[Alt-photo] Re: DAS

[sanking at clemson.edu]

All things being equal there is no doubt but that a thicker tissue is
capable of giving a much stronger relief effect than a thinner tissue,
though there is a diminishing point of return.

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












> 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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