From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/23/03-12:58:17 PM Z
Kerik Kouklis wrote:
>
> Basically, the idea is that dichromate is added in powdered form in
> proportion to the amount of gum in the solution. Ed mentioned 20% Ammonium
> Dichromate (2 gm in 10 ml gum). That is in no way a standard.
There seems to be some confusion in this thread about the amount of
dichromate in a given mix using this method relative to the amount of
dichromate used in a traditional 1:1 gum mix. Sandy looked at the amount
of dichromate in relation to the amount of gum and called it 20% and
said that was less than saturated ammonium dichromate and asked why are
so many gum printers using more dichromate than is necessary?
The thing is, we generally don't calculate the dichromate percentage
that way, as the amount of dichromate relative to the amount of gum. We
generally calculate it as the amount of dichromate relative to the
amount of water, and we add that solution to the gum but don't add the
gum to how we're figuring the percentage of dichromate.
Using the information from Ed's notes as a basis for comparison: 10 ml
gum, 2 g dichromate, 2.5 ml water. If you add the water and the gum and
figure the 2 g dichromate as a percentage of that total, you get about
16% (I'm ignoring the pigment for purposes of discussion here and also
ignoring the difference in specific gravity between gum and water). If
you had the same amount of total emulsion (12.5 ml) in the traditional
1:1 mix; this would be 6.25 ml gum, 6.25 ml water, and for saturated
ammonium dichromate, there would be approximately 1.7 grams of
dichromate in this mix, in other words, less than in the Melvin mix; 1.7
g vs 2 g, or 13% vs 16% of the total emulsion, not counting pigment.
So the difference between the two is not less dichromate, but less
water. I've never had a problem with staining, even though I use
saturated dichromate, so wouldn't have any reason to change what I do,
but it's an interesting idea. As to whether it matters whether you add
the dichromate dry or already mixed with the requisite amount of water,
there's no logic of science that says it would matter whether you added
the dichromate dry and then added the water, or made the solution first
and added it to the gum. The mystique associated with adding it "dry" is
simply that, a mystique, IMO.
Katharine Thayer
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