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 substantial. 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. Chris Christina Z. Anderson Assistant Professor Photo Option Coordinator Montana State University Visual Communications Building Room 220 Box 173350 Bozeman Montana 59718 406.994.6219 CZAphotography.com
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