PMG Gels

Conservation Specialties

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• Photographic Material

Group

• Photographic Materials Group

Section

• Materials
• Treatment Techniques and Procedures

Page Information
Date initiated	February 2019
Contributors	Luisa Casella, Amparo Escolano, Amber Kehoe, Michelle Sullivan, Stephanie Watkins

Introduction

Since the 1980s, rigid and viscous gels have been successfully used by conservators to treat paintings and objects. Beginning in early 2000's, a steady uptick in the documented use of gels to treat works of art on paper has also been observed. Given the inherent sensitivities and complexities of photographic materials, gel treatment of photographs has been more limited and remains in an experimental phase. Interest among photograph conservators, however, is strong and will no doubt lead to innovation, wider application, and many more case studies to populate this section of the PMG Wiki. The focus of this page—PMG Gels—is to provide the reader with an introduction to the types of gels available, potential applications within photograph conservation, possible advantages over traditional treatment methods, and useful tips and tricks. For a broader discussion of gel types and gel chemistry see the general AIC Wiki entry on Gels.

Gels and Photographic Materials

With practice and experience, gels can be a versatile tool and can be very easy to use. They may be useful in a variety of applications including, but not limited to, the following:

Advantages of Gel Treatment

• Many gels are naturally-derived and non-toxic (e.g. agar-agar, agarose, gellan gum); dependent upon preparation, some gels may not be considered chemical waste and subject to special disposal (e.g. an agarose gel prepared with deionized water).
• Gels are easily made and do not require specialized tools unfamiliar to most conservators.
• Gels may be preserved with refrigeration and by placing cast gels between sheets of Mylar, in polyethylene bags, lidded Petri dishes, etc., to exclude as much air as possible.
• Many gels are compatible with modified aqueous solutions and some organic solvents, making it possible to chemically "tailor" gels to suit the needs of a given treatment.
  • Buffers to maintain a specific pH.
  • Adjusted aqueous solutions (hypotonic, isotonic, and hypertonic) to control swelling.
  • Chelators to target metallic components of staining and/or surface soil.
  • Reducing agents and oxidizing agents (oxidizing agents are not compatible with all gels as they may degrade the polymer).
  • Enzymes (must be added after the gel solution has cooled to prevent protein denaturing).
  • Organic solvents (the gels discussed in this section have greater affinity for polar solvents: ethanol, isopropanol, acetone).
• Gels typically make it possible to use smaller quantities of reagent or solvent than traditional immersion protocols.
• Rigid gels may be cut to shape for precise, targeted application in local treatment.
• Larger sheets of gel may be cast for overall treatments (e.g. controlled bathing of brittle object).
• A function of pore structure, rigid gels exhibit restricted lateral flow and can function as a "sponge".
  • Rate of solvent diffusion and re-absorption is dependent upon the polymer concentration of the gel (weight %) and casting thickness; higher concentration, thin-cast gels diffuse solvent more slowly.
  • Gels offer longer dwell times, affording reagent solutions and solvents more time to work.
• Gels may be used in-situ to treat mounted or bound photographic materials.
  
  

Factors to Consider

• Residues
  • All gels, even rigid gels, leave some residue, however minimal; more research is needed to understand which residues are problematic. Thin Japanese tissues may be used as an effective barrier layer to minimize residue deposition.
• Sensitivity of emulsion or binder layer to moisture (e.g. swelling, crack propagation, solubility, etc.); though moisture delivery is controlled, gels should be considered a wet treatment.

Applications

Gels can be applied in a variety of applications in Photograph Conservation including, but not excluded to:

Testing:

• Surface pH can be measured using small plugs of agarose and a pH meter.
• Surface conductivity (ionic concentration) (mS or µS/cm) can be measured using small plugs of agarose and a conductivity meter.
• Gels can be used for solubility testing, mimicking actual treatment time with longer applications than traditional aqueous spot tests.
• Gels can be used for organic solvent testing with precise application and longer dwell time than traditional spot tests.
  
