Yes, you should use a little chrome alum in the gelatin for oil printing and you
should even try a small amount of dextrin (potato starch, 10% of the gelatin
amount) to give the gelatin a certain 'tooth' which makes the ink adhear better.
The chrome alum is esp. required if you want to use the oil print for transfers.
My favourite formula is:
A
15% gelatine sol. 160,0 ml
15% starch sol. (Dextrin) 16,0 ml
B
6% acetic acid sol. 6,0 ml
1% chrome alum sol. 8,0 ml
Keep A and B at 60-65 C and slowly add B to A with constant stirring. Take ca.
40 ml to coat one sheet 50x70 cm. If the gelatin gels in the beaker allready,
your solution wasn't hot enough. Rewarming is impossible. Beakers and brushes
are difficult to clean even in hot water after the gelatin has gelled. Let them
soak in a solution of detergent used for the washing mashine first.
Klaus Pollmeier