[Alt-photo] Re: DAS

[Sandy King]

Bear in mind that how deeply the exposing light can penetrate into a carbon tissue depends both on pigment loading and the actinic filter of the dichromate sensitizing agent (which is determined by the concentration). A thick tissue with very high pigment loading will give a thin carbon relief that will have no relief. So to optimize final relief it is necessary to balance the thickness of the gelatin layer and pigment loading so that when the sensitized tissue is exposed the light is able to penetrate nearly all of the way to the substrate. Needless to say, the contrast of the exposing negative must also be carefully matched to the strength of the sensitizer.

In practice my monochrome carbon tissue has a wet coating height of .9mm, which on dry down measured about .14mm. When this tissue is sensitized with dichromate solution of the appropriate strength, and then exposed with a negative of the right contrast range, the exposing light penetrates virtually all the way the substrate. You can tell this on warm water development because there is virtually no soluble pigment remaining on the substrate when it is stripped from the print. 

It is possible to make very thin carbon tissue that is so heavily pigmented that it is not capable of giving any appearance of relief. 

Sandy










On Apr 18, 2013, at 5:31 AM, Kees Brandenburg wrote:

> 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
> 
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