Myth: Chromium pigments are incompatible with Gum Bichromate

All you have to do to prove to yourself that this is a myth is to print with chromium pigments.

I suspect this myth came out of a misunderstanding about the chemistry of the process. Because chromium is an active participant in the gum process, perhaps someone mistakenly supposed that any pigment with chromium in it would interfere with the reaction, and then that misconception just got repeated down through the decades. But the chromium in dichromate is Cr(VI) which is highly reactive in the presence of colloid and UV, whereas the chromium in the pigments is a lower valence, functionally inert to the process.

Here are three chromium pigments: chromium oxide green (PG 17; anhydrous chromium sesquioxide), cerulean blue (PB 36; cobalt chromium oxide), and viridian (PG 18; hydrous chromium sesquioxide). These are test prints; they are all slightly overpigmented in an attempt to make them print as dark as their nature will allow, and the cerulean is underdeveloped because I was in a hurry that day, but it should be evident regardless that these pigments print just like any other pigments; there's nothing inherent in them that makes them incompatible with the process. They are all fairly weak pigments, but this is true of these pigments whether they are used in the gum process or not; it's also true of many pigments that don't contain chromium; and, at any rate, pigment strength is a completely different issue than whether a pigment is compatible with the gum process.

Copyright Katharine Thayer, all rights reserved