U of S | Mailing List Archive | alt-photo-process-l | Re: bichromate gum

Re: bichromate gum

Hi Don,

So glad to see you want to experiment!!

Yes, I have done this. The key is to add just water to the gum/pigment mix instead of dichromate at time of coating to make the coat equally as thin.

I did not find any advantage to doing it this way. The initial idea was, to Demachy or Maskell's way of thinking as well as others, that this way was faster and the whites stained less. But soon in time, the following gum printers questioned the speed factor at least, and the double steps of the process made it less desirable as well. Dichromate by itself is not light sensitive--only when it is in the presence of organic substances. But sized paper does have organics on it (gelatin) and the keeping ability of the paper is not all that great. It was thought that one could sensitize a bunch of paper at once and then coat it later with pigment, but the paper turns brown after a while as the dichromate reacts with the gelatin in dark reaction.

The other thought was that each molecule of dichromate that was spread all over the paper would react with each molecule of gum arabic in a more even way and thus the layer would be more tenacious, more smooth, less grainy, etc. etc. This, too, I think is disproven in that one molecule of dichromate can react with many, if i am not mistaken--Ryuji would know for sure.

But for a one coat gum there is nothing wrong with doing it this way and I did find it a bit contrastier which might be a good thing for some people. As a general process, though, time and practice proved it not necessary.

I followed mentions of this method through the 400-some sources I read through for the gum book and compiled a chronological listing of books and dates it was practiced so it was of interest to me, too. It was at one point the best thing since sliced bread...around the time of Barnet's book for sure.

----- Original Message ----- From: "Don Sweet" <don@sweetlegal.co.nz>
To: <alt-photo-process-l@usask.ca>
Sent: Monday, July 07, 2008 3:53 PM
Subject: bichromate gum

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
Don Sweet