Ceramics

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THIS ENTRY IS A DRAFT

Sao Earthenware Pot (2008.128), Worcester Art Museum, Worcester, Massachusetts, Austin S. Garver Fund

Ceramics are objects made from clay (or clay mixed with other materials) that are subjected to high heat. This application of heat causes irreversible changes in within the clay body, rendering the form permanent. Ceramics can take the form of pottery (utilitarian vessels), sculpture, casting cores for bronzes, or architectural elements such as tiles, pipes and bricks.

Generally speaking, ceramics are brittle and weak under shearing or tensile stresses but very strong under compression. In addition, they can withstand very high temperatures and are not prone to chemical erosion. They consist of both crystalline and amorphous phases, though the ratio of these phases is determined by the specific mineral composition.

Materials and technology[edit | edit source]

History[edit | edit source]

Ceramic technology in its simplest form has been around since the Neolithic period. There are a host of regional differences in available materials, forms, style and technology that have developed in different parts of the world, making the ceramic arts highly variable across time and space.

Materials[edit | edit source]

Clay[edit | edit source]

Kaolin, a primary clay

The primary component of the ceramic body is clay, though it is seldom free of other naturally present or intentionally added materials in smaller quantities (see Tempers and Fluxes below). Clay (the hydrous silicate of aluminum) is the decomposition product of sedimentary mineral deposits, and is characterized by microscopic, plate- like particles with a large surface area to volume ratio. The size and shape of the particles, which varies from deposit to deposit, are the keys to this material’s utility. When just the right amount of water is added, the plate-like particles easily slide past one another, making clay easy to manipulate into almost any shape. After this “free” water (as distinguished from water that is chemically bound to the clay minerals) is again lost to evaporation, the resultant form is hard enough to handle without deforming and can be finished and fired to permanently set that shape.

Primary clays (or residual clays) are found in the same location as the original mineral deposit from which they have weathered. Kaolin is an example of a primary clay. They are typically low in plasticity (ie. lean or short), coarse, and may contain impurities depending on the composition of the parent rock.

Secondary clays (or sedimentary clays) have been transported from the location of the original deposit by wind or water and redeposited. As a consequence of this relocation, secondary clays tend to have a greater number and variety of impurities (and therefore are darker in color) but have higher plasticity (are fat or long clays) and finer particles than primary clays.

Tempers[edit | edit source]

Tempers are materials added (or inherent) to the clay that modify its working properties and the characteristics of the fired ware. Mineral tempers (such as sand) reduce the amount a ceramic shrinks during the firing process and will therefore prevent cracking and warping in the finished product. Grog, or previously fired and ground ceramic material, is often used to this end in high-fired ceramics. Organic tempers (for example, grasses or dung) are added to create porosity in low-fired ceramics. When the ceramic is fired, the organic material burns out and leaves small pockets through which gasses can escape, preventing explosions in the kiln.

Fluxes[edit | edit source]

Fluxes are alkaline additives (or components inherent to the clay) that serve to reduce the melting point of the silica content of clay, glaze or overglaze pigments. Fluxes include iron oxides and calcia (from naturally occurring impurities in the clay or ground shell, for example) for low temperature firings, and potash, soda, and magnesia (often contributed by feldspars) for high temperature firings.

Types of ceramic wares[edit | edit source]

Ceramics are often categorized based on the characteristics of the clay from which they were formed, the temperature at which they were fired, and the resultant structure of the ceramic body.

•Greenware is a term to describe vessels that have been shaped but have not yet been fired, and are, therefore, not technically ceramics.

•Leather-hard describes wares that have lost their free water, but not their chemically bound water (approximately 15% water content remains). It is at this stage that many finishing techniques are done (see below). Like greenwares, leather-hard vessels are not technically ceramics.

•Bisque (or biscuit ware) describes a ceramic vessel after it has lost its water of crystallization during a preliminary firing. This is the point at which reversion to a lump of clay is no longer possible.

•Earthenwares are made from coarse clays and are quite porous and completely opaque. They are fired in a range from 700-1200°C. Although many earthenwares are glazed to minimize the porosity, some, like terracottas, are left unglazed.

