On Sat, 18 Nov 2006, Christina Z. Anderson wrote:
Also, as of late I have found a number of unusable pigments listed (not chrome yellow--that was recommended) with these gum printers. One, "blanc d'argent", was said to, in a number of sources a. prevent insolubility (Rouille Ladeveze) b. darken and c. hold onto dichromate stain and turn yellow (German texts). So I guess the "certain pigments don't work" started at the beginning of time...and since we don't have certain of these anymore, it is a moot point. Blanc d'argent is lead white or flake white...I can't seem to find it in a watercolor paint.
Chris, lead white is used AFAIK only as an undercoat, eg., a primer for oil painting, where it has good qualities (or used to -- see Ralph Mayer, et al), but wouldn't be used in watercolor because it's opaque (which is one reason it's undercoat -- to hide the tan linen). But lead white yellows with age anyway -- no dichromate necessary.
Zinc & titanium whites stay white better. IME Zinc works much better in gum than titanium, tho that could I suppose depend on multiple factors... including the part in your hair and the brand.