  

Humidification (with intent to release, detach, or separate):

• Overall humidification of photographic materials.
• Local or overall humidification of paper-based attachment to the recto (face) or verso (back) of photographic materials (e.g. hinges, mount, etc.). For additional information, refer to Releasing, Detaching, and Separating and Unmounting entries.
• Paper-based and RC photographs blocked to glazing, other photographs, or paper. For additional information, refer to Releasing, Detaching, and Separating and Unmounting entries.
• Blocked negative bases (not appropriate for all types).

Cleaning (with intent to reduce or remove):

• Unwanted, aberrant marks on surfaces such as signed mats, boards, some photographic material surfaces
• Adhesive residues
• Local stains or tidelines
• Overall discoloration

[>>>Editor's Note/Goal: Please link to various application/treatment PMCC procedure sections after applications listed are more finalized.]

How were gels applied successfully to treat photographic materials?

• Please list material used, applied to what type of photographic material, how applied, and why application worked well.

What did you try that didn't work so well with photographic materials?

• Please list material used, applied to what type of photographic material, how applied, and why application failed.
  

Gels Used in Conservation

The International Union of Pure and Applied Chemistry (IUPAC) defines a gel as a "non-fluid colloidal network or polymer network that is expanded throughout its whole volume by a fluid." Gels may be classified as either "physical gels" or "chemical gels." Physical gels form by ionic bonding, hydrogen bonding, and/or van der Waals forces between polymer chains. Chemical gels form by covalent bonding or crosslinking of polymer chains. [1] Agar, agarose, and gellan gum form physical, thermoreversible gels.

Rigid Polysaccharide Gels

Types of Rigid Polysaccaride Gels

• Agarose
  • Electronically neutral, naturally occuring polysaccharide capable of forming a clear, rigid gel.
  • Derived from agar-agar, a polysaccharide extracted from the cell walls of certain species of red algae (Gelidium and Gracilaria); the second component of agar-agar is agaropectin, a sulfate-containing, anionic polysaccharide.
  • Due to purification process, agarose is typically more expensive than other polysaccharide gels.
  • Agarose can be prepared using polymer concentrations ranging from <1% (weight/volume) to 12% (weight/volume).

• Gellan Gum
  • Anionic, naturally occurring polysachcaride capable of forming clear, rigid or opaque, flexible gels, dependent upon type of gellan gum used.
  • Gellan gum is an extracellular secretion of the bacterium Sphingomonas elodea [2].
  • Cations must be added during gel preparation to promote gelation and formation of a robust gel network.
  • Gellan gum is available in two forms:
    • Low-acyl, creates rigid, brittle casts; low-acyl gellan gum is chemically-modified form of native gellan gum in which a fraction of the acyl functional groups have been removed (type used most commonly by conservators).
    • High-acyl (native form), creates soft, flexible casts (used by some textile and book conservators).
      

Cellulose Derivatives

Methylcellulose (Methocel), hydroxypropyl methylcellulose (Klucel), Carboxy methylcellulose (CMC)

• Water soluble synthetic cellulose derivative. Created from plants combined through heating with various salts. Dissolves to form clear viscous solutions that can release some alcohols and water.
• For more information, see AIC Gels page under "Viscosity Modifiers", [3], and Adhesives section.
  

Chemical Hydrogels

• Polymer materials that can retain large amounts of water without dissolving usually because of physical or chemical cross-linking of the structure [4]. Some of these materials may be PVA subjected to sequential freeze-thaw cycles creating an open amorphous structure that can hold and release liquid (Hoover, Maccarelli, 2020).
  • Benefits to conservation are that some solvents can be loaded into the structure.

Making Gels

Agarose

Preparation:

• Prepare all casting materials before starting gel preparation; gel solution will begin to set quickly once removed from heat source, especially when preparing higher weight percentage concentration gels.
• Prepare gel in a heat-resistant container (e.g. tempered glassware, Nalgene beaker, silicone beaker).
• Add dry, powdered agarose to beaker.
• Add deionized water or modified aqueous solution (e.g. pH- or conductivity-adjusted solution).
• Stir or swirl to combine.
• Cover beaker with watch glass, plastic wrap, or silicone lid to prevent excessive evaporation during heating.
• Heat agarose dispersion in microwave or on hot plate until solution fully hydrated; solution will appear clear with a temperature near boiling (85°C).
• If using a microwave, heat in short intervals to prevent boiling over; between intervals, stir gel solution to ensure even heating and hydration.
• Once solution is fully hydrated, quickly cast into a heat-resistant container (e.g. fiberglass tray, Petri dish, etc.) or onto a sheet of Mylar to create a thin-cast.
  
