Re: Gum hardening: top down experiment

Marek, thanks for your response. My patience for this discussion was
coming to an end just as your post popped into my mailbox; you have
revived me. A couple of comments embedded below:

On Apr 11, 2006, at 7:41 AM, Marek Matusz wrote:

> Katharine,
> I was very impressed with your results and thanks for contributing
> to this discussion in a positive and constructive way. Looking at
> your scans I would conclude that heavily pigmented gum layers on
> unabsorbant substrate do harden from the top down, just like other
> dichromated colloids.
> I see very nice tonal gradations in the print exposed from the
> bottom. The three variables (gum, dichromate and pigment ratios)
> are not optimised, but at this point I am looking for illustration
> of principles, rather then perfect prints.

Thanks, I agree.

> I did a similar experiment last night. I coated a heavily pigmented
> and thick layer of gum on a transparency material that I use to
> print diginegatives (HP brand). This brand has a nice sandy feel to
> it, so I though it would help to hold the gum. I exposed coated
> pieces for twice my usual times, one through top, the other from
> the bottom.

For whatever it's worth, I exposed the front-exposed print on mylar
for 6 minutes, which is 4X my usual time for that negative, and the
back-exposed print for twice the time of the front-exposed one (12
minutes).

> The piece exposed from the top flaked off rather quickly leaving no
> image. The was no image that I could see at any point. The piece
> exposed thorough the bottom once in the water started behaving like
> a carbon print, where the colloid was dissolving in water, rather
> then flaking off. After about 3 minutes I was excited to see a full
> tonality image, with beautiful tonal gradations. Unfortunately the
> image continued to develop even afet I took it out of water and
> hanged it to dry.

This is exactly what happened with the front-exposed print in my
experiment; it looked okay, although very high-contrast, when I took
it out of the water, but by the time it was dry the hardened gum had
melted and puddled on the mylar. (This is not what usually happens
when I print on scuffed mylar using a more normal emulsion).

> This morning there was only a faint image left on the piece of
> transparency. I will give it another try with much longer exposure
> and perhaps lower dichromate to get more depth of UV penetration
> and hardening and higher Dmax.
>

I look forward to your report,
Katharine

>> From: Katharine Thayer <kthayer@pacifier.com>
>> Reply-To: alt-photo-process-l@usask.ca
>> To: alt photo <alt-photo-process-l@sask.usask.ca>
>> Subject: Gum hardening: top down?
>> Date: Mon, 10 Apr 2006 11:33:03 -0700
>>
>> Okay, I've coated a very thick, very heavily-pigmented gum
>> emulsion on mylar and printed it from the front and from the
>> back. A couple of comments before I give you the URL:
>>
>> (1) though the emulsion was very heavily pigmented, two things
>> resulted in not a very deep DMax: (a) the fact that I used ivory
>> black, a transparent pigment (if I were to do it again, I'd use
>> lamp black) and (b) the fact that it's printed on a transparent
>> material and was scanned as a transparency, with the light
>> shining through it. But the thing to note is, be that as it may,
>> the DMax is about the same in both prints.
>>
>> (2) there's a light brown pigment stain (ivory black is a
>> brownish black) in both prints that is probably a function of the
>> heavy pigmentation. It hardly shows in the prints themselves, but
>> for some reason was accentuated in the scanning.
>>
>> (3) I don't honestly know what to make of the results. If you
>> look just at the prints on mylar, you'd have to conclude that
>> back- printing is much superior to front-printing, at least for a
>> thick coat on mylar. But if you compare the back-printed print on
>> mylar to the regular front-printed gum print (using a less
>> heavily-pigmented emulsion) on paper (at the bottom of the page),
>> it's hard to claim that the back-printed print is superior. But
>> since they are on different materials, it's apples and oranges.
>>
>> So I guess if I were forced to draw a conclusion from this rather
>> inconclusive test, I'd say that if you are going to print on
>> mylar using a very thick and heavily pigmented emulsion, then
>> you'll probably do better printing from the back. But if you're
>> printing on paper, you can get fine results printing from the
>> front with a less pigmented emulsion.
>>
>> http://www.pacifier.com/~kthayer/html/topdown.html
>>
>> Katharine
>>
>>
>
>
>
Received on Tue Apr 11 09:22:18 2006