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

Re: gum thickness and French check

Thank you, Jean! So that means a true 60% gum solution and not a 37.5%! Wow.

Ha, ha, over the weekend I was piddling around in a bunch of different gum experiments while printing and I finally observed this one thing I had read about. In the German books I have had translated for me, there was a term "flagging" which for the life of me I never knew what it was. I even asked my original gum teacher (who is Austrian) what it is and he explained it to me but I had never observed it.

This weekend I accidentally forgot to add the water part of the equation to my gum/pigment/di mix (I don't use am di at 30%, but cut it with an equal amount of water so it comes out to a 15% am di). When the gum prints went into the water for development I finally observed "flagging". The layer, instead of flaking off as I usually see happen, came off in a chunk here and there, and at the edge of the chunk there were these little tears of wavy veils that waved like little flags! It was worth ruining three large gum prints to observe this--and, technically I didn't waste anything but time because I just brushed the whole layer off.

Speaking of ruining prints, I did so to 8 large prints over the last week for various reasons. I sized unshrunk Rives BFK and FAEW and printed on it, thinking it would be dimensionally stable as I have read. NOT. Both papers shrink 1/16 inch horizontally and vertically so I will NEVER again skip the shrinking step. Those were ruined. Maybe with small prints this is fine, or in a humid climate, but 16x20 in MT, even with sponging the back of the prints with water, not for this babe. Besides, the shrinking step with a bunch of baby hangers and a shower pole is not a biggie.

Second, I got this hairbrained idea to try out a yellow ink only negative--yellow being oddly enough a very dense pigment to UV light. Well guess what--the second coat, have you ever tried to see registration with a yellow negative on top of a blue layer? Don't try it. 4 more large prints in the trash along with 3 large pieces of Pictorico. Impossible.

Third, I decided to coat a couple sheets of paper with a layer while my negative was printing out, thinking I would be accurate with the size of the layer area. When I went to print, my coated area was not long enough. Tried to resurrect it with another layer. In the trash. Oh well.

Christina Z. Anderson
Assistant Professor
Photo Option Coordinator
Montana State University
Visual Communications Building Room 220
Box 173350
Bozeman Montana 59718
----- Original Message ----- From: "Jean Daubas" <jean.daubas@wanadoo.fr>
To: <alt-photo-process-l@usask.ca>
Sent: Monday, November 20, 2006 8:10 AM
Subject: 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