  

Tools (not all necessary for all procedures - heating, casting, testing, using):

• Clean water source (deionized, distilled)
• Cellulose ethers, agarose, and gellan gum powders
• Scale for measuring out amounts of dry powder
• Graduated cylinders for accurate measuring of liquids (remember, beaker markings are approximations)
• Beakers, other glassware, or silicone measuring cup for mixing and microwave safe heating
• Heat source: microwave, stove (with pot/pan), heated magnetic stirrer (and stir bars of appropriate length and size)
• Silicone spatula, whisk, or spoons
• Hot pads or gloves (if sensitive to heat and cooking in glassware)
• Poly(ethylene terephthalate) film (Mylar Type-D, Melinex 516, thin mil preferred as barriers or evaporation covers; more rigid preferred as gel casting support)
• Plastic wrap, plastic or glass containers to slow down evaporation of methyl cellulose or gel castings
• Glass or acrylic glazing sheeting
• A cleaned work surface (counter tops: glass, acrylic, melamine, granite, stainless steel, etc.)
• Heat resistant cups or large trays. Size of gel needed dictates casting surface (full sheets versus sectional work)
• Matboard or acrylic sheet scraps (long strips) useful for casting specific thicknesses of rigid gels
• Nitrile gloves
• 3mm or 4mm biopsy punch with plunger (plunger is essential for easy use)
• Dark colored glass or ceramic square, or dark colored sample wood flooring (smooth)
• Horiba brand LAQUAtwin-EC either 22 or 33 Compact Conductivity Meter
• Horiba brand LAQUAtwin-EC either 22 or 33 Compact pH Meter
• Smooth, poly(ester) webbing (e.g Bondino, Hollytex/Holytex, Remay) or nylon fabric (Cerex®)
• The thinnest Japanese papers with small interfiber pore dimensions and without furnish additives, such as Gampi Usouyo, Gampi paper silk, or Usa-gami (9g/m² or thinner) (to reduce residues).
• Photographic grade blotters
• Scalpel, spatula, Teflon knife, soft small brush, plastic spoon, plastic tweezers
  

Considerations and Tips:

• Rigid polysaccharide gels require complete contact (without any air gaps) to be effective.
• A higher percentage gel usually releases moisture more slowly; therefore a longer application time period often needed.
• A lower percentage gel usually releases moisture more quickly: therefore a shorter application time period often needed.
• Casting thickness is also a significant factor in solvent release from rigid polysaccharide gels. Thinner gels will likely dry out more quickly than thicker ones of the same percentage.
• Sequential application of thinner gels in a shorter time frame may soften an adhesion better than fewer, but thicker gel applications in contact for a longer time.
• Application of thicker gels may taker longer, yet may use less gel amount overall than with repeated thinner gel applications.
• Agarose powder is the most expensive of the gels.
• Agarose gels that partially dry out can be rehydrated by immersing them in the same solution that was used to prepare them. When removing gels from the hydrating solution, blot the surface with a towel. Rehydration will not change the initially prepared concentration of the agarose.
• Agarose scraps of the same percentage (concentration) that are still moist can be reheated to create another casting. Repeated heating can drive off water, potentially slightly changing the percent range, however. If precision is not essential, small amounts of additional water can be added as desired.
• Making gels without added preservatives is safer for use with photographic materials.
• Use gels and poultices shortly after casting, such as day before or day of working.
• Cast-gels can be stored flat in the refrigerator for a time while in plastic zip-lock style baggies of appropriate size. Larger cast-gels can be rolled between two sheets of poly(ethylene) terephthalate film (e.g. Mylar Type-D). Eventually gels will mildew and mold and also potentially change pore size as they still slowly dry out, so be diligent about use or disposal in a timely manner.
  
