For multiple coat gum prints, it’s necessary to preshrink the paper in order to maintain registration throughout the printing process. Unfortunately, preshrinking removes any sizing when the paper was manufactured. Resizing the paper involves soaking it in a gelatin bath and then hardening the gelatin onto the paper.
Since it’s such a time consuming and laborious project, I often will size several sheets (20-30) at a time. I set up a clothesline to dry the paper.
I boil water enough to fill a tray a place the paper in the hot water one sheet at a time. After 10-15 minutes I flip the stack of paper over and remove each sheet individually (first in first out) and hang by clothespins on two corners.
Once the paper is completely dry (usually I wait overnight) I combine 6 boxes (each box contains 4 envelopes) of Knox gelatin (available in most grocery stores) with 6 liters of water (one liter per box of gelatin). While gently stirring, I empty all 6 boxes of gelatin into about ½ of the amount of water and let it sit for about 15-20 minutes to let the gelatin “swell”. I add the remaining water and heat until about 140° (but not hotter) while stirring constantly. I pour the heated gelatin into the tray and immerse each sheet (turning each sheet over once to coat both sides). After about 10 minutes I flip the whole stack over and remove each sheet (first in first out) one at a time and hang to dry.
To squeegee the paper involves removing enough of the gelatin to leave an even coat on the paper. Removing too much is just as bad and not removing enough and will result in an uneven coat, which will cause staining and other problems later during printing. It takes practice and is a tricky to do alone. I’ve made a squeegee system using blocks of 2×4, a couple of clamps, and an old mop handle. Using a hole saw larger than the circumference of the mop handle (to allow it to spin freely), I cut a hole in each 2×4 block at approximately the height of the tray.
The clamps hold the blocks in place while the mop handle rests slightly above the lip of the tray. As I pull each sheet out of the tray, I position the tray with enough room to squeegee the paper between the tray and the mop handle and hang each sheet to dry with clothespins on two corners.
Hardening the sizing
Once the paper is dry the sizing must be hardened soon after. I’ve been using Glyoxal as a hardener for over 10 years instead of Formaldehyde (the US National Toxicology Program recently described formaldehyde as “known to be a human carcinogen Glyoxal is cheap, less toxic and works extremely well (my sizing can hold up to a dozen or more layers) and can be found at Photographers’ Formulary.
I mix up about 20 ml of 40% Glyoxal per liter of tap water and sometimes add ½ teaspoon of baking soda per liter. I fill the tray with the Glyoxal solution and add the paper one sheet at a time while turning each one over to coat both sides. After about 10-15 minutes I flip the entire stack over a remove each sheet (first in first out) individually and hang to dry with clothespins on two corners.