•Stonewares (or vitreous chinas) are fired in a range from 1200-1350°C, and are more dense-bodied than earthenwares.

•Porcelains are vitreous ceramics with translucent white bodies. They are fired in a range from 1220-1450°C.

•Hard-paste porcelain (or true porcelain) was developed in China around or before 600 CE and is made from naturally occurring materials. Traditionally, this is kaolin (a primary clay whose light color is due to its low iron content) and petuntse (also known as chinastone, a mixture of feldspar and quartz).

•Soft-paste porcelain was developed in Europe around 16th century in an attempt to imitate Chinese porcelain. The recipe includes glass or glassy frit – a manufactured ingredient not found in hard-paste porcelain.

Technology[edit | edit source]

Preparation of the clay[edit | edit source]

After a clay is mined, it will likely require further processing to refine its qualities. Impurities can be removed by filtration or through levigation, a process whereby particles of different sizes are separated in a series of water baths. Afterwards, excess water can be allowed to evaporate and the clay can be left to age. This process increases a clay’s plasticity by allowing the particles to settle into alignment with one another. Before creating a ceramic object, the clay mass must also be compacted and the moisture within it evenly distributed. This is accomplished by wedging or pugging, done by repeatedly slamming the clay against a surface to eliminate any trapped air.

Creation of the structure[edit | edit source]

When clay is wet and plastic, there are any number of ways it can be shaped into the desired final form. Hands are the most important tools but some techniques also employ paddles, rollers, cutting wires, forms, molds, pottery wheels, etc.

Handbuilding describes several different vessel construction techniques – basically, those that do not employ a pottery wheel. Vessel walls can be gradually raised from a ball of clay by pinching between the fingers, or they can be built up with a rolled length of clay coiling upwards from the base (coil-built). In slab construction, vessels are formed with rolled or cut slabs assembled with slip or wet clay. Any seams resulting from the latter two techniques are usually smoothed out after the basic structure is complete.

The use of molds and forms enable the rapid creation of a series of nearly identical wares. By incorporating decorative patterns or motifs into the mold or form, a potter can simultaneously shape and decorate a vessel. Flat slabs of clay can be paddled over a form or pressed into a mold. The clay can also be made fluid with the addition of water and poured into a mold.

Another, later development in potting technology involves the use of a pottery wheel – a variously powered spinning platform on which a vessel can be raised. Wheel-shaped or thrown pots can be made much more perfectly round than handbuilt vessels.

More modern techniques include extrusion, powder pressing and lathe turning.

The Johns Hopkins Archaeological Museum blog on Recreating Ancient Greek Ceramics contains extensive information on ceramic technology.

Decoration[edit | edit source]

Structural[edit | edit source]

After the basic structure of a vessel has been established, the clay is allowed to dry to a leather hard state. It is at this stage that further refinements and embellishments can be made. Trimming, or fettling is done to smooth the form. The surface can be incised or impressed with designs or textures, or can be burnished to a high sheen. Additional elements such as handles, spouts or feet can be separately formed (either by hand or with the aid of molds) and attached, or luted, to the body of the vessel with clay slip. When relief decorations are molded and applied in this way it is called sprigging.

The color of clay can also be utilized for decorative effect. For example, clays with differing mineral content (and, therefore, different colors) can be layered or wedged together until they are marbled together. Beautiful patterns can be exposed by selectively cutting away layers or cross-sections. This technique has several different names: millefiore, agateware or neriagi.

Surface[edit | edit source]

Decorative slips, or fluid slurries of clay, can be applied in a multitude of ways to add color to a vessel. The slip can cover the entire pot or can be locally applied to the surface with brushes, quills, combs or other tools. For example, the Ancient Greeks employed slips painted onto the surface and fired in carefully controlled environments to create the sharp and highly contrasting designs found on red-figure and black-figure vessels. Slip can also be inlaid into recesses prepared in a clay body. In another technique called sgraffito, a contrasting colored slip is applied overall and when dried, is carved away to expose the clay of the vessel body.

