Re: bichromate gum
On Tue, 8 Jul 2008, Don Sweet wrote:
I don't recall the "Barnet" text cited (tho my memory goes even as I write) but have read MANY gum texts of the period, and marveled/wondered at the apparent "keeping qualities" of the old gum emulsions. In due course, however, I arrived at the explanation -- or, if you like, *an* explanation or two....Christopher James' new book mentions, as an untested idea, something that I read recently in a Barnet text from about 1898, namely that you can put the dichromate coat on after the size has hardened and leave the paper for some time before putting the paint/gum coat. From memory the Barnet text (too dear to buy) suggested the resulting image would be more contrasty with less highlight details than a conventional gum print. Can anyone cast any further light on this idea, or suggest any starting points for further experimentation?
Most frustrating were the reports of keeping coated paper "in a tube" or otherwise for WEEKS ! That was paper coated with pigment & sensitizer combined but not exposed. However that was potassium dichromate (ususally at *5%* since the saturated solution -- about 12?-- was diluted) AND the chemicals of the period were no way as pure as our chemicals today, a factor we often forget.
And, a propos, I remember reading something (about a million years ago) at the Visual Arts Workshop library in Rochester about attempts to duplicate a formula in one old book which failed and failed and failed, until they mixed the emulsion in a COPPER pot-- so that residues of copper were incorporated... on the assumption that early lab equipment was or included copper... (either the explanation or they were wearing their lucky shoes that day).