Cinnabar green, Brunswick green)	opaque
Chromium oxide green (Chromic oxide. Reading green)	opaque
Cobalt green (Cobalt zincate. Zinc green, Rinmann’s green, Saxony green)	opaque
Emerald green (Copper aceto-arsenite. English green, Paris green, Schweinfurt green, Veronese green)	translucent
Green earth (Ferrous silicates + aluminum and magnesium. Terre verde, Bohemian earth, Verona green)	translucent
Malachite (Basic copper carbonate. Mineral green, mountain green Synthetic: green verditer, green bice)	transparent
Permanent green deep (Hydrated chromium oxide + barium sulfate)	transparent -
Phthalocyanine green (Phthalocyanine. Monastral green, thalo green, heliogen green)	transparent
Verdigris (Basic copper acetate)	transparent
Viridian (Hydrated chromium oxide. Guignet’s green, emerald oxide of chromium)	transparent, but thick layers can be opaque
Zinc chrome green (Zinc chromate + ferric ferrocyanide)	opaque
Blues
Azurite (Basic copper carbonate. Mountain blue, copper blue. Synthetic * : blue verditer, blue bice)	translucent
Cerulean blue (Cobaltous stannate)	semi-opaque -
Cobalt blue (Cobalt aluminate. Thénard’s blue, azure cobalt. Adjectives: light, dark.)	semi-opaque -
Egyptian blue (Calcium-copper silicate. Blue frit, Pompeian blue)	transparent
Indigo * (Plant extract indigotin; also exists as the synthetic thioindigo)	translucent
Indanthrone blue (Anthraquinone)	transparent
Lapis lazuli (A complex sulfur-containing sodium aluminum silicate. Natural ultramarine, azure)	transparent -
Manganese blue (Mixed crystal barium sulfate-permanganate)	translucent
Phthalocyanine blue (Copper phthalocyanine. Monastral blue, heliogen blue, thalo blue, fast blue)	transparent
Prussian blue (Ferric ferrocyanide. Berlin blue, Chinese blue, Milori blue, Paris blue)	transparent
Smalt (Cobalt silicate in the form of glass frit. Saxon blue)	transparent
Ultramarine blue, artificial (Sodium aluminum silicate-polysulfide. French ultramarine, French blue)	transparent -
Violets
Cobalt violet (Cobalt arsenate [paler], cobalt phosphate [deeper], or a mix of the two)	translucent
Dioxazine violet (Oxazine. Dioxazine purple, carbazole violet)	transparent
Manganese violet (Thought to be manganese ammonium phosphate. Permanent violet)	translucent -
Mars violet (Synthetic iron oxide)	opaque
Quinacridone violet (Quinacridone)	transparent
Ultramarine violet (Sodium sulfosilicate. Ultramarine red)	translucent
Blacks
Bone black (Carbon + calcium phosphate, calcium sulfate and salts)	semi-opaque, but used to glaze metal leaf
Ivory black: today virtually synonymous with bone black.
Lamp black (Pure carbon, with possible oily components. Carbon black)	opaque
Manganese black and manganese gray (Manganese dioxide)	opaque
Mars black (Ferroso-ferric oxide. Iron oxide black, iron black, mineral black)	opaque, but high tinting strength; used to glaze metal leaf
Slate gray	transparent -
Spinel black (Contains copper and manganese)	opaque
Vine black (Mostly pure carbon. Spanish black, blue black, cork black)	semi-opaque

Key: + Format: pigment name (approximate composition; alternative names), *questionable permanence, - shifts towards opaque in some water-based media

Opacity: opaque, semi-opaque, translucent, or transparent

Exploiting Opaque and Transparent Pigments When Inpainting

Introduction

Since the colors of many historic paintings are built up through the use of numerous relatively transparent paint layers, some conservators try to imitate the layered technique of each painting as closely as possible, feeling that it results in the best inpainting match. Others may fear that glazes will not have acceptable color permanence for inpainting.

Determining whether the original passage was painted with opaque or transparent pigments gives the conservator the possibility of achieving an inpainting that matches not only the color but also the depth and luminosity of the surrounding original paint. Observation of Van Heemskerck’s flesh tones suggested that his mixture of azurite, yellow ochre, vermilion or iron oxide red, and lead white pigments could be useful in inpainting renaissance flesh colors. In various proportions, you get all shades of pink, grey, purple, light brown, etc. This mixture is luminous, as opposed to a mixture of white and black or white and umber (Woudhuysen-Keller, personal communication, 2005).

Glazing with transparent pigments can be used as an alternative to mixing pigments together. For example, Titian’s technique of creating a very dark green by applying a green glaze over a dark red layer could be useful in inpainting (Woudhuysen-Keller 2005).

Applying a transparent or translucent final glaze over a slightly pale area of inpainting is a method used by many conservators to achieve an exact match to the original. For example, John Brealey recommended underpainting a large loss in a translucent oil painting with the pure color in tempera and then glazing in varnish medium with raw sienna and ivory black as a transparent “discolored linseed oil” color (Stoner, personal communication, 2005).

