Gelatin sized paper

From Silvergrain Labs

Many home-made photosensitizing solutions are coated on paper to make prints. Each process prefers different substrate conditions, such as sized or unsized, and pH buffering of the paper. Processes like silver-gelatin and dichromated colloid processes are preferrably coated on gelatin-sized paper.

Contents 1 Paper base 2 Sizing 2.1 How to make sizing solution 2.2 Polymer Addenda 2.3 Biocide Addenda

Paper base

Since the paper generally experiences wet handling, the paper base is generally made from cotton rug, rich of alpha-cellulose, rather than wood pulp. The paper is also preferrably internally sized with dimeric alkylketene derivatives to increase the wet strength and regulate the absorption.

Sizing

In order to further decrease the absorption and smooth out the paper surface, the paper surface is typically coated with a sizing solution containing 3% gelatin in water. The gelatin is hardened by adding a hardener to the sizing solution before coating, or the coated paper is allowed to dry and then hardened by soaking it in hardening bath.

How to make sizing solution

1. Soak 30g hard gelatin (preferrably of 220 bloom or greater) in 1.0L of cold water
2. Wait until the gelatin is fully swollen
3. Slowly heat the solution to 40C while stirring
4. Optionally add: surfactant, biocide (thymol, 2-phenylphenol, phenol, p-chloro-m-cresol, etc), antifoaming agent (t-butyl alcohol, etc.), pH buffer, etc.
5. Add hardener immediately before coating

Hardener is added at 0.5 to 2% of the dry gelatin weight. For example, for 30g dry gelatin, 0.15g to 0.6g of glutaraldehyde or bisepoxide would be appropriate. This translates to about 6ml of 2.5% glutaraldehyde solution, or 0.6g of glycerol diglycidyl ether in the sizing solution.

Polymer Addenda

Sometimes, it is preferred to increase the adhesion between sizing layer and its adjacent substrate as well as the sensitizing solution layer. This can be achieved by replacing a part or whole of the gelatin with trimellitiated gelatin or phthalated gelatin (collectively called acylated gelatin). These modified gelatins contain increased number of carboxyl groups, and they exhibit excellent adhesion to various substrate surfaces (glass, paper, latex subbed polyester film, etc.). These gelatin sizing solution should be hardened with a suitable bisepoxide or chrome alum, instead of or in addition to glutaraldehyde, since the latter is incapable of crosslinking the acylated gelatin molecules.

Another kind of polymers adde to sizing is poly(methyl methacrylate) and poly(ethyl methacrylate) in a suitable binder, such as poly(vinyl alcohol-co-vinyl acetate). These polymers are commonly used as acrylic medium, and they are readily available. Examples are Rhom and Haas Primal AC33 and AC35. Primal is called Rhoplex in the U.S. market. Primal AC35 is preferred for sizing layer, and is available in small quantity from Kremer Pigmente.

Biocide Addenda

Gelatin solution is readily susceptible to bacterial and fungal attack. Therefore, if the sizing solution is to be kept for later use, a small amount of biocide should be added.