From: Katharine Thayer (kthayer@pacifier.com)
Date: 04/19/03-06:16:18 AM Z
Christina Z. Anderson wrote:
>
> Another thing: in PhotoAquatint Demachy says to mix your gum arabic up
> to 30-35%. However, in a talk he gave to the RPS in March 1898 he counsels
> to mix the gum to a baume of 18 to 20, using 1 part gum to 2 parts water. I
> assume this is a 50% solution, correct? Or is my scientific ratio knowledge
> in need of an update?
Alberto Novo wrote:
>
>
> Christina,
> I regret but I think so... it is about 30-35%!:
> 1 part + 2 parts = 3 parts; 1/3= 33.3%
>
> What is unclear in some recipes is what a "part" is: if it is a volume,
> surely 1 part of powdered gum arabic weights less than 1 part of water, so
> the resulting concentration will be below 30%.
Christina answered
> Thanks, Alberto,
> But then why do we call a 30% solution mixing 300g in 1000 ml water?
> Because it seems here Demachy is mixing 300g gum in 600ml water, correct?
> Not taking into account weight but just matching grams to mls. But then of
> course, the final amount of solution will be greater than 600ml because the
> gum arabic will take up some space but not 300ml water space, so that's
> where I get totally confused--you don't end up with 900ml of gum.
I think there are maybe two sources of confusion here. First, the issue
of volume vs weight. Alberto is correct that in some recipes it's not
clear whether the proportion is by volume or by weight. I don't myself
know whether Demachy is referring to volume or to weight, but since
Christina is weighing the gum rather than measuring it by volume, which
seems right to me on the face of it at least, I will assume that we're
talking percentage by weight. (For our purposes, water can be assumed to
weigh 1 gram per ml.) Proceeding under that assumption:
Demachy says in one place to mix the gum at 30-35%. Then at another
place, he says to mix it at one part gum to two parts water, or 33.3%,
as Alberto pointed out. In other words, Demachy was simply narrowing
down a vague 30-35% to a specific 33.3% and change. If you wanted to
end up with 1000 mls, for a 30% solution you would use 300 g gum; for a
33.3% solution, you would start with 333.33 g gum. If you're determined
to stick with 300 g gum as a starting point for the 33.3% solution, then
you're right, you would end up with 900 mls total solution.
Hope that helps,
Katharine
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