Glazes are vitreous or semi-vitreous coverings on ceramics. Glazes may have multiple functions, including waterproofing a porous ceramic and creating a decorative layer that contributes color or gloss to a ceramic body.

Slip glazes: Slip glazes are made of ceramic material. Slip glaze must have a lower fusing temperature than the ceramic body it is applied to in order to form an effective glaze. Slip glazes may be sintered or vitrified depending on firing temperature. They may be applied by painting, extruding, or dipping, and may be burnished in the leather-hard state.
Egyptian paste: Egyptian paste, also called faience, consists of silica sand, lime, small amounts of clay, sodium carbonates and bicarbonates, and copper oxides. As the paste dries, the carbonates and dissolved copper oxides are brought to the surface. When fired, the paste self-glazes to produce a glossy turquoise blue surface color.
Lead glazes: Pure lead glazes are colorless and do not contribute opacity on their own. They have relatively low firing temperatures, enabling their coloration with a variety of metal oxides to create a wide palette of colors. They are fluid enough to penetrate porous ceramics well. Boron may be added to increase their hardness, also increasing their firing temperature. First use: Ptolemaic Egypt c. 300-200 BCE.
Lead/tin glazes: Lead glazes become opaque and white when white tin oxide is added. Lead/tin glazes have been widely used to cover and whiten darker ceramics, often in imitation of Chinese porcelain. Lead/tin glazes have a low firing temperature and may be tinted with a range of metal oxides. Lead/tin glazes coat the ceramic rather than penetrate it and shrink as they cool.
Salt glazes: Salt glazes are made by adding sodium chloride to a kiln at around 1000°C. The salt dissociates and sodium condenses on the ceramic. It creates a glassy glaze layer with a characteristic "orange-peel" texture.
Feldspathic glazes: Feldspathic glazes are the major glaze used in European "hard paste" porcelain and Chinese porcelain. They require high firing temperatures, which may be lowered with the addition of calcium.
Luster glazes: Luster glazes have a metallic sheen or opalescent appearance when in very thin layers.
Ash glazes: Ash glazes have high firing temperatures, which may be lowered with the addition of slip. They have a glasslike appearance and highlight the texture of the ceramic body.
Enamel or overglaze colors: metallic oxides with firing temperatures lower than that of the glaze (usually 700-900°C)

Glaze colorants

Metal ions and the colors they produce:

Chromium +3: green

Cobalt +2: deep blue

Cobalt in excess: black

Copper +2: blue, green

Copper +1: red

Iron +2: blue, green

Iron +3: brown, amber

Manganese +3: purple brown, violet

Manganese +2: pink, red

Nickel +2: green, yellow

Titanium +3: purple

Glaze application (ie. dipping, pouring, grounding, hand painting, transfer decoration, spraying)

Underglaze decoration: The decoration and the overall glaze are usually applied to a sintered ceramic in the same step, requiring only two firings of the ceramic. The limitation of underglaze decoration is that as firing temperatures increase, few glaze colorants retain their color. At firing temperatures for porcelain, only cobalt blue and copper red function as underglaze colorants (usually fired between 1200 and 1400°C).

Overglaze decoration: Overglaze is applied over a base glaze, and may be fired in one or several separate firings depending on what is appropriate for a particular color. This allows for a full range of colors, even on high-fired wares.

Bole

Cold painting with oil or lacquer based colors

Gilding

Firing[edit | edit source]

Oxidizing and reducing atmospheres, biscuit and glost firings, temperatures, types and features of kilns

Identification[edit | edit source]

Thin-section microstructure of unetched 99.9% Al₂O₃ ceramic in polarized, transmitted light. A full-wavelength rectifier provided the color contrast.

While a simple visual examination, or examination under a binocular microscope, can provide a great deal of information about an object, there are several analytical techniques that can shed further light on ceramics. When used in conjunction with relevant art historical information, information gleaned from these techniques can indicate the ceramic fabrication method and process conditions, aid in authentication, and may even contribute provenance by correlating objects to specific geological deposits. Further, conservators use these techniques to determine the type and degree of deterioration and to predict or evaluate the efficacy of different treatments.

