[Alt-photo] Re: DAS

[Kees Brandenburg]

Hi Peter,

That's very interesting information and completely in line with my observations. This also strengthens my hypothesis that ferric carbon is the absolutely winner concerning relief. It's also in line with Charles remarks about DAS sensitized tissue gelatin thickness optimization. The DAS sensitized tissue that printed best for me is a #200 coating rod coated tissue with relatively much pigment.

The lower pigmented, hand coated 1 mm wet height tissue printed much softer, with less dmax and the relief was stil almost absent. The heavy selfmasking lengthens the tonal scale significantly but also prevented digging deep in the layer. With this tissue though it's very nice printing in 2 or 3 layers with different exposure times!


I will do some more tests with a #90 rod, I don't have the #120 Charles mentioned. Also cutting down Gelatin/DAS ratio might bne an interesting path to go.

Thanks very much for doing these tests!

Kees


On 23 apr. 2013, at 04:29, Peter Friedrichsen <pfriedrichsen at sympatico.ca> wrote:

> 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