I'm not the kind of person who would cry nyah nyah as I would never want
anyone to cry it on me, but you may recall my saying some months ago that
your 30 to 90 minute exposures for gum printing were suspiciously long. So
let us consider the present perplexity as an opportunity ;- ) to at last
solve the mystery:
Fluorescent bulbs are rated for 7000-9000 hours, and even at death the
speed loss isn't all that noticeable. That's not the cause. "Regular"
fluorescents (which Tom mentions) age similarly, but you'd never get a
speed of one to two minutes with them -- they're much slower. And despite
Sil's suggestion of lower voltage at home, my tests of gum with bulbs at
different wavelengths suggest the difference is a factor of 3, not 200.
As for the number of bulbs, if your bulbs at home are clean, having two
less of them would hardly make a difference.
Ammonium dichromate will lose from 10 to 20% speed in a year's time, so
that is not the cause, either.
The first possibility comes to mind is that your dichromate was
contaminated, or there was some other error in compounding it. This is
easily tested by taking some of the school dichromate home and using it to
print with all other factors constant. (Was yours mixed with distilled
water? Are you sure it was distilled? We've had "distilled" water with
sealed cap at school turned out when tested by the chem dept. to be tap
water.)
Similarly your gum arabic (or other colloid), which also could be
contaminated or faulty, also easily tested.
One other possibility that comes to mind is that although you say the
frames are the same, perhaps somehow the glass on yours at home got to be
the UV resistant kind. (Gypsies come in at night while you're sleeping &
take your good glass and leave their gypsy glass behind. Ooops, apologies
to gypsies on the list. I mean gypsies from the old fairy tales.) So take
a frame from school to test at home (and lock it up at night).
And how about a REALLY remote possibility -- I mean a stretch. I had a
student once whose paper fogged for gum entirely when she sized it at her
home in Vermont when there was some kind of spore in the air. Do you have
strange spores or mold in your house?
But consider the following: Are your speeds at home for other processes
more or less normal? If so, the field is narrowed, either your dichromate
or your colloid would most likely be at fault.
We trust you'll let us know when you solve the problem -- surely this is
a good one for the books.
Cheers,
Judy
On Sun, 22 Jun 1997 SCHRAMMR@WLSVAX.WVNET.EDU wrote:
> 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
>
>