A scumble of opaque pigments, applied thinly enough to be semi-transparent, can create a dull, bluish, or hazy effect. For example, in an adjustment of another John Brealey technique, mixing raw sienna with white makes a good starting translucent color to inpaint black cracks in blue skies. Due to Rayleigh’s law, the milky “scumble” of those colors turns bluish and often matches the surrounding sky. The color can, of course, be adjusted with ultramarine or viridian, etc., according to the sky (Stoner 2005). Naples yellow can be used to scumble over darker paint and create this bluish effect, also known as the turbid medium effect. Wehlte commented that Prud’hon used Naples yellow scumbled over brown underpainting for shadows of flesh tones.

Titanium white has such high opacity that scumbles are easily made by combining it with translucent or even transparent pigments.

Mixtures Exploiting Opaque Pigments

Naples yellow is an opaque pigment that can be substituted for whites in mixing. A mixture of yellow ochre and cerulean blue can be useful for aged, warm sky colors.

Mixtures of titanium white, vine black, and yellow ochre can be useful to inpaint pale sky blues, possibly with less risk of metamerism problems than when blue pigments are used.

Chromium oxide green and Venetian red make a good opaque “dirt” color (Stoner 2005, attributed to J. M. Brealey).

Mixtured Exploiting Transparent Pigments

A glaze of Indian yellow plus vine black can imitate a yellowed varnish.

A general glazing mixture consisting of ivory black, burnt sienna, and Indian yellow is helpful to imitate discoloured varnish or for the final glaze on top of tempera, as well as having many other uses. With more burnt sienna, the tone is warm; with more Indian yellow and black, it is cooler and more greenish. It is a very versatile mixture (Massing, personal communication, 2005, attributed to J. M. Brealey).

Mixtures of Indian yellow, alizarin crimson, and ivory black can be used to create deep, transparent browns. Mixtures of black, Indian yellow, and earths are useful for glazing.

A range of transparent darks that can mimic dirt, discolored varnish residues, etc., can be mixed with alizarin, Indian yellow, and pthalo green. By varying the proportions very slightly, you can make a warmer or cooler and more or less golden brown (Gifford, personal communication, 2005).

Dark glazes can also be mixed using alizarin, Indian yellow, and Prussian blue. Vary the proportions to create all shades of transparent brown, gold, purple, or black (Woudhuysen-Keller 2005).

Phthalo blue or viridian can be added to deepen blacks. A transparent black can be mixed from dioxazine violet and viridian.

Quinacrinone Gold (Golden PVA) is totally transparent with great tinting strength. It can warm up a black dye color (for example, Nigrosin from Kremer pigments is a cold black that can be warmed up by the gold), or it can add that “discolored linseed oil” look to your mix. For 17th-century paintings, burnt sienna and viridian together are another great starter for aged linseed oil matches (Stoner 2005).

Raw Sienna and Ivory Black make a good transparent “dirt” color (Stoner 2005, attributed to J. M. Brealey).

Hooker’s green in Charbonnel and Bruno di Garanza from Maimeri will make any shade of degraded/not-so-degraded copper resinate green (Metzger, personal communication, 2004). (Maimeri lists the components of Bruno di Garanza as viridian, diarilide yellow, and anthraquinone. Hooker’s green is generally a mix of Prussian blue and, traditionally, gamboge or, more likely today, synthetic yellow pigment.)

Plesters’ greens are replacements for copper resinate greens designed to have good permanency by Joyce Plesters, formerly of the Scientific Department, National Gallery, London. Plesters transparent green consists of 7 parts by weight Indian Yellow (Winsor and Newton synthetic) plus 2.5 parts by weight Monastral Green (I.C.I. grade CS). (Plesters Opaque Green: see in the section below.) Mix the dry pigments well together on a ground glass sheet (as large as possible) with a palette knife. Then add a few milliliters of methylated alcohol to give a fairly wet paste. Grind very well with a muller. Allow to dry out thoroughly in a warm place, preferably overnight, then scrape from the glass plate and muller. Wet and rub out again a small sample and if it is not completely homogeneous and streaks of any one pigment are visible, repeat the wet grinding. Pass the thoroughly dried pigments through a fine sieve and pack into dry warm jars.

Mixtures of Opaque Plus Transparent Pigments

Adding 1– 2% Titanium white to a transparent pigment can make the paint more opaque when this is desired.

Plesters opaque green (see above): 10.5 parts by weight yellow ochre (Ralph Nye and Biddle), 1.5 parts by weight cadmium yellow pale (Winsor & Newton), and 1 part by weight Monastral green (I.C.I. grade GS). Mix following the instructions above.

