A home I use a bank of 6 FL bulbs as a light source. Last semester I
constructed a bank of 8 bulbs (same bulbs, same manufacturer, same wattage,
etc)for use by my students. I use ammonium dichromate for gum printing and
had been using a bottle of saturated solution that was about a year old. When
we got to gum printing in my class I mixed up a new bottle of saturated
ammonium dichromate for the students to use. When asked by my students for
exposure times I said that I had been using exposures of 30-90 minutes at home
but to run some test strips. To my surprise the test strips and the subsequent
prints they made suggested that exposure times of 1/2 to 2 minutes were
correct.
The print frames they were using were identical to the ones I use at home.
Their negatives were about the same density. I even brought in one of my
negatives to try and sure enough, 1 min exposure compared to 30 min at home.
The distance between the print frames and the bulb bank are the same both at
school and at home.
I have thought of two possibilities: 1. I have been using the bulbs in the
light source at home for about two years, but I don't burn them for more than
about 2-3 hours a week. The bulbs at school were new. However, its hard for me
to believe that two additional bulbs or new bulbs as opposed to two year old
bulbs could make that much difference in exposure time. 2. Does ammonium
dichromate solution lose sensitivity with time? I keep it in a brown bottle
in a dark place. If it does, how often should one prepare new ammonium
dichromate solution?
I would greatly appreciate any thoughts/experience on this subject.
Bob Schramm