PMG Releasing, Detaching, and Separating

Conservation Specialties

Conservation Specialties Analysis and Imaging Preventive Care Education and Professional Development Workplace Practices About Conservation Lexicon

• Photographic Material

Group

• Photographic Materials Group

Section

• Condition Issues
• Treatment Techniques and Procedures

Page Information
Date initiated	March 2020
Contributors	Barbara Lemmen, Amanda Maloney, Stephanie Watkins, Margaret Wessling

Purpose of Releasing, Detaching, and Separating

• Many types of photographic materials, but often gelatin containing ones, can become attached to each other in a stack, to adjacent papers whether in a book or stack, or to glazing, often to glass, while in a frame. This section addresses various methods and materials for releasing these attachments.
• Photographs in an album or personal scrapbook may be partially attached to a page, attached on front and back of pages, or overlapped with other materials, both paper and photographic. Photographs can become only partially attached to "Magnetic" adhesives.
• Paper materials, such as labels, and pressure-sensitive tapes and post-it-notes can be applied to photographic materials and become difficult to remove safely.
• This section does not cover overall unmounting of photographs from secondary supports, yet some of the materials and techniques used for removal are the same, perhaps applied on a smaller scale.
• Some photographic material is intentionally adhered to other materials, as are face-mounted photographs. Treatments aimed at separating intentionally adhered photographic materials are not covered in this section.

Attached Photographs and Negatives

Photographs and negatives can become adhered to other photographs, paper, or glazing (glass or acrylic). It is common for loose photographs and negatives, such as silver gelatin developing-out papers (DOP), some Polaroid processes or others with a coating or adhesive on their surfaces, to become adhered to each other or other adjacent materials while in a stack or to matting and glazing materials while framed if the humidity is high enough or a water event occurs. This process is called "blocking".

Photographic glass negatives can also become attached to each other. Separating glass from glass remains problematic without viable treatment methods. Research and reliable, safe treatment solutions are desired.

Why Materials Become Attached

• Image and binder layers may swell and increase tack in the presence of moisture (humidity or water).
• Heat may cause attachment to adjacent materials.
• Pressure (light, but constant) may cause attachment to adjacent materials in combination with moisture or heat.
• Fungus (mold/conidia) may grow through the photographic material weakening it while also "binding" the stack together.
• Varnishes, coatings, and adhesives may be present on front, back or both sides of a photograph, or on material in contact with the photograph, such as in a closed album or scrapbook, or in a stack.
• People intentionally attach pressure-sensitive tapes and post-it-notes, not understanding the deleterious effects of these adhesives.

Attachment Mechanisms

The two types of attachment forces are:

• Adhesion is the tendency of dissimilar particles or surfaces to cling to one another (from WIKIpedia)
• Cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another (from WIKIpedia)

Lowe, in 2015, investigated the mechanism of attachment specifically between:

• Gelatin silver developed-out paper (DOP) photographs and picture glass
• Gelatin silver developed-out paper (DOP) photographs and themselves in a stack (blocked)

Lowe's published research (Lemmen and Lowe 2017) remains the best in-depth investigation into the science behind blocking mechanisms with photographic materials. The authors noted that because of the process of moisture and heat introduction, the gelatin layer is modified. If the gelatin swells above the glass transition temperature, the gelatin increases tackiness and can be rubbery (the gelatin becomes "adhesive"). If the photograph is positioned against another material, the two surfaces may attach. The attachment can be avoided if the photograph and adjacent material are separated while the gelatin is still moist and swollen.

• Mechanical bonds can form between softened gelatin and paper.
• Mechanical bonds can form between adjacent softened gelatin layers.
• For blocked prints, the drier the gelatin becomes, the less likely clean separation will occur.
• For glass and gelatin print attachments, the adhesion changes from electrostatic (single ion exchange between two atoms) to covalent bonding (ion pair sharing between two atoms) within a few months time.
• Gelatin may more likely adhere to uncoated picture glass than various coated picture glass types (like those with UV coatings).