Finally, when it is necessary to match a brilliant color, an observation by Ralph Mayer may be useful. He pointed out that mixing white with a transparent pigment results in the best clarity of tone. Using alizarin as an example, he describes the effect “as if each particle of white were surrounded by an envelope of the transparent red…Tints or mixtures of opaque colors are usually duller.”

Janine Wardius

Submitted September 2007

Metamerism

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We tend to think of color as an inherent physical characteristic of a substance, and to an extent that is true. However, external changes can dramatically affect the hue that a substance appears to have. The appearance of a color is not independent of the conditions in which it is viewed. When a conservator is trying to mask damage with invisible inpainting, this may create difficulties. A loss hidden to perfection in one set of viewing conditions may stand out in stark contrast to the surrounding paint in others. This phenomenon is called metamerism.

Viewing Conditions

Illumination

Different types of changes in the viewing conditions can have an effect. The most common, perhaps, is a change in the source of illumination under which the painting is viewed. This is because the spectral power distribution, or the relative amounts of different wavelengths of light that each light source contains, varies considerably. For example, daylight has a fairly even distribution of the different wavelengths of visible light. Tungsten, on the other hand, has a relatively small amount of short wavelength or blue light, and a very large amount of longer wavelength or red light. If you matched a color in the natural light of the restorer’s studio and then put it back on the wall in a museum gallery with light provided by a tungsten bulb, much more red light would be shining on it. A retouching that had a little more red in it than the original paint might have matched perfectly well in the even light of daylight. However, with the extra red light provided by the tungsten bulb, it will reflect more red and will no longer match.

Viewer

Another factor is the viewer. Different people can have biological variations in their color vision. In addition, the lens of the eye becomes progressively yellow with age. A retouching made by an older conservator may prove visible to a much younger viewer.

When Photographed

Metamerism can also be a problem when a work of art is photographed. A freshly restored painting, photographed for the exhibition catalog, suddenly displays every damage with its metameric retouching. The relative sensitivity to color of color film is different from that of the human eye. Therefore, colors that match to the eye may not match in a photograph. Digital photography may offer a means to correct these problems by adjusting the hue on the computer after the image has been captured.

Preventing Metamerism

Because you cannot know under what varying conditions a painting will be viewed in the future, the only way to prevent metamerism from occurring is to match as closely as possible the reflectance curve of the original with the reflectance curve of your retouching. Where you know which pigment or combination of pigments the artist used, and can obtain the same ones, this is not a problem.

However, that is rarely the case. In addition to the problem that some pigments are no longer available or have been found to be unstable, the explosion of available modern pigments and their proprietary formulations have made this process extremely difficult. In addition to the increase in the number of possible pigments used, many applications of paint are complex mixtures of pigments. Michael Swicklik et al. have devised a method, in a relatively simplified way, of analyzing the reflectance of the original and determining a retouching color or color combination that would have a similar reflectance curve. Practically speaking, this is not always possible, as not every conservator has access to a spectrophotometer.

Alternatively, if you know or can make an educated guess as to the original pigment(s), if this pigment is no longer available you can use published reflectance curves of the original and of modern pigments to choose a pigment with a similar reflectance curve. If you know the painting will be viewed under different sources of illumination, check your retouching under all those different light sources. The medium does not significantly affect the shape of a paint’s reflectance curve. Adding white to a pigment increases the overall level of reflectance but does not change the shape of the curve and therefore does not have an effect on metamerism.

Jane Tillinghast Sherman

Submitted May 2009

Vendors

Pigments

Kremer Pigments
247 West 29th Street
New York, NY 10001
Phone: (212) 219-2394 or (800) 995-5501
Fax: (212) 219-2395
Website

Sinopia
321 Seventh street
San Francisco, CA 94103
Phone: (415) 824-3180
Fax: (415) 824-3280
Website
sinopiasf@sinopia.com

Natural Pigments
Phone: (888) 361-5900
http://naturalpigments.com

PVAc, AYAA, and AYAB resins

Conservation Materials Limited
1395 Greg Street, Suite 110
Sparks, NV 89431
Phone: (800) 733-5283

Additional Resources Consulted

Feller, R. 1959–1966. The quarterly reports of the National Gallery of Art for the Mellon Institute, Pittsburg. Vols. 4–7.

Howard, H. 2003. Pigments of English medieval wall paintings. London: Archetype Publications.

Kremer Pigments Catalog 2005.

Kremer, G. 2008. Private communication, Nov. 14.

Mayer, R. 1991. The artist’s handbook of materials and techniques. 5th ed. New York: Viking Press.

Metzger, C. 2008. Personal communication.

McGinn, M. 2007. Personal communication.

Sinopia Catalog 2007.

Thomas, A. W. 1980. Colors from the earth: The artist’s guide to collecting, preparing, and using them. New York: Van Nostrand Reinhold Company.

Wardius, J. 2008. Short unpublished paper on the subject of dry pigments.

Endnotes

^{Conservators who are interested in learning more about this are encouraged to attend the Mastering Inpainting Workshop that Bernstein teaches or to contact Jim directly.}