Typical exposure times for the UltraStable Color pigment films in a 26-1K plate burner ranged between 3 to 5 minutes. The need for a 25 minute (or more) exposure time baffles me. The Hardener No.3 absorption peak is 335nm and perhaps your light is emitting little energy of that nature. Alternately, the problem may be sensitizer (concentration, mixing technique or ?) related. Here's an example of how the sensitizer was incorporated into the UltraStable materials:
Originally Posted by gmikol
To make 250 ml of emulsion:
1. Add 1.5 g of Hardener #3 to 50 ml of water at 85F and QS to 60ml.
2. Slowly add sensitizer prep to 190 ml of a 13% pigmented gelatin prep at 105F.
Hope this helps..