Gelatin-acrylic sizing provides a reliable, non-toxic alternative to traditional paper sizing for gum bichromate printing. Marek Matusz explains how combining gelatin with acrylic medium improves adhesion, reduces pigment staining, and produces consistent multi-layer prints. He compares several formulas, recommends his preferred mixture, and shows how the sizing also works well for cyanotype, palladium, and gum-over prints.
WARNING: This is for experienced and advanced users. Read the MSDS (Material Safety Data Sheet) before attempting to use chemicals and use the correct safety equipment. Hazardous chemicals are used.
The first step in creating a gum masterpiece is always paper selection. You pick the surface, maybe you like some brands, or maybe you already have an image with cyanotype of palladium on it.
“The invisible component of the paper surface, the size will dictate a number of features of the finished print.”
It dictates how well the gum will stick to the paper, how tonal separations will develop and how pigment stain or lack thereof will manifest itself during the process, especially with multiple gum passes.
There are a number of practitioners that print without sizing, but even that might not be quite true as they might use a palladium or cyanotype layer that might act as a size. Some papers might actually have enough external size and can be used as such. I have printed successful gum prints on unsized BFK Rives, but the number of pigments and gum layers might be limited in this approach. There are endless papers out there and just as many sizing combinations for commercial papers, but most manufacturers don’t actually disclose the sizing chemistry, so it is quite difficult to rely on it. We have all discovered a paper that works so well, only to realise that the manufacturer changed something and it no longer works for us. Case in point is Arches Platine paper that in the last several years encountered manufacturing issues, that have been supposedly fixed, followed by some new issues and fixes and now apparently the paper is to be discontinued all together. There will always be somebody out there who says “nothing wrong with my sizing method”, and that is certainly true. No changing their mind.
Gelatin crosslinked with formaldehyde, glutaraldehyde, and glyoxal is probably the most widelyused as a size. Formaldehyde being the oldest described in literature and widely used, but itcarries a number of HSE issues. As an industrial chemist, I can not imagine myself working withformaldehyde without proper PPE (Personal Protective Equipment), and that includes adequateventilation. I would discourage any non-chemists from the use of formaldehyde. My size to go inthe past has been diluted Gamblin PVA. It is very convenient and quick, but as I switched tousing PR122 (Kremer Quinacridone Pink, transparent, PR 122) as my magenta, I have discovered that this pigment will stain with PVA as a size. Some other pigments can stain slightly as well (like some brands of Phtalo, Pb 15.3). It can also impart a plasticky feel to the paper at higherconcentrations, which some users object to.
I have tried using plain acrylic (with different dilutions), but it never worked for me. My new go-to size is a mixture of acrylic medium and gelatin. I buy it from Artcraft Chemicals and Bostick & Sullivan. Both are totally miscible and form stable solutions that do not separate. I have kept the mixed solution overnight (it gels like a regular gelatin), remelted and used it again with no issues, although I try to make just enough for one session. Both materials are non-toxic and widely available. My acrylic medium of choice is Liquitex matte professional medium (Figure 1).
It is an inexpensive, high-quality product that can be purchased everywhere. Matte medium imparts a little bit of tooth to the paper, which I like for gum printing. Gloss medium might be preferred if you want to use the mix as a varnish. Gelatin is also miscible with Gamblin PVA, and I have made a few prints using this combo, but will not go into details in this article.
There are a number of combinations of gelatine and acrylic medium that I will describe here and why to choose one of them.
I started with a 2% warm gelatin solution (250 bloom, photo grade) and mixed it with acrylic medium. I never tested food-grade gelatin, but I don’t see any reason why it could not be used as well.
- Size A: Add 5 g of acrylic medium (shake well before use) to 100 grams of warm 2% gelatin solution, stir and use.
- Size B: Add 7 g of acrylic medium to 100g of 2% gelatin solution and mix.