Factors to Consider

• Photographs adhered to other photographs, and photographs with attachments cannot be used, nor appreciated, as the image is obscured.
• If adhered to glass, the photograph can usually be seen, yet might be in physical danger from sharp edges of broken glass, or weight of overall glass depending on the size of the glazing.
• Not all attachments will release, nor release cleanly.
  • The type of bond created between the gelatin and the adjacent surface dictates treatment success.
  • The type of photographic surface (glossy, matte, etc.) and the surface of the adjacent material affects the attachment strength.
  • The age of the material and length of attachment may affect separation attempts.
    • Weakly formed bonds may separate more easily during treatment.
    • Strongly formed bonds will likely sheer a paper support, leaving an island of photographic material adhered to the adjacent material.
• Fibers, coatings, or image layer may remain attached to the adjacent materials.
• Photographic media layers may become solubilized and be lost from the event or from the treatment procedure.
• Fungal (mold/conidia) may grow in moist conditions, weakening the photographic binder and support, if organic in nature. Stains from these biologic materials may also occur throughout the structure of the photograph.
• Physical distortion may occur during the process of attachment and be problematic to reduce after treatment.
• Physical distortion may be necessary to aid in separation. This may cause permanent physical damage to the photographic image layer and support.
• Physical damages such as skinning or tears of the support may also occur either from expansion and contraction cycles, or from physical attempts at separation.
• Once separated, the sheen or color of the photograph may have changed. These changes are sometimes permanent in nature; sometimes reducible in appearance.

Techniques

Often determining the reason for and understanding the mechanism of attachment is key to choosing the best swelling or softening detachment, and removal method.

Mechanical Approaches

• Gentle Force Dry, resin coated (RC) photographs blocked in a stack can sometimes be separated with gentle force due to the coatings inherent with these materials. If they don't release easily and cleanly, or if they retain moisture in the center of the stack, moisture methods can be tried.
• Mechanical techniques may be required once the attachment is softened.
  • Insert a smooth, non-sticking tool, such as a plastic or Teflon "knife" to help sheer away the connection near the attachment. Insert a thin, smooth protecting material, such as silicone-release Mylar during separation.
  • Slow, steady application can reduce the amount of potential stretching and stress to the gelatin image layer.
• Scrape gently against the glass between the attachment with a new razor blade.
  • Mat-cutter blades are best as they are smooth and have thin, acute angled bevels.
  • Blades used can be fashioned into a wooden stick (or end of a wooden or bamboo spoon) to extend the tool range if necessary (von Waldthausen).
  • Continuous motion creates better separation results.
• Paper splitting Resin coated (RC) papers may require invasive paper-splitting to facilitate detachment, achieved using very sharp, thin flexible knives whether working dry or wet (Lemmen, Albright). Again, steady, continuous motion creates better separation results.

Moisture Approaches

Humidification remains the safest method of swelling and detaching photographs adhered to other materials. Additional and more detailed information on use and approaches are listed within the PMCC Unmounting and Humidification, Drying and Flattening sections. All methods may need to be used in conjunction with mechanical methods.

• GORE-TEX (felted membrane)/Damp Blotter Pack
  With GORE-TEX, and similar synthetics, such as Sympatex and Evolon, water passes in small molecules when in direct contact with paper. Developed by Norris (1989) and McCabe, this approach remains the safest moisture introduction method for softening and releasing adhered photographs from other photographs, adjacent paper, or glass. Whether applied locally or overall, it is also the method that works most often, and thus, perhaps most used. The procedure occurs working on the back of a paper or paper-plastic support.
  • Prior to the introduction of GORE-TEX felted or smooth membranes to the conservation profession, a damp blotter pack, whether applied locally or overall, was used by conservators to introduce moisture in a similar manner.
• Poultices and Gels
  Capillary action requires even, continuous contact. An ultra thin Japanese paper placed between the gel and the photographic surface can reduce any potential residual gel materials while allowing capillary delivery of water.
• Ultrasonic humidifiers/Nebulizer
  These machines produce moisture vapor. They can be locally used or used overall in conjunction with domed chambers and plastic tents whether small or large.
• Steamers/Steam pencils
  Useful when extremely fast and aggressive humidification is needed or for softening attachment and residual adhesives, however, more components of photographic prints can be adversely affected.
  • Gently warming a slight dampened blotter with a tacking iron or heated spatula can temporarily "steam" an attached area facilitating separation.
  • Steaming techniques are sometimes helpful in partially reversing physical damages such as dents when used in conjunction with mechanical methods, and partially restoring surface gloss.
• Misting/Water spray
  Another traditional and faster method of dispensing water. Can deliver water too fast without sufficient absorption of the support.
• Brush/sponge
  Localized treatment to introduce moisture carefully, with much control over quantity and location of water delivery.
• Immersion
  Partial or complete immersion. Temperature can be varied from cool to warm. PH levels and conductivity levels can be adjusted to fit the parameters of the material in question.
  • Some pressure-sensitive tapes, such as acrylic-based adhesives and stage-3 deteriorated rubber adhesives, can swell and release during prolonged water immersion or gel application.