Thin-section petrography uses a polarized light microscope to identify the mineral content of a 30 μm thick, epoxy-mounted and polished ceramic thin-section based on several optical properties. These include the size, shape, color, pleochroism, refractive index, optical symmetry and cleavage characteristics of individual grains, as well as the abundance of and relationships between grains. Stains and dyed epoxies can be employed to highlight particular minerals and to more easily assess porosity in a ceramic structure (Reedy 2008).

Neutron Activation Analysis (NAA) also requires a small (~50 mg) sample which is powdered and irradiated. The artificial radioisotopes that result are detected, giving both qualitative and quantitative measures of major, minor and trace elements. Using this chemical fingerprint, ceramics can be related to known geological deposits.

Thermoluminescence (TL) dating involves the measurement of radiation accumulated in the crystalline structure of a ceramic since the time of its last firing (Aitkin 1985). Dates are given as ranges. The accuracy of the technique can be affected by x-ray radiography, so TL dating must be considered prior to any radiography of the ceramic.

Many archaeological ceramics suffer from contamination by soluble salts (nitrates, chlorides and sulfates) (Buys and Oakley 1993). The presence of soluble salts can be detected with a conductivity meter or by the use of microchemical testing (Odegaard, Carroll and Zimmt 2000), and can be addressed by desalination if necessary.

Deterioration[edit | edit source]

Factors causing ceramics deterioration can be divided into four broad categories: flaws in the materials, design or circumstances of manufacture; use; if archaeological, the burial environment; and the circumstances while in storage or on display.

Flaws in materials, design or manufacture[edit | edit source]

•Too much temper or the wrong temper will cause the ceramic to be intrinsically weak and prone to breakage.

•“Lime popping” may occur if the ceramic temper includes calcium carbonate. When heated during firing, calcium carbonate (CaCO₃) loses its bound water and converts to simple calcium oxide and carbon dioxide (CaCO₃ → CaO + CO₂). If the calcium carbonate then rehydrates to lime (Ca(OH)₂), it will expand, causing the ceramic to crack or spall.

•Air bubbles can create structurally unsound areas.

•Uneven or fast drying can create microfractures and structurally unsound areas.

•Seams of slab- built and coil-built vessels are inherently weak, and may crack during drying or firing. If not well-applied, components such as applied handles, bases, or low-relief decorations may also develop cracks at their seams during drying or firing, or the seams may fail altogether.

•The clay may “spring” (or release tension, with resulting cracks or breaks) if the ceramic was dried unevenly or dried under tension.

•Mismatched thermal expansion coefficients of the glaze and the ceramic will create an inherently unstable interface between the decoration and body.

•Faulty proportions of glaze ingredients can result in unstable glazes that become microcracked, cloudy or “corroded.”

•Incorrect firing atmospheres can produce unwanted results. For example, white lead glazes fired in a reducing atmosphere rather than an oxidizing atmosphere can reduce to metallic lead, which is black.

Use[edit | edit source]

•Normal use or accidental mishandling can result in mechanical breakage or abrasion.

•Deliberate or ceremonial damage, such as ritual breakage (Abend et al. 2010) or “kill holes,” may be introduced.

•Contact with water, food or other contents, soot, nicotine, iron mounting hardware for architectural elements, and so forth, can introduce stains.

•Foreign additions such as grout can disfigure the object and can expose the ceramic to detrimental salt compounds, acidity, or alkalinity.

•The introduction of salt compounds from contents, or from airborne sources such as sea air or pollution, may result in weakening or fragmentation of the ceramic. If the ceramic is subjected to cycles of drying and wetting, the salt compounds within the ceramic pores will crystallize and mobilize in alternation, producing severe pressures on the ceramic body and ultimately causing cracking and surface spalling.

•Fire can discolor, crack, break and warp ceramics.

•Long-term exposure to air, acids, or alkaline agents such as cleaning agents, may produce “crazing” or microcracking of glazes.

The archaeological burial environment[edit | edit source]

•Soil pressure may induce mechanical breakage and abrasion.

•Paint, slip, or glaze may detach if poorly matched to the ceramic body, and under damp burial conditions, the surface tension of the water may affix detached paint, slip or glazes more firmly to the surrounding soil than to the ceramic body.

