As long as you use the gelatin just to size your paper for gum printing, the
food gelatin will do, because you harden it anyway before applying the gum (or
gloy). Although food gelatin may be of varying quality, inconsistant results may
rather be due to different thicknesses of your coatings or varying quality of
the gum solution. 50 Bloom seems to be very low however and you should chose the
250 Bloom type, giving your coating more strength.
If you use gelatin to create an image, like with carbon transfer, the Bloom
index is very important, as it describes its rigidy under standard conditions. I
quote from F. W. Wainewright: "Physical Tests for Gelatin and Gelatin Products"
in The Science and Technology of Gelatin, London/New York/San Francisco 1977,
p. 509:
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In 1925, Bloom developed an instrument to measure gelatin gel rigidity. The
design of the instrument was such that it has remained basically unchanged and
has become the standard instrument for gelatin testing throughout the world.
Essentially, the Bloom gelometer determines the weight required to make a 0.5 in
diameter, flat bottomed punger depress the surface of a gelatin gel 4 mm. The
weight is applied as a stream of lead shot, the total of which is subsequently
weighed to the nearest gram. This weight is expressed as the *Grams Bloom* or
*Bloom* for the sample under test.
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Anyway, that's no explanation, why I got these damned big air bubbles right in
the middle of my carbon prints today...
Klaus Pollmeier