From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/04/03-09:00:24 AM Z
> Christina wrote:
>
> Maybe the reason am di gets a bad rap in gum sometimes is people don't
> > realize how fast it is, and overexpose, and thus their image gets
> > overprinted highlights.
>
Since the above sentiment is one I've often expressed, this might be a
good place for me to jump into this discussion.
Somewhere in this thread I thought I read the assertion (although I
can't find it now) that potassium dichromate is generally considered
the dichromate of choice for gum printing, which was a surprise to me.
It wasn't clear whether that assertion referred to the historical
literature or to current practice. I would say it was certainly true for
the early gum printers, but I'd be surprised if it was true of today's
gum printers. I can only make a stab at the proportion of gum printers
using ammonium vs potassium dichromate, but my guess would be that
considerably more than half of modern gum printers use ammonium rather
than potassium dichromate.
In the early days of gum printing, it's my understanding that potassium
dichromate was really the only one that anyone used. Ammonium dichromate
wasn't widely available, and it was generally considered to be
dangerously explosive, so would have been avoided at any rate. When gum
printing was "rediscovered," ammonium dichromate was readily available;
the idea that it could blow up on you at any moment had gone the way of
other outdated and discredited ideas, and those facts combined with its
increased speed made it the sensitizer of choice for most modern gum
printers.
Only in the last couple of decades has the idea been promoted in some
circles that potassium dichromate is preferable, or even that ammonium
dichromate is "no good" for gum printing. The idea behind this assertion
is that ammonium dichromate "promotes" dichromate staining, whereas
potassium dichromate prints clear colors untainted by dichromate stain.
However, this assertion is easily disproven and certainly doesn't
constitute a general rule; many gum printers (including yours truly) use
ammonium dichromate with no dichromate stain whatever. My
investigations have led me to believe that the use of very intense
graphic-arts lights has created this misunderstanding. For some very
intense lights, it's probably true that the speed of the ammonium
dichromate outruns the speed of the light and with that particular
light/dichromate combination, it's impossible not to overexpose the
print, resulting in inevitable dichromate staining. So yes, folks using
a very intense, hot light may find that they need to use the slower
potassium dichromate to keep from overexposing, but folks using
different lights will find that they can print beautifully in clear
colors with ammonium dichromate.
But I would say that this controversy has affected only a small corner
of the gum printing world, and that everyone else has gone on happily
printing gum with ammonium dichromate without a problem and without even
being aware that there was ever a controversy about ammonium dichromate.
As for the argument that it's useless to compare speeds etc of the
different dichromates because at the same percentage solution they act
the same, that argument smells of red herring, or some kind of fish at
any rate. The fact that you can dilute ammonium dichromate down to 10%
and it will be as slow as potassium dichromate at that strength, isn't
nearly as relevant to this discussion as the fact that you can't
possibly saturate potassium dichromate to 27% and make it as fast as
ammonium dichromate. The important thing to understand about the
difference between dichromates is that at saturation, ammonium
dichromate is significantly faster than potassium dichromate. Ammonium
dichromate can be made slower by dilution, but potassium dichromate can
never be made faster by addition of more compound to the solution. So
the argument that the relative speed of the dichromates is simply a
function of concentration rather than an inherent quality of the
compound, doesn't hold water (so to speak). Ammonium dichromate at
saturation IS faster than potassium dichromate. The fact that it can be
made slower by diluting it to potassium dichromate's maximum
concentration may be useful to folks who bought ammonium dichromate and
find that at saturation it's too fast for their lights, or others who
want to adjust the speed of dichromate by changing the dilution, but is
irrelevant to the discussion of the relative inherent speed of ammonium
vs. potassium dichromate.
Katharine Thayer
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