Re: Gum and Solutions

Chris wrote:

>
> Sandy, am I mistaken or haven't you done the step wedge test to
>determine what kind of speed change happens with dilution of dichromates?
>Kosar (if I dare mention his name) says "Although only very small amounts
>(0.5%) of dichromate are necessary for the photochemical hardening of
>dichromated colloids, the sensitivity increases almost proportionally with
>the dichromate concentration." Scientifically this may be so, but
>practically do you find this to be the case? Or does it follow some sort of
>shouldered curve?
>Chris

I made this test with dichromate and gelatin, not gum, but I have
experimented enough with gum to believe that the results in terms of
printing speed would be similar if one used the same standards for
all of the tests. I have always been surprised that most people seem
to use such strong dichromate solutions in gum since there does not
appear to be any practical reason to do so. I looked at Sam Wang's
formula and unless I am reading it wrong he appears to be using about
a 3% dichromate solution, which seems much weaker than what most
other folks seem to use.

In any event I provide below is the data from a recent test of a
medium contrast carbon tissue with various strength dichromate
sensitizers. The tests were made with a Stouffer 4X5 step wedge,
exposed with a bank of BLB tubes.

Strength ES Speed Point
1/8% 1.05 0.6
1/4% 1.33 1.0
1/2% 1.57 1.3
3/4% 1.79 1.4
1% 1.92 1.4
2% 2.08 1.4
4% 2.20 1.4
6% 2.32 1.4

The sensitizer was straight potassium dichromate. ES is exposure
scale and indicates the negative density range that would be required
for this tissue with the sensitizer strength indicated. Speed point
is based on the silver convention of 0.6 over B+F, but with carbon
would be the same if based on Dmax since the curve is so straight.

What you can tell from the above that speed increases quickly as the
strength of the dichromate sensitizer is increased from 1/8% to 1/2%,
but above 1/2% it stabilizes.

Sandy
Received on Sun Nov 23 13:02:43 2003