I'm going to size some cold press watercolor paper with gelatin. I have in my possession and want to use chrome alum, not formalin or glyoxal.
Is it necessry to include the alum in the gelatin, or can I soak the sized paper in it after the fact. How severe is the staining, as mentioned above by R Shaffer?
On my first attempt at sizing used alum - I went a bit over the top and added way too much potassium chrome alum and the paper came out with a very distinct blue tint. I now keep a bottle of 2% solution to hand and add 2ml per 100ml of gelatin (1g gelatin per 100ml water). At this dilution, the colour is barely perceptible when wet, and disappears on drying.
If you need a larger volume to soak your paper in, the sizing solution can be reheated two or three times.
That seems to echo what Luis Nadeau says in his book on carbon. He also notes that manufacturers who use this method wait no less than a month for the gelatin to fully harden. That seems excessive... and impractical.
Can I soak a plain gelatin sheet in some % of chrome alum, or is incorporating it in the emulsion the only way to keep the stain at an imperceptible level?
To be honest, I'm no expert on sizing, but.... I would have thought it possible to do a subsequent soaking in a ~0.5% bath. However, if Nadeau states a month or more to reach fully hardened with chrome alum combined, then I would have expected a similar or longer time for a secondary bath.
I have successfully hardened both Knox's gelatin and gelatin from Bostick and Sullivan with formalin and with Glyoxol, which is less ordorous than formalin. I've used this treatment for carbon, gum, and salted paper, although I prefer arrowroot starch for salted paper prints. I think aging the paper is important when making carbon prints, but not necessary for salting or gum.