• While warm, gels can also be cast into a variety of containers and tubes, such as piping (C. Stravroudis, 2013) or clear plastic syringes with the ends cut off, to create a stain stick (M. Brockman 2020) that has an added component of tri-sodium citrate chelator. The gel can be easily handled while in the container, then the syringe plunger exposes the used amount to remove. After use, cover the exposed end with a plastic sheeting, in addition to plastic bagging within refrigeration, to extend the life of the gel. Thin slices or sections can be trimmed off the larger cast for use as needed, also.

Low-acyl Gellan Gum

Preparation:

• Prepare all casting materials before starting gel preparation; gel solution will begin to set quickly once removed from heat source, especially when preparing higher weight percent concentration gels.
• Prepare gel in a heat-resistant container (e.g. tempered glassware, Nalgene beaker, silicone beaker)
• Add required volume of deionized water to beaker
• Add calcium acetate (0.4g/L) to deionized water; dissolve completely
• Slowly add low-acyl gellan gum powder to minimize clumping; a sieve or flour sifter my help add gel powder in a slow, diffuse manner
• Whisk to combine; whisking will be more difficult and clumpling more problematic with higher concentration gellan gum preparations (i.e. 3-4 % weight/volume)
• Cover beaker with watch glass, plastic wrap, or silicone lid to prevent excessive evaporation during heating
• Heat gellan gum dispersion in microwave fully hydrated; solution will appear clear with a temperature near boiling
• If using a microwave, heat in short intervals to prevent boiling over; between intervals, stir gel solution to ensure even heating and hydration
• Once solution is fully hydrated, quickly cast into a heat-resistant container (e.g. fiberglass tray, Petri dish, etc.) or onto a sheet of Mylar to create a thin-cast.
• Working temperatures around 36°C or lower at 1.5%.
• More structure to the molecule increases rigidity.

Troubleshooting Preparation:

• If gellan gum dispersion is especially clumpy, allow the mixture to sit for several hours (or overnight, if time permits).
  
  

Considerations and Tips:

• Gellan gum is often substituted for agarose because of cost. Sometimes the materials can be substituted depending on application, yet know, the two materials work in different physical and chemical ways. Choose the gel for the specific application as appropriate.
• Gellan gum gel needs complete contact (without any air gaps) to be effective.
• A higher percentage gel usually releases moisture more slowly; therefore a longer application time period often needed.
• A lower percentage gel usually releases moisture more quickly: therefore a shorter application time period often needed.
• Thickness of gel casting may also be a factor. Thinner gels may dry out more quickly than thicker ones of the same percentage.
• Sequential application of thinner gels in a shorter time frame may soften an adhesion better than fewer, but thicker gel applications in contact for a longer time.
• Application of thicker gels may taker longer, yet may use less gel amount overall than with repeated thinner gel applications.
• Gellan gum is gelled with alkaline salts (calcium and magnesium), so may not be appropriate for working directly against all photographic material processes. Using gellan gum gels to remove outer boards is likely very safe. A conservator's experienced judgement is required for safest approach.
• Making gels without added preservatives is safer for use with photographic materials.
• Use gels and poultices shortly after casting, either the same day or the day after.
• Cast-gels lacking preservatives can be stored flat or rolled between Mylar sheets in the refrigerator for a time as long as they are properly covered to prevent evaporation. Eventually gels will mildew and mold, and potentially change pore size as they still slowly dry out, so be diligent about use or disposal in a timely manner.

High-acyl Gellan Gum

Preparations
Troubleshooting Preparations

Cellulose Ethers

See the Adhesives section for information on material preparation advice.

Chemical Hydrogels

Products and Specifications:
Listing here does not constitute endorsement of any kind. The specific products listed here are some, but not all, of the products conservators found useful and effective. Many other products are likely available. Proprietary manufacturing can change recipes at any time. Please be knowledgeable about the materials used and how applied. Damage can occur with improper making and application. All transactions are between you and the businesses you use and do not involve AIC, PMG, or any of it's officers or members.

• Brand: Nanorestore [5] .

Preparations:
Troubleshooting Preparations:

Tips and Techniques for Working with Rigid Polysaccharide Gels

Casting Thickness

• For even casting, create a jig by placing long strips of the same thickness around the casting area. The platen used must be able to extend past the strips for easiest handling and use. Strips can be made of Coroplast® (cut with the flutes, not across), acrylic, 2-, 4-, or 6-ply mat board, or other. Hot gel tends to adhere to paper readily, so more frequent replacement is necessary if mat board strips are used.
• Thickness can be controlled by determining the volume necessary to cast into a flat-bottomed container of known dimension, such as a Petri dish, Pyrex tray, or heat-resistant tray.
  

