I too use fluorescent bulbs (F25T8-350BL). These are the clear (blue when
powered) "black lights". I use mine more per week than you do, and change
1/2 of them every year (so each bulb is used for 2 years). I do no see any
drift or problem with this usage. You didn't mention if your bulbs were
"Black light" (high UV output) or standard type. Perhaps the standard
(household bulbs) vary more with use??
tomf2468@pipeline.com (Tom Ferguson)
>Here is something that has been puzzling me for some time. Perhaps the list
>members with great gum experience can explain this.
>
>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