•Biodeterioration may occur, including attack by macro-organisms such as lichens, but also microbial attack (Seaward 1989). Bacterial activity, particularly sulphate-reducing bacteria, can produce black staining.

•Insoluble salt compounds (commonly referred to as caliche or limescale) may deposit on the surface.

•Soluble salt compounds may be introduced into the ceramic pores from ground water, agricultural run-off, seawater, or other sources. If the ceramic is subjected to cycles of drying and wetting, the salt compounds will crystallize and mobilize alternatively, producing severe pressures on the ceramic body from within the pores, ultimately causing cracking and spalling.

•Manganese dioxide accretions may obscure the surface (O'Grady 2005).

•Contact with iron or copper may introduce staining. Standing water, some organic materials, and a variety of other substances may also introduce staining.

•In an alkaline burial environment, some glaze components may leach, with resulting crizzling or detachment of the glazes.

•In an acidic burial environment, certain types of temper may dissolve, resulting in a drastically weakened ceramic body.

•In a waterlogged burial environment ceramics may experience "well rot", or a weakened ceramic body due to the dissolution of temper and binders. If the object is actually unfired clay rather than fired ceramic, the clay may rehydrate and slump, or lose its form altogether. Wet burial environments foster biological growth.

•Exposure to fire can crack, warp and break ceramics. If the ceramic is under reducing conditions during the fire, iron-based red slips can become black, and if under oxidizing conditions, black slips can become red.

The storage and display environment[edit | edit source]

•Poor handling may result in mechanical breakage or abrasion.

•Inadequate padding and improper stacking in storage may result in abrasion, failed joins (if previously repaired), and breakage.

•Improper mounting materials may cause staining and physical damage. Improper materials include some adhesives, some silicones, some waxes (including “earthquake wax” or “museum mounting wax”), and some dyed textiles.

•Improperly padded or adjusted mounts may cause abrasion or stress fractures.

•If the RH is high, biological growth may occur.

•If the RH is fluctuating and the ceramic contains soluble salts, salt damage may occur (spalling, powdering, cracking and even complete disintegration).

•Exposure to air pollution can introduce the ceramic to compounds that convert to soluble salts.

•Airborne particulates, including dust, encourage biological growth. Dust accumulation necessitates cleaning, raising the likelihood of accidental breakage. Dust can become ingrained in pores or cracks in glaze, becoming difficult or impossible to remove. This is particularly true of coal-containing dust generated by early heating systems.

•Incorrect storage materials, including many padding materials, can become more acidic with age. Some storage materials and storage furniture, notably some wooden storage cabinets, offgas compounds that convert to soluble salts (Halsberghe et al. 2005)

The Michael C. Carlos Museum website provides useful K-12 teaching materials, including this tutorial on salts.

Conservation and care[edit | edit source]

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.

To find a conservator, please visit AIC's Find a Conservator page.
To learn how you can care for your personal heritage, please visit AIC's Resource Center.
For general recommendations, please refer to the Objects wiki article on Conservation Practices or for general conservation work practices, please refer to the main AIC wiki section on Work Practices.

Documentation[edit | edit source]

Written and photographic documentation should be undertaken for conservation treatments. Condition reports of ceramics should note any condition issues such as cracks, losses, and previous repairs. Guidelines for the photographic documentation of ceramics and other three-dimensional objects can be found on the AIC Guidelines for Practice.

A manually drawn horizontal intersection, or profile line, may be appropriate in situations such as archaeological sites. 3-D scanning and modeling may be useful in complex reconstructions. Identification of previous restorations may be aided with x-ray radiography, examination under ultraviolet and infrared radiation, and a range of other analytical techniques.

Preventive conservation[edit | edit source]

For material/object type specific issues regarding recommendations for storage and display, handling, inhibitive conservation measures, supports, mounts, labelling, transport, condition surveys, monitoring, etc. In order to reduce overlap, general preventive care issues should refer to or be discussed in the appropriate Preventive Care section of the main AIC wiki.