Alcohol and Solvent Approaches

If varnishes and similar coatings or pressure-sensitive tapes are the reason for the attachment, choosing an alcohol or organic solvent that softens the attachment is helpful to facilitate release. Alcohols and solvents can be introduced locally and overall. As always, please use these materials in a well-ventilated environment while wearing the appropriate personal protection equipment for the materials chosen.

• Chambers
  • Pre-humidifying photographic material is sometimes helpful before employing other methods. Humidification can be overall or locally applied.
  • Large or small chambers can effectively slow down evaporation of alcohols and solvents during treatment.
  • Large or small chambers can introduce some solvents as a gas rather than a liquid. Ensure that blotters or cotton within the chamber cannot fall or will not drip onto the surface of the photograph.
• Gels
  • Gellan gum and agarose gels can soften attachment areas locally or overall. Used locally, they have an advantage in being able to be shaped to match the attached area.
  • Some solvents can diffuse into pre-made agarose gels over a few hours time. Likewise, solvents can be brushed onto the surface of a gel. Used in conjunction with a small cover (inverted wide-mouth small glass jars work well), solvent infused gels can help soften material or dissolve varnishes and pressure-sensitive tape adhesives.
• Emulsions
  • Emulsions have the advantage of being able to be engineered with smaller amounts of solvents, and engineered to contain multiple cleaning systems simultaneously, including water with potentially surfactants, alcohols or solvents.
  • Emulsions have a disadvantage as they must be cleared from the area of application.
• Nebulizers
  • Because of the molecular size generated, this machine application should be used with caution and with full personal protection equipment.
  • Working within a fume hood or near a vented small dome or tent can be helpful to contain the mist created.
• Direct liquid applications
  • Locally applied with small brush or sponge, purified water (distilled, deionized) or 1:1 ethanol water solution. Wait a few minutes for bond around edge to soften. Mechanical force, and use of a non-sticking knife or spatula may be required to peel apart. This method sometimes works better with fiber-based photographs than resin coated (RC) photographs.
  • Overall immersion can soften and release materials either by the action of swelling, or combined with splitting in the water. However, immersion does not always work, and attachment of larger areas of the photograph can occur during drying. Separating the non-attached sections from the adjacent material with interleaving materials such as stiff, this poly(ester) webbing can help reduce additional attachments during the drying process. If attached to glass, drying with the glass face-down is also beneficial.

Differential Expansion or Contraction Approaches

• Heat: Hot air guns, hairdryers, heated air pencils, heated spatulas, tacking irons
  • Warming and softening heat-sensitive materials can result in separation. Heat has helped release developing-out papers (DOP's) from glass (von Waldthausen, Lemmen and Lowe, 2017). However, the conservator's judgement and caution is warranted. Heat can melt, shrink, scorch or burn, or crack various photographic materials. Heat can also harden certain materials, such as adhesives, making separation more difficult.
  • Carefully regulate the temperature of heat applications during use. Plug tools into a rheostat temperature control unit while working. Hot air pencils should be used at the lowest possible settings as they can easily damage materials.
  • Focused, heat applications at lower temperatures can result in gentle temperature increases.
  • Direct heat application, even at lower temperatures, can be too hot for weakened and aged, historic photographic materials.
  • Working on top of a protective intermediary layer, such as silicone release paper, or a thin blotter or paperboard, can diffuse heat applications. Templates of isolating material can protect adjacent materials when working locally.
  • Even though softening may help with releasing adhered materials, complete separation will likely entail physical separation methods also. Smooth, heat-resistant tools, such as Teflon ^(TM), Delrin^(R), Turcite^(R), or Glide dental floss, may be helpful for separating materials while the adhesive or attachment bond is softened, yet can also cause damage without proper application. Physical separation may involve stretching or distorting a plastic or paper-plastic photograph that may cause cracking and other damages occurring soon or in the future.
  • Portions of the adhered material may release, and others may not, resulting in tearing or splitting of the emulsion layer from the base material.

