RE: Gum hardening: top down experiment

Hi Katharine, Marek, Sandy, Judy, and whoever interested,

Thanks for your efforts and sharings on your findings.

About the tests, however, I have a few suggestions:

Katharine, you probably shouldn't expose longer than usual and expose the
front and back differently because that will affect interpretation. Let's
try to step back and look at our hypothesis first. Let's say we assume that
hardening is from top to bottom (we don't have to agree on this, but we can
take any view and start from there). The test then, is to expose it not too
much from both side. If hardening is from top to bottom, the one exposed
from the front will not reached the base, so it has nothing to hold on, so
it will flake off, whereas the one exposed from the back will grab the base,
so it will have some image. But if expose a lot (or strongly), then probably
both will have an image because even the one exposed from the front will be
exposed sufficiently to reach the base. Now the contrast of the two might be
different, and so we might start to make different inference from there, but
that is not the original goal of the test.

Suppose that we expose it normally, and get a fine image for the one exposed
from the back but not for the one exposed from the front, we pretty much
proved our hypothesis. But suppose we don't get an image from either one,
then we need to adjust. It might be that the mylar might be cutting off a
lot of UV, so we need to expose more. But if we expose more, is it a fair
comparison because the front one doesn't have the UV blocking by the mylar?
So in order to have a reasonable test, we need to increase expose but also
cover the front side by a piece of mylar during exposure.

Our tests seem to be something in between. The fact that the image continue
to dissolve after development seems to suggest that exposure is not enough
so there is still a lot of unhardened gum (Sandy, to answer the question of
why this is happening in gum but not in carbon, the difference in mechanism
is that gelatine doesn't dissolve in cold water whereas gum does).

My suggestion would be to expose perhaps a step tablet with double the
amount of time used for exposing through the back, but when exposing the
front, cover it with a piece of mylar.

And