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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.
Chris
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....
Dirk-Jan
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