• Cold: Dry ice and acetone slurry
  • Depending on the potential surface coatings, or the manner and size of attachment, applications of a mixture of crushed dry ice and acetone may release adhered photographic materials from glass. The procedure relies on two adhered materials having different thermal expansion rates: Abruptly changing the temperature can result in the two items releasing from each other. The smaller the area of attachment (approximately <1/4 inch or <0.5-1cm), the more likely this approach may work.
  • Preliminary trials using this method for releasing resin coated (RC) prints from glass were not consistently successful with the smallest areas more likely separating than larger areas (Lemmen). Results from further tests and on more materials is needed.
  • CAUTION! Dry ice should never be handled with bare hands! For the conservator's safety, personal protection equipment, including goggles or face shield, and protective leather or thermally-insulated gloves, are advised when handling dry ice. A respirator is also advised when using dry ice and acetone. Both materials require good ventilation during use, disposal, and storage.
  • This procedure may fail and the glass might break during the process. For the safety of the photograph and easier clean-up, place a layer of poly(ester) plastic (such as high density poly(ethylene): HDPE) or poly(ethylene) terephthalate (Mylar, Melinex) on top of the glass before working.
  • Crushed dry ice is mixed with acetone to a workable "slurry with a slushy consistency" (Mass, 2020) then applied locally in the area of attachment (on top of the barrier laying on top of the glass). If the procedure works, the release should be quick, with a distinct sound (a "pop") occurring (Dion, 2020).

Tips: Photographs Attached to Glass

• Scan or digitally capture the image before treatment begins as a safeguard backup of image retention.
• Humidification remains the easiest and most effective method of softening and releasing. GORE-TEX, made of polytetrafluoroethylene (ePTFE), with a 1/16" felted polyester side was available prior to 2002 and if stored without creasing, remains useable. Sympatex (polyester) felt and Evolon CR (70% polyester/30% polyamide non-woven micro-filament textile) can be used as replacement materials for GORE-TEX.
• 2 - 3% Gellan gum gel or 3 - 5% agarose gel can soften prints very effectively through slow humidification when applied on the back of the attached area, assuming the photograph is not mounted to a thick card. If mounted, mechanical thinning of the mount is usually required for this technique to work.
• Silver gelatin developing-out paper (DOP) photographs from mid-twentieth century onward tend to release using moisture. Silver gelatin photographs produced in the early twentieth century, with notoriously thin or non-existent baryta layers, tend to disintegrate rather than release when introducing moisture. "Specked" attachment around a larger attached area may indicate a difficult or impossible release.
• In some cases, breaking the glass close to an area of partial attachment can allow easier and safer access to areas that are adhered.
  • Insert soft, yet rigid supports gently between the photograph and glass before scoring and snapping the glass.
  • A professional-grade glass cutter works best and are obtained from framing stores or suppliers.
• For fiber-based developing-out paper (DOP) photographs, the vibrations sometimes dislodge gelatin prints from glass (from Sylvia Verselli, 2003 workshop).
  • Strike with the ball end of a glass cutter, near an edge of the bond. The amount of force required is unknown for each individual situation. Be prepared for this method to fail, yet the glass breaking into pieces or shards while still attached to the photograph.

Tips: Photographs Attached Together

• Humidification remains the safest and easiest swelling and release method. GORE-TEX, made of polytetrafluoroethylene (ePTFE), with a 1/16" felted polyester side was available prior to 2002 and if stored without creasing, remains useable. Sympatex (polyester) felt and Evolon CR (70% polyester/30% polyamide non-woven micro-filament textile) can be used as replacement materials for GORE-TEX.
• For small personal commercially processed gelatin developing-out paper (DOP) prints, both colored and black-and-white, GORE-TEX (or other), blotter stacks, and gel sheets can be applied and kept moist while in a zip-lock style baggie or very small covered, photographic trays.
  • For separating large batches of photographic prints, it is helpful to pre-cut GORE-TEX felts (or other) or gel sheets, poly(ester) webbing, very lightweight Japanese papers, and blotters slightly larger than standardized photographic processes, yet smaller than the container chosen.
  • Additional small poly(ester) webbing sheets can be inserted into areas not currently attached, or previously released to reduce the chance of reattachment.
  • GORE-TEX felt (or other) or gels require continuous contact to work best. Minimizing air in the container allows dampened felts and gels to retain moisture longer.
  • Check periodically, but this process often takes a few hours.
  • Once humidification begins, it is common for several photographs to detach in succession. Steady, quick work is required while the attachment bond is soft enough to release.
• 2 - 3% Gellan gum gel or 3 - 5% agarose gel can soften stacks of blocked prints and sometimes negatives, very effectively through slow humidification. Thin barrier papers and poly(ester) webbing can be used to minimize any gel debris on the surfaces. If the photographs are not moldy, agarose gels can be cleaned, rinsed, and rejuvenated in same conductivity and pH baths/trays of the original gels for several hours or over night and thus re-used each day until the project is complete. It is assumed whatever pollen, salts, or debris found is likely the same throughout a single stack of blocked materials. Re-using the same gels for different projects is not recommended as unknown or undesirable contaminants may be transferred. Judgement and experience of the conservator is required.

