> Ammonium dichromate will lose from 10 to 20% speed in a year's time, so
> > that is not the cause, either.
On Sun, 22 Jun 1997, Charles Berger wrote:
> This may/may not be accurate. The light sensitivity of dichromate
> solutions are greatly influenced by the pH of the solution. The highier
> the pH, the lower the sensitivity. Over time, all dichromate solutions
> will show an increase in pH (whtether stored in a dark bottle or not)
> until they get to the point where they are virtually insensitive to
> light. Also, if the stored dichromate solution has turned a dark brown
> over time, it will act a a light-filtering dye, preventing actinic light
> from reaching the sensitized gel/gum layer.
>
> I have found that the best way to maintian the speed of dichromate
> solutions is to regularly "top-off" old solutions with freshly made
> ones and to carefully monitor the pH of the solutions.
Hi Charles and all,
Yes, I was struck by lightning, really & truly, about which more in my
next, but first things first:
My point wasn't about how "to maintain the speed of dichromate", but what
happens to the speed of dichromate that has just sat on the shelf for a
long period of time, which was what Bob was wondering about.
I have no reason to doubt your information about pH rising as the
dichromate ages, and that could in fact be the cause of the slowing down.
I've never been involved with the pH of any dichromate, although perhaps
I'm missing yet another one of gum's charming ways.
I simply reported what my tests showed when I mixed a new batch of
dichromate while I still had a bit in the bottle that was a year or so
old. The new was maybe 20% faster.
I have the tests, and I even know what file they're in (I think), so if
anyone doesn't believe me, he or she can come by for a look....
I'll add that now that I'm printing only gum my dichromate gets used up
quicker, so those tests may never happen again. However I generally find
an enormous adjustment is possible by how long you develop, so a 20%
difference in speed isn't as serious as it might be in another medium.
cheers,
Judy