My initial testing was to put size A and B on a glass plate and let them dry. Both mixtures dried to transparent layers. The next day, I immersed the glass plates in warm water and observed more swelling from the A size. Not surprisingly, the gelatine is less cross-linked with less acrylic. As I increased water temperature to maybe 115-120 (the hottest my hot water heater allowed), size A finally dissolved, but size B stayed intact. I figured both will probably be quite fine on paper that has more tooth to grab size with temperatures around 90F as my maximum processing temperature. I have tested both, and they are excellent, stain-free size options that feel very organic in nature. My typical procedure is to allow the papers to dry overnight, but I have also stored, sized and dried papers for months. Preferred coating method for me is to spread size on a full sheet of paper with a 4” Richeson brush using perpendicular strokes. Start by pouring about 28 grams of size for a full-size sheet. It takes about 60 seconds to brush it all out. Make sure there are no dry spots and the size has not pooled somewhere. The amount of size depends a bit on your temperature, humidity and type of paper. Let the paper sit horizontally for a few minutes before hanging it to dry so there are no drip marks.
Some additional combinations that I have tested are as follows:
- Size C: 100g of 3% gelatin and 11g of acrylic medium (my preferred combination)
- Size D: 100g of 3% gelatin and 20g of acrylic medium
- Size E: 100g of 3% gelatin and 30g of acrylic medium
They all seem to work OK, although I noticed an increase in grain during development with increased acrylic concentration. Paper surface also becomes more plastic and matte in nature. There is some effect on speed/contrast with the proportion of acrylic in the size. My standard working practice is now Size C, which is close to size B with just more material in one pass. Size C needs to be occasionally warmed up if you are coating larger batches of paper. A few seconds in the microwave work best.
My favorite paper is Fabriano Artistico Extra White, 140lb. in cold and hot press variety, which I always pre-shrink in warm water and dry before sizing. It is best to use freshly prepared gelatin solution. Gelatin can be stored in a refrigerator for a few days, but it will eventually grow mold. My preservative of choice for gelatin solution and long storage is Consan 20, a common garden fungicide. For use, mix 10 cc of Consan 20 with 90 cc of water. Use 1cc of that diluted solution /100 cc of gelatin solution. Gelatin solutions preserved with Consan can last for weeks at room temperature. They are simply remelted before use.
Since I have started using this gelatin-acrylic size a few years ago, I have made hundreds of gum bichromate prints. Cyanotypes, palladium-platinum prints, and perhaps others can be sized with this mixture in preparation for gum-overs. Dichromate stain in gum printing with this new size mostly clears with 20-30 minutes of development time. Alternatively, it can be cleared with any standard method used in gum printing. Examples of pigment testing with size C on Fabriano Artistico papers are illustrated in Figures 2 and 3. Figure 2 illustrates pigment staining of iron oxide pigment on unsized paper (left) and the lack of staining with paper sized with C (right). Figure 3 is a test for a standard four-colour CMYK pigment combination for gum printing on paper sized with C.
Pigments from left to right are: Carbon black pigment paste from Calvin Grier. His pigment black, Pbk7 is a unique product with very low stain and neutral tint. There is nothing quite like that on the market. I strongly recommend this product. Next, primary magenta PR122 also from Calvin. I like to use transparent yellows, for example. PY129 (very transparent yellow pigment from Daniel Smith watercolor line) green gold, PY 129 and last Pb15.3 (cyan available from a variety of sources). I am also including some examples of finished prints in Figures 4 and 5 to illustrate how stain-free the prints can be with multiple gum passes.
Gelatine-acrylic size is also an excellent medium to size existing cyanotypes as a foundation for gum-overs, but it can also be used as a size before the cyanotype process. Figure 6 is an example of a cyanotype printed on Fabriano Artistico paper sized with size C.
You just have to brush the cyanotype formula a little bit longer to allow full penetration of the size layer. The white square on the print is an opaque patch to test for paper white/stain. You can see the base paper-white in that square with no stain.
Figure 7 is a print made on Fabriano Artistico HP extra white paper, 140 lbs sized with formula C. Three layers with different black pigment concentrations and different exposure times are used to build density and detail with a single negative. It is important to have no staining to build B&W images with highlight detail and very dark shadows.
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Oh this is amazing. I’ve also found success using this mix for carbon transfer prints. I like this over the pva sizing or gelatin hardened sizing cos you get the benefits of both in one.