Tips: Emergency Triage Situations

For more information specific to photographic materials, please see PMCC's Emergency Response, Salvage, and Recovery Techniques.

Photographs Attached to The Glazing

• Materials should be kept damp by wrapping in plastic provided a conservator will be able to treat them within 48 hours.
• Prioritize anything stuck to glass for immediate treatment. If the glazing is acrylic, the photograph should release on its own once it is dry. The surface will likely still be altered or distorted where it was stuck.
• Immerse in water: ideally the photograph should lift off without a great deal of force. Do not pull a photograph off at an acute angle that can cause cracking in the emulsion.
• If immersion is not a possibility, place release materials under the areas that are not stuck and place thin spacer material such as blotter between the glazing and the release material to try to encourage gradual release and prevent more areas from adhering.

Photographs Attached to Themselves

• If wet and fused in a stack, gently peel apart resin coated (RC) or commercial color snapshot photographs at a low angle only if possible. Do not force separation. Some image loss may occur.

Photographs Attached to Plastic Enclosures

• Materials should be kept damp by wrapping in plastic provided a conservator will be able to treat them within 48 hours.
• Cut three sides of the enclosure and peel the plastic carefully off the emulsion surface using an acute angle while keeping the photograph flat. Avoid pulling wet photographs from their protective enclosures. Release slides also, as plastic and paper mounts may also be stuck to the enclosure. Be especially careful of acetate materials as they may be extremely sticky.

Tips: "Magnetic" Scrapbook Pages

• Check the tackiness and discoloration of the exposed adhesive before choosing a removal plan. The less tack and darker the adhesive, the more tenaciously cross-linked and bound the attachment is likely to be. Likewise, the more faded or stained the photograph is along the adhesive lines will indicate level of adhesive penetration.
• Plastic-backed photographs generally are easier to remove with dry mechanical removal methods than paper-based photographs.
• Double-sided attachments can be challenging to reduce curl and distortion of the photographs during removal. Some thicker pages can sometimes be split depending on the quality and condition of the paper.
• Sometimes the attachment can soften with slight warming, then "slicing" the adhesive away with flexible, thin materials such as dental floss or mono-filament fishing line. A single, non-stopping motion is best when separating.
• Mechanical removal of the adhesive from the back with a crepe/crepeline eraser is sometimes possible depending on how cross-linked the adhesive has become.
• If the adhesive has hardened, scraping with a scalpel is sometimes possible. Be careful not to skin or chip the back of the photograph or cause plastic distortions from pressure on the back.

Tips: Pressure-sensitive Tapes

• Identifying and understanding the different types of pressure-sensitive tapes and their possible adhesive types informs which treatment approaches may produce the best result.
• Carefully consider the condition of an aged photographic image layer before applying water, solvents, heat or mechanical force. The robustness of the material can determine whether chemical or mechanical processes used to remove pressure-sensitive tapes and post-it-notes are possible.
• If a pressure-sensitive tape is newly applied, mechanical means may occasionally remove pressure-sensitive tapes from plastic surfaces without the use of solvents.
• Organic solvents and heat used to remove and reduce pressure-sensitive tape adhesives can alter the physical and chemical characteristics of the image layer and supports of photographic materials. Heat and solvents can distort and solublize plastic surfaces.
• Be careful using alcohols and solvents with dye-based colored photographic materials as some can alter the color dyes, causing dye migration and color shifts (e.g ethanol can fade chromogenic photographic materials over time, van Waldthausen from Lemmen and Lowe, 2017).
• Remember which photographic processes dissolve in which solvents! Likewise, before use, test any plastic tools and supports with solvents chosen.
• Depending on the photographic process and the robustness of the image layer, it can be safer to use tweezers to lift and peel back carriers rather than inserting a plastic or metal knife when removing pressure-sensitive adhesives, especially tacky ones, from the photographic image side.
• Prepare to remove both carrier and adhesive residues within a short period of treatment if not at the same time, or during the same treatment session. Removing the carrier, thereby exposing the adhesive to air, can make the adhesive more difficult to remove the longer it is exposed. If the removal cannot be completed in one time period, temporarily "adhering" a Mylar or silicone-release sheet or Mylar may help reduce adhesive hardening and oxidation.
• Solvents brushed onto or diffused into gels cut to shape or created within emulsions are helpful to minimize residual adhesives.
• Treatment effectiveness of minimizing these materials can be tracked using a long-wave ultraviolet light (roughly 400-315 nm) such as an LED flashlight.
• Accept that it may not be possible to fully remove all components of pressure-sensitive tapes from organic materials and some plasticizers and adhesive residues may remain. There can be a point of diminishing returns with tape and adhesive removal.

