U of S | Mailing List Archive | alt-photo-process-l | Re: gum thickness and French check

Re: gum thickness and French check

Hi Christina !

Replying about your question about the French expression "pour cent".
For me, there's no doubt it means the final percentage of combined products :
- the solution "40 pour cent" means : a total 100 ml of combined solution contains 40 g
- in the same manner, the "10 pour 100 "potassium dichromate means a total of 100 of combined solution contains 10 of the chemical...

Hope it helps...

Cheeers from France
Jean Daubas

----- Original Message ----- From: "Christina Z. Anderson" <zphoto@montana.net>
To: "Alt, List" <alt-photo-process-L@usask.ca>
Sent: Saturday, November 18, 2006 3:51 PM
Subject: gum thickness and French check

One of the items that I have been looking into is the thickness of gum
solution--when 14 baume (288g gum arabic per liter) became "the norm". Gum
solution seems to have gotten thinner through the years. Back around 1900
it was more often "2 in 5", 40 g to 100ml, and as high as 60g to 100ml. I
measured this out and when adding 60g to 100ml you get about 155-160 ml of
combined solution, so essentially that would translate to the equivalent of
375g gum arabic in 1000ml water--30% more than the 288g/1000ml considered 14
baume. They're still mixing it at time of use 1:1 with dichromate.

I finally received a copy of the very first gum book ever--Rouille Ladeveze
Sepia-Photo et Sanguine-Photo, 1894. How fun to read this. In there the
solution is "40 pour 100". I want to make sure I am correct in my
assumption that this means to add 40 grams to 100ml (resulting in a combined
solution of 140ml) and not a combined solution of 100ml. Is this how a
French chemist would state it nowadays? It also says to make a potassium dichromate at "10 pour 100".

I routinely use a thicker gum when doing my gum prints anyway, preferring to
mix it from scratch. However, I have a couple liters of commercial gum that
I have been using up and the thickness between a 60/160 and a 288/1000 is

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.

Christina Z. Anderson
Assistant Professor
Photo Option Coordinator
Montana State University
Visual Communications Building Room 220
Box 173350
Bozeman Montana 59718