Re: Demachy and Maskell's postscript

Christina Z. Anderson wrote:
>
>
> The thought at the time was that the dichromate would be on the surface of
> the paper and better secure the halftones because it would harden from the
> bottom up. It's interesting, because I read in solarplate literature (not
> sure what the sensitizer is in solarplate) that it hardens from the base up.
> Isn't that odd? Why would light hitting the surface harden the depths first
> and not the top? But that is a digression.

Actually, the light doesn't hit the surface and bounce off, it
penetrates the coating layer. The hardening can only occur where the
dichromate and colloid are in close proximity to each other, so the
hardening would occur only at the bottom of the gum layer where it
touches the dichromate, not higher up in the gum layer where there are
no available dichromate ions for the gum to react with. There may be
some diffusion of the dichromate into the gum layer where it is wetted a
bit by the gum layer, but I personally doubt it travels very far into
the gum layer.

But Demachy and Maskell gave a different explanation: "The rationale of
this system would appear to be that each molecule of the pigmented gum
with which the dried bichromate paper is coated aborbs, or is in contact
with, just its molecule of bichroamte and no more. the rest of the
bichromate, protected by the coating of colour, is probably very little
acted upon." This passage should be read as an example of historical
color but shouldn't be taken seriously as a description of what happens.

>
> I did find it was more contrasty, therefore looked sharper, and did not find
> more stain either in pigment or dichromate.

Interesting. Rather opposite of what I found. I certainly found the
potassium dichromate more contrasty (objectionably so) than the same
negative printed with ammonium dichromate, but this greater contrast
(mainly caused by the loss of highlights and midtones) was the same in
both the mixed emulsion and the separated emulsion.

>
> To me, the method did not make a bit of sense because it required two steps
> instead of one, and if there weren't benefits, why bother.

Agreed. I'm not interested in this for any interest of using it in
practice, but for the purpose of determining whether the assumptions
that it rests on are sound.

Katharine
Received on Mon Apr 11 11:26:23 2005