Tips: Hinge Removal

A useful old hinge removal technique comes from Nora Velensek, (Akademie der Bilden Künste Stuttgart) that was shared with Debra Evans, who shared it with Margaret Wessling (shared with permission). The methylcellulose gel poultice is easily removable, and "drier", therefore less likely to overly wet the paper beneath. Using Methocel A4C, create between a 2-7% solution, heat. While at room temperature, form the mixture into a ball, then cover it tightly with plastic cling wrap. Leave overnight. Next day, while wearing clean gloves, form the stiff gel into 1/2 inch (1.3 cm) diameter cylinders. Individually wrap each rod into fresh plastic wrap, then placed into a ziplock-style bag and into a lidded-plastic covered container for storage in the refrigerator. Airtight storage will prolong the shelf life. Slice off lozenges for use. Thickness of the slice and length of time to apply the poultice will vary by situation.

Specific Note-Worthy Situations

• Photographic album black mounting paper can leave dye stains and fibers imbedded within a gelatin matrix after separation and release.
• The coating on "erasable" typing papers can form a particularly tenacious bond with gelatin layers, whereas an uncoated bond typing paper may more easily release.

Beyond Separation, Post-release Guidelines and Recommendation

• Once separated, surface cleaning may be required.
• Once-separated, the material may remain swollen or distorted. Drying may be immediately necessary requiring pressing either locally or overall. See Humidification, Drying and Flattening.
• Once separated, the surface sheen may be irregular, often ferreotyped in the area of attachment, or the materials dented or creased. Careful use of steam application or slight alkaline water can sometimes ameliate these conditions, somewhat. Complete reversal to a seamless surface should not be expected.
• Once separated, dried materials can be isolated to reduce the likelihood that re-attachment may occur. Isolation can be placing a photographic or negative in Mylar sleeves, Mylar or paper folders, or creating mats or inserting glass separators when framing.
  • However, during water events, paper-based photographic materials can swell and distort beyond the air-gap space and distance provided by a traditional 4-ply mat, a double- or 8-ply mat, or plastic frame spacers.

References

• BPG. 2020 (1992 printed version). PCC Hinge, Tape, and Adhesive Removal for paper-based photographic material.
• Dion, Judy. 2002. "Notes on Treatment Procedure: Removal of Glass from a Painted Surface". Unpublished student research paper for Winterthur/ University of Delaware Art Conservation Program.
• Dion, Judy. 2020. Personal email exchange with Stephanie Watkins, 23-31 March 2020.
• Edwards, Gwenanne. 2011. Works of Art on Paper Adhered to Glass: Approaches to Conservation Treatment. Unpublished student research paper, Buffalo State Art Conservation Program.
• Edwards, Gwenanne. 2020. Personal email exchange with Stephanie Watkins, March 2020.
• Gels overview on AIC-WIKI; See also PMCC's PMG Gels
• Lemmen, Barbara and Emma Lowe. 2017. Blocked Photographic Prints: Adhesion and Treatment. Topics in Photographic Preservation, AIC: Washington, D.C.: Volume 17 (2017) 29-50.
• Mass, Jennifer. 2020. Personal email exchange with Stephanie Watkins, 16 April 2020.
• Norris, Debra Hess. 1989. The Removal of a Silver Gelatin Photograph Adhered to Glass. AIC: Washington, D.C.: Topics in Photographic Preservation, Volume 3 (1989), 86-91.
• Warda, Jeffrey, Irene Brückle, Anikó Bezúr, and Dan Kushel. 2007. "Analysis of Agarose, Carbopol, and Laponite Gel Poultices in Paper Conservation." Journal of the American Institute for Conservation 46(3): 263-279.

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