Interventive treatments[edit | edit source]

Cleaning[edit | edit source]

Mechanical, solvent, chemical, aqueous, poultices, pastes, or gels; reduction of surface dirt, grime, accretions, or stains; removal/reduction of non-original coatings or restorations; etc.

Stabilization[edit | edit source]

Consolidation[edit | edit source]

Desalination[edit | edit source]

Desalination is the process of removing salt compounds (compounds consisting of an anion and a cation, such as sodium chloride or sodium nitrate) from objects. The presence of excess salts in an object can be dangerous to the object's structural integrity and stability. If the relative humidity in the object's environment cycles between high and low, any soluble salts in porous materials will cycle through liquid and solid phases. When solid, the salts expand, putting large pressures on the object; when liquid, they become mobile and move to new locations via capillary action, eventually causing additional damage. Characteristic soluble salts damage includes flaking glazes or slips, powdering, spalled, or lost surfaces, and micro- or even macro cracking. Archaeological ceramics from maritime sites are especially susceptible to salts damage due to the high levels of salts in their saline burial environments.

Standard ceramics desalination treatment involves soaking the ceramics in water to remove enough of the salts so that remaining salts pose an acceptably low risk. The specific salts species, ceramic pore size, firing temperatures, ceramic thicknesses, surface treatments, and the degree of storeroom RH fluctuations factor into the length of the desalination process needed. There are several ways to assess the endpoint of the treatment. A common way is via the use of a conductivity meter that measures the total salt content of the desalination bath. After normalizing the measurement for ceramic mass and water volume of the bath, monitoring conductivity over time permits the conservator to know the rate at which the salt is coming out. For ceramics that are not from archaeological marine sites, experimental results indicate that adequate desalination may be achieved in time periods ranging from under an hour to several days. ^[1]^[2]

Archaeological ceramics from maritime environments require more extensive and lengthy treatment; years of desalination is common [1]. For maritime artifacts, desalination is often achieved through a series of staged baths, sometimes initially using a mixture of tap water and seawater. After the conductivity of the bath has plateaued, meaning the extraction rate has slowed, the conservator moves the object to the next staged bath where the ratio of tap water is increased and the seawater is decreased. This offers the conservator the ability to regulate the process, allowing the salts to be removed from the object at a safe rate that will not destroy the object's structural integrity.

Structural treatments[edit | edit source]

Removal of deteriorated previous structural repairs, structural fills, joining, etc.

Aesthetic reintegration[edit | edit source]

Aesthetic reintegration involves disguising, to varying degrees, the fills and cracks in a restored vessel to allow them to blend in with the ceramic body. Considering the level of aesthetic reintegration is an important final step in any ceramic treatment. Reintegration can include creation of fills to compensate losses, inpainting of such fills, and finishing of the surface. Matching restorations to better integrate them with the surrounding pieces can involve the toning, texturizing, and painting of such fills. Treatments often aim to improve the viewer’s interpretation of an object. Reintegration should, however, be carried out to the minimum degree needed, with sufficient evidence to support the restorations (Buys and Oakley 1993). As with all conservation treatments, aesthetic reintegration should be fully documented, particularly so that original materials may be distinguished from the restorations.

Considering Object History
Finding an appropriate solution for reintegration will be dependent upon the object’s history and context, and should be decided on a “case by case” basis (Elston 1990). This is a decision that should be discussed among different parties, such as the object’s owners and perhaps art historians, curators, or others who can offer insight into the piece’s original appearance. Factors such as the original “purpose” of the object and its history will also be important considerations. The aesthetic reintegration applied to an art object to be displayed in a fine arts museum will likely be very different from that applied to an archaeological object, for example.

Materials

Materials used should be as durable, but also as reversible and retreatable as possible. See structural treatments for more on fill materials used for structural repairs. Commercial vinyl and acrylic fill materials are often used to fill fine cracks and provide easily sculptable surface for aesthetic reintegration. For more on this see: Craft and Solz 1998. Synthetic resins bulked with fillers (such as fine silica or cellulose powders) are also commonly used for fills. Plaster of Paris (calcium sulfate) also has a long history of use in conservation and is still commonly used, especially for archaeological ceramics. See Koob 1998 for discussion of obsolete fill materials commonly used in restorations of the past.