Cutting and Shaping

• While on a Mylar or other rigid support (Plexiglas, glass, ceramic tile), a scalpel blade, minarette, or micro-spatula tool can accurately cut and shape the agarose or gellan gum gel sheets to the exact shape desired for treatment.
• A template of the shape can also be cut from the thinnest Mylar. Remember that the gels require continuous contact to work, therefore, templates work better to isolate larger, rather than smaller, stain or adhered areas.
  

Regulating Evaporation

• As gels dry, the pore size diminishes increasing the capillary action.
• Remove gels before they become too dry as they can adhere to many porous surfaces, becoming attached.
• Covering gels with wide mouthed containers, or clear sheeting such as high density poly(ethylene) (HDPE) or poly(ethylene terephthalate) Mylar, Melinex) can slow down the evaporation rate without adding unwanted pressure to the gels.

Solvent Introduction

• Agarose gels can be cut into workable pieces and soaked in solvents or solvent solutions for several hours to overnight. The nature of the solvent or solvent mixture and polymer concentration (pore size) will determine length of time needed. Blot gently on both sides before use.
• Solvents can be brushed applied to agarose and covered to slow evaporation; brush in the center of the gel mostly to avoid solvent running down the sides and into your photograph. This method requires a steady hand for small applications.
• Working with proper fume extraction and personal protective equipment, solvent may be added to gel in the sol (fluid) state prior to casting. Research flash point, ignition point, and other pertinent solvent information before adding solvent direct to hot, fluid gel.

Suppliers

Below is the list of suppliers used for the workshop Introduction Rigid Gels for the Conservation of Photographs taught by Michelle Sullivan and Amber Kehoe, during the PMG Winter Meeting in February 2019. Please note: this is not an endorsement of specific companies but rather a guideline for possible sources.

• Agarose (A6013) - Millipore Sigma
• Calcium acetate monohydrate (402850) - Millipore Sigma
• Triammonium citrate (A1332) - Millipore Sigma
• Diammonium citrate (247561) - Millipore Sigma
• Low-acyl gellan gum - Talas or Le Sanctuaire

Videos and Playlists For Gels

• Stavroudis, Chris and Getty Conservation Institute. 2013. Measuring surface pH and conductivity using water drop and agarose plug methods - YouTube. [Getty Series] (accessed 15 July 2020). Refer to the entire Getty series videos on gel preparation and use.

References

• Brockman, Madison. 2020 (July 13). "The Stain Stick." Syringe-cast agarose handout (pdf). BPG-PMG joint tips session, AIC virtual conference.
• Gels, AIC-WIKI page. Many additional references located on Gels page are not replicated here.
• Gels, 2019, Feb 20, Introduction to Rigid Gels for the Conservation of Photographs, Workshop with instructors Michelle Sullivan (J. Paul Getty Museum), and Amber Kehoe (then, a third-year intern Winterthur/University of Delaware conservation program); held in conjunction with the 2019 Joint AIC-PMG and ICOM-CC Photographic Materials Working Group Winter Meeting at the Penumbra Foundation, NYC, USA.
• Hoover, Caroline, and Laura Maccarelli. 2020. Nanogels: An Investigation into the use of Nanotechnologies for the Cleaning of a Painting by Ernst Ludwig Kirchner. 2 July 2020, Paintings Specialty Group session: AIC virtual annual conference.
• Ortega Saez, N., Arno, R., Marchetti, A. et al. 2023. "Towards a novel strategy for soot removal from water-soluble materials: the synergetic effect of hydrogels and cyclomethicone on gelatine emulsion-based photographs." Heritage Science 11, 78. https://doi.org/10.1186/s40494-023-00916-5
• Sullivan, M., et al. 2014. New approaches to cleaning works on paper and photographs. Proceedings of the 2014 Conference of the Association of North American Graduate Programs in Conservation of Cultural Heritage.

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