U of S | Mailing List Archive | alt-photo-process-l | Re: Stock Paint:Gum Solutions?

Re: Stock Paint:Gum Solutions?

DJ et al,
This whole thing used to confuse me so much until one book from early 1900s addressed the fact that a "30%" gum solution (meaning 300g gum + 1000 ml water but sometimes some authors meant 300g in a total volume of 1000ml) wasn't truly 30%.  Then I went to just the "practical" or "cooking" method of worrying about gum printing--e.g. "2 in 5" and whatnot.
But DJ brings up a great point--the people who measured pigment by weight.  Pigments vary so much by weight, and a lot of time the weaker pigments (e.g cerulean blue) weigh a ton and the stronger pigments (e.g. thalo) weigh very little.  Like comparing the weight of ground up rock to the weight of a vat dye.  I found that a light pigment weighed 5g and a heavy 9 g. given the same volume, and weight did NOT correspond to covering power or saturation.  So I quit using weight as a form of measurement as well.
But is is a good point Kees started the discussion on the recipe people use, or the concentration of gum solutions. These discussions can cause some clarifications between 'hardcore' chemists, and the people who are not really 'into' chemistry. It seems that this discussion has helped clarify the importance of being very clear in what you mean when giving directions for stock-solutions (and then I won't even get started on the difference of using tubes of 15 ml of paint vs the weight of the different colours ;-), or other directions for the use of certain chemicals....