Mimicking the Original Piece

Aesthetic reintegration involves mimicking the object’s original appearance on some level. Polishing with abrasives to retouch the surface or using particular fillers so that the restoration matches the texture or reflectance of the adjacent areas may be considered. Paints (lightfast, applied with a paintbrush or airbrush) may also be used to provide the appropriate color or approximate the original texture and other surface details. Regular patterns (such as a repetitive motif) may be extrapolated if deemed appropriate. While components such as sheen, color, and texture may be adjusted to closely match the original piece when deemed appropriate (see Surface Treatments, below), slight distinctions between the original and the restoration may also be used to allow the viewer to readily distinguish the two.

See Two Approaches to Vase-Painting Restoration on the Getty website.

Surface treatments[edit | edit source]

Polishing, coatings, etc.

Other treatments[edit | edit source]

Historic repairs and re-treatment[edit | edit source]

Most archaeological and historic ceramics collections contain historic ceramics repairs. In some cases, those repairs are valuable in themselves. In archaeological collections, pre-depositional repairs are evidence of use value, and should never be removed. Similarly, a good case can often be made for retaining repairs in historic collections that were introduced during the period of original usage. Repairs made by early conservators are valuable documentation of early conservation practices, and may be of significant value historically. The traditional Japanese technique of Kintsugi, or repairing ceramics with gold leaf powder in lacquer, is an art in itself. Kintsugi repairs, which are intended to honor the damage, are considered to enhance the value of the ceramic and would normally not be reversed.

However, modern conservation ethics stipulate that repair materials should be detectable and reversible, and should not damage the object. Historically, repair materials have not always met those criteria. Ceramics collections frequently contain inappropriate repairs that are likely to cause damage to the object. As a result, conservators regularly re-treat previously restored ceramics in order to prevent, arrest or repair damage caused by the old repairs themselves.

Historic ceramics adhesives[edit | edit source]

Adhesives for ceramics repairs should not only be non-damaging and reversible, but must be strong enough to support the weight of the ceramic, are ideally insensitive to temperature fluctuations, and may also need to be be water-resistant and heat-resistant. Until the 20th century, adhesives with those characteristics were limited. Some historic repair adhesives are listed below.

•Animal adhesive: Historically, easily obtained. Initially strong and transparent. However, animal glue darkens, shrinks, loses adhesion and becomes less soluble over time, eventually resulting in a repair that is aesthetically unacceptable, weak, susceptible to failure in high humidity, and difficult to reverse without damage to the substrate.

Identification: Aged animal glue ranges in color from transparent to dark brown. It fluoresces bright white under UV light. It is soluble or somewhat soluble in hot water, and may retain an animal glue smell when wet. Spot tests: Protein with copper (II) sulfate (Odegaard, Carroll and Zimmt 2000 144 – 145).

•Bitumen or asphalt: An ancient repair material in some cultures. Usually considered to be an important part of the object that should be protected.

Identification: Bitumen and asphalt appear dark brown or black. They may fluoresce orange under UV light. They are soluble or partially soluble in petroleum distillates (petroleum benzine, stoddard's solvent).

•Cellulose nitrate: A popular ceramics repair adhesive since its introduction in the late 19th century with extensive use until the present day, primarily due to the fact that it can be easily obtained off-the-shelf. Cellulose nitrate was used by virtually all archaeologists working in the American Southwest during the 20th century. Cellulose nitrate remains reversible, but it has been demonstrated to shrink, embrittle, discolor and lose adhesion over time (Selwitz 1988). Additionally, off-the-shelf formulations generally contain additives that can leach and discolor. Some conservators report that they have not observed the expected instability with aging and its suitability for conservation continues to be debated.

Identification: Cellulose nitrate appears transparent or transparent yellow. Under UV, cellulose nitrate appears yellow, milky white, or yellow-green: it can be dull, or it can be bright. It is soluble in acetone. Spot test: test for nitrates with dipheynalamine (Odegaard, Carroll and Zimmt 2000 164 – 165).

