[Alt-photo] Re: DAS

[Kees Brandenburg]

I think it's true that for a high relief it's not neccesary to coat very thick layers. Exposure 'colors' the dichromate or DAS itself, resulting in masking. But I have seen very prominent reliefs, maybe more the result of carefully finetuning the pigment amount in the gelatin. Also I found very high reliefs in feric sensitized carbon. Maybe the feric sensitizer is masking less. What also could be an explanation for the somewhat shorter scale ferric carbon has.

About heavy tissue: my small format tissues I coat 'freehand' with a comb, quick and easy, directly on a small sheet of yupo, whitout any borders. It's a very fast method, I coat a batch of 32 sheets in half an hour. But with this method it's impossible to coat thin layers. Another downside of this method is that a lot of pigmented gelatin goes down the drain.

-kees

On 18 apr. 2013, at 10:08, Charles Berger <fotocmb at gmail.com> wrote:

> The light restraining effect of dichromate (or DAS) in a  gelatin emulsion
> keeps the light from penetrating very deeply into the film regardless of
> excess coated film thickness.  Even a clear (no pigment) film with a weak
> (say 1%) sensitizer will have far less than a 1 mil depth of hardening.
> More sensitizer (or more pigment) will further limit hardening depth .
> 
> The apparent "relief"  of carbon tissue has more to do with local
> variations in the surface tensions of the processed and dried film layer
> than with infinitesimally slight physical differences in d-min and d-max
> areas.
> 
> Charles