[Alt-photo] Re: DAS

[Kees Brandenburg]

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