•Epoxy: Occasionally used on larger ceramics where great strength is needed; used elsewhere because of its insolubility and water resistance, particularly on ceramics that will continued to be used. Can be too strong for the ceramic, resulting in breaks just beyond the glue line. Generally insoluble or only partially soluble in very strong solvents, and is difficult to reverse without damage to the substrate.

Identification: Epoxy appears transparent or milky white. Under UV, epoxy is bright white. It is generally insoluble; it may swell in methylene chloride. Spot tests: look for nitrogen with CaO2 and pyrolysis (Odegaard, Carroll and Zimmt 2000 142 – 143. The test is published as a test for protein, but it is a test for bound nitrogen); look for a false positive with the PV(OH) test (see "White Glues" below).

•Natural plant resins or gums: A historic repair material in some cultures. May be considered to be an important part of the object that should be protected.

Identification: Resins and gums range in color from transparent to brown. Under UV, there is a range of fluorescence from no fluorescence to yellow-green: it can be dull, or it can be bright. Resins and gums are soluble in water or ethanol.

•Plaster: Plaster can be inpainted easily, and for that reason has been used not only as a fill material but also as an adhesive. However, while plaster has good cohesive strength, plaster has little adhesive strength, and plaster used as an adhesive is unsafe. Plaster can leach sulfates into the ceramic, producing soluble salt damage, and is difficult to reverse without damage to the substrate.

Identification: Plaster is opaque white. It is insoluble, but it may soften in water. Spot tests: if lime plaster, will test positive for carbonates (Odegaard, Carroll and Zimmt 2000 102 - 103); if gypsum plaster, will test positive for sulfates (Odegaard, Carroll and Zimmt 2000 124 – 125).

•Shellac: Initially strong and transparent. However, shellac darkens, shrinks, loses adhesion and becomes insoluble over time, eventually resulting in a repair that is aesthetically unacceptable, weak, unsafe, and difficult to reverse without damage to the substrate (Koob 1984).

Identification: Aged shellac is glossy dark brown or black. It usually fluoresces orange under UV light, but may also fluoresce yellow, green, or white. If unaged, it is soluble in ethanol; aged, it may be only slightly soluble in ethanol or in an acetone/ethanol mixture.

•Staples: a strong prehistoric repair technique, used into the 20th century and still in use in some cultures. Holes are drilled into the ceramic on either side of the break line, and the pieces are joined using ties or metal staples through the holes. Ties can be made of many materials including natural fibers, manufactured twine or leather, or more recently, plastic. Metal staples are frequently lead because of the ease of manipulation of that metal; ferrous staples are also common. In some cases a waterproofing agent is additionally applied across the break line. The technique is obviously destructive to the ceramic fabric and aesthetically obtrusive; however, in many cases, a staple repair is an interesting or culturally significant historical feature in its own right, and should not be reversed.

Identification: Visually apparent. Spot tests: Test for lead using lead spot test papers (Odegaard, Carroll and Zimmt 2000 66 – 67). Test for ferrous alloys with a magnet.

•White glues: “White glues,” “school glues,” “craft glues,” or “carpenter's glues” are easily obtained, and for that reason, have been extensively used until the present day, although their working properties are less than ideal for ceramics. Chemically, these glues may be several things. Early white glues contained an animal protein (casein). Today, most white glues are a derivative of poly(vinyl alcohol) (PVOH) plus additives. White glues are popularly believed to be water soluble; however, solubility depends on the formulation. White glues formulated to remain water soluble normally do not have enough adhesion to be appropriate for conservation. Other white glues are initially acetone soluble, but the eventual insolubility of many white glues is well documented. Many off-the-shelf white glues ultimately become difficult to reverse without damage to the ceramic substrate. The additives in off-the-shelf white glues additionally make them unsuitable for conservation.

Identification: Visually, white glues are transparent or milky white. They may be water soluble, acetone soluble, or insoluble. Under UV, they show a range of fluorescences: milky white, bluish, greenish, yellowish, or no fluorescence. Spot test: test for PV(OH) derivative with KI/I2 and glacial acetic acid (Odegaard, Carroll and Zimmt 2000 166 – 167). Note that this test gives a false positive for epoxies.

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