[Alt-photo] Re: DAS

[Charles Berger]

Hello Peter,

Thank you for this valuable information.  It illustrates a great deal about
the penetration of UV light into a sensitized emulsion and accounts for
differences in exposure "speeds".

Do you know what type of lamp was used to exposure the tests?
Multi-spectral lamps (Metal Halide/Mercury Vapor) emit varying wavelengths
of varying intensities (% of emitted light).  A Series 0 lamp, for example,
which is used for most platemaking applications, emits 0% at  320 -360 NM
(DAS peak sensitivity is 335NM) while 11% of a Series 7 bulb is in the 320
- 360NM  spectrum.  Both lamps, of course, emit large amounts at 365nm, but
comparisons of emitted light absorptions may be affected by the secondary
wavelength outputs of the lamp.

In any case, it does underscore the need to find an exposure bulb that
matches the specific spectral sensitivity of the materials for maximum
penetration and minimum exposure.

Charles


On Tue, Apr 23, 2013 at 12:35 AM, Kees Brandenburg
<workshops at polychrome.nl>wrote:

> 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
>
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