On Apr 27, 2006, at 9:14 AM, Katharine Thayer wrote:
>
> On Apr 26, 2006, at 2:56 PM, Marek Matusz wrote:
>
>
>
>> The total volume of the solution is incidental as it only serves
>> to dissolve everything and make the solution spreadable. Once the
>> gum dries, it is the weight of dry components that will determine
>> final properties of the mixture.
>>
>
> Seems plausible in theory, but given my own observations over time
> I wonder if this is so. I might have to drop my cleaning and
> sorting and try mixing a different dichromate solution to see if
> less dichromate in the same amount of water has the same effect. If
> so, then I'd have to agree that the reduced DMax in #3 is caused by
> something other than more water in the emulsion.
Marek, I'm glad you're still working at this. This afternoon I took
the time to do this experiment, to satisfy my own curiosity re your
assertion that it's the weight of dry components, rather than the dry
components as a proportion of the total volume, that determines the
properties of the emulsion. I found, at least in this experiment,
that neither of us was exactly right; the answer fell somewhere in
between.
To review: last, I showed a back-exposed print made with 2.5 ml of
33% paint/gum mix and .5 ml of saturated dichromate, in other words .
135 g dry dichromate in 3 ml total volume coating. I found this mix
to be too contrasty. Then I showed a print made with 2.5 ml of the
same paint/gum mix and 2.5 ml of saturated ammonium dichromate, or .
675 g dry ammonium dichromate in 5 ml of total emulsion. I found this
mix to give insufficient DMax, which I attributed to the extra water
in the coating, and to be a bit too low in contrast.
Today I mixed some ammonium dichromate at 1/5 saturated, to give
(theoretically) the same dry weight, .135 g ammonium dichromate, in 5
ml total coating. In other words, this tests whether I was right in
attributing the reduced DMax in last week's print to the extra
water. This print came out between the earlier two prints (neither
too contrasty nor not contrasty enough) with DMax also between the
other two. It could have benefited by a slightly longer
development, but it's just a test print, after all, and it's
obviously on the right track as far as the tonality. But weirdly,
there's a graininess to it that isn't in the other two prints, which
I can't explain. It doesn't show well in the jpeg, but the right hand
and the face are very granular in quality.
For further explorations, I need a better negative (this one I'm
using wasn't particularly intended to produce a long tonal scale) and
will continue along this line of thinking: about 33% paint/mix,
about 5% ammonium dichromate. It's interesting that I need much more
pigment and much less dichromate when back-exposing on
transparencies, to get the same kind of print I get with less
pigment and more dichromate, front-exposing on paper. I don't quite
see why that would be, but it seems to be, nonetheless. I added this
print to the page I showed last week, for comparison.
http://www.pacifier.com/~kthayer/html/topdown4.html
One more caveat: I exposed all of these for 2 minutes in the sun,
but of course differences in the UV intensity at different times
could be responsible for some of the differences.
Oh, and by the way, I got some new inkjet transparency material
today, which I like for negatives but which doesn't work for beans
for printing gum on, either front or back. So it seems to be hit or
miss as to which brands of inkjet transparencies work for this.
Katharine
Received on 05/01/06-06:28:14 PM Z
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