Judy,
I have not had good luck with dry pigment. It requires a lot of work to
arrive at a good dispersion in the gelatine. If your student insists on
using it, do the following:
1) grind throughly with a mortar and pestle, using a bit of
glycerine to help disperse the pigment more evenly, finally adding about
100ml of gelatine, and grind again.
2) add the above mixture to the gelatine solution and mix in a
blender for 5-10 minutes on one of the lower speeds. (I would not use this
container in the future for margaritas)
3) the blender will kick up a huge amount of bubbles so the
solution must be set aside for several hours to allow the bubbles to go
down. I place it in an ice chest filled with water at about 105 degrees F.
It may be necessary to add a few mls of alcohol as a defoamer.
All in all, dry pigment is a lot more trouble than sumi ink and watercolor
pigments and it is very difficult to get a tissue that does not have a lot
of little pigment grains which will show on the final print, especially in
the highlights.
Sandy