[Alt-photo] Re: DAS

[Peter Friedrichsen]

Kees,

I went ahead to try to measure the amount of UV transmitted through 
some of these sensitizers+gelatin.

I used a UV exposure unit that exposes gum dichromates in 4-6 
minutes, and cyanotypes in about 10-12 minutes. I placed a UV sensor 
under a glass slide coated with these emulsions -pigment free.

For roughly the same film thickness of gelatin+sensitizer, I found 
that DAS is a much stronger UV absorber at 6% of dried gelatin than 
potassium dichromate at 30% of dried gelatin. In fact, only about 14% 
of UV at 365 nm can get through the DAS+gelatin film vs 
PotDich+gelatin. Significantly more UV passed through Ferric ammonium 
citrate/Gelatin than either of these. Even after full exposure 
(curves were plotted), the UV transmitted through the DAS+gelatin was 
still much less at about about 16% of the PotDich+gelatin.

I am thinking that this strong UV blocking ability of DAS would limit 
the depth to which the gelatin could harden to produce a relief. It 
would also suggest a more compressed scale.

Peter Friedrichsen

At 07:21 AM 19/04/2013, you wrote:
>Hi Sandy,
>
>These remarks are exactly what I observed. I think the yellowing of 
>the DAS/dichromate/ferric-ammonium citrate plays together with the 
>pigment concentration an important role in relief height. In my 
>observation the relief of a DAS sensitized tissue is lower, when 
>compared with a dichromate sensitized tissue, both with the same wet 
>height, pigment concentration,  optimized sensitizer concentrations 
>and printed with the same negative.
>
>Im my testing with fairly thick 1 mm wet height tissue and a process 
>optimized negative, relief is highest with ferric carbon, then 2 % 
>dichromate sensitized carbon, and lowest with DAS sensitized carbon. 
>I think the real advantage in using DAS apart from the (very 
>important!) low toxicity, lies in fullcolor and layered monochrome 
>work and the enourmous advantage of presensitization.
>
>Kees
>
>
>On 18 apr. 2013, at 17:50, Sandy King <sanking at clemson.edu> wrote:
>
> > 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
>
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