sizing for gum tests

Hi:

I continue playing around with sizing techniques for gum.

I haven't been happy with tray sizing - my results have been inconsistent
so I've been playing with brush sizing - actually roller sizing.

I've been applying the size with a foam roller with a fine nap finish.
The roller leaves small bubbles, so I finish the size with a paint pad - a
7 inch wide pad. The pad has a fine layer of bristles on a 1/4 thick
piece of foam. Produces a nice even coat.

I use chrome alum as a hardener and add it to make a concentration of 0.5%
of the size mixture.

I decided to be more scientific and introduce only one variable at a time.

I tried three sizes techniques, single coat gelatin, double coat gelatin
and single coat casein. Gelatin was mixed one envelope of plain knox food
grade in 250 ml water, left to sit for 1/2 hour heated to 50 C and then
add chrome alum.

The casien - I put 30 g of scientific grade powdered casein in 300 ml
water, add 100 ml ammonia (household grade), and chrome alum (at 0.5%)

Double coat gelatin - I applied gelatin, let it dry for an hour, and
applied another layer. Single coat gelatin and caesin - applied a single
layer and dried.

I coated one small sheet of each with the same gum/dichromate/pigment mix.
Exposed to a test negative and a stouffer 21 step tablet, at the same
exposure time.

The double coated gelatin gave the best results, clear highlights and
prints 9 distinct steps on a stouffer 21 step tablet

single coat gelatin produced mottled highlights. The gradation between
the steps was very different compared to the double coat gelatin. The
difference between the steps is not pronounced. The single coat gelatin
size produces a much more contrasty image. Almost no detail in the darker
regions compared to the double coat gelatin. Hard to see a differnce in
the steps. Prints a very short tonal range.

The single coat casein size, produced results very similar to single coat
gelatin, but it produced a denser black, and somewhat more detail in the
highlights, but still had mottled highlights. Contrast appears to be
better than single coat gelatin, but not as good as double coat gelatin.
Looks like casein sizing might support an image with a longer tonal scale.

Will try a double coat casein size and see how it compares to double coat
gelatin size.

Gord

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Gordon J. Holtslander Dept. of Biology
holtsg@duke.usask.ca 112 Science Place
http://duke.usask.ca/~holtsg University of Saskatchewan
Tel (306) 966-4433 Saskatoon, Saskatchewan
Fax (306) 966-4461 Canada S7N 5E2
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Received on Tue Jan 24 10:19:40 2006