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Re: [BKARTS] percent solutions? mc, wheat, gelatin



Amy,
Tablespoon-per-cup formulas can be scaled too! One tablespoon is 1/16 of
a cup, so a formula calling for, say 1 tbsp/cup is 6.25% (1/16 or 100%)
V/V.

Now I will point out that this is only an approximation, and one that
gets worse for higher percentages, since % V/V really means percentage
of solute within the finished solution, whereas the formulas are usually
given as amount of solute as a percentage of the (pure) solvent. Few
recipes encountered in paper and book arts are of the form "mix powder
with [less than the full amount of water], then top up with more water
to [some specific volume]."

More concretely, the above formula likely produces more than a cup of
finished solution, so the actual V/V percentage would be less than 6.25%

To convert from V/V formulas to W/V, you need to know the bulk density
of the powder. Sometimes this is listed on a specifications sheet, and
sometimes on the MSDS. Lacking either, I have found that assuming a bulk
density of 1 g/cm3 is close enough. The only powders in our shop that
are far from this are titanium dioxide and iron oxide based pigments.

Does either of these approximations matter? Probably not. Most recipes
have plenty of leeway in them, often ending with a recommendation to add
extra water to suit. Differences in the water quality (e.g. mineral
content) can have dramatic effects on the viscosities of these
solutions, and individual recipes vary from one to another by more than
the error in these approximations. As well, other technical changes in
the usage of the solutions can adjust for varying solution strengths,
for example by adjusting the temperature at which the solution is used.

Unfortunately this general lack of technical rigour means that each
artist must collect their own set of custom-tweaked favourite recipes by
trial and error.
-Kevin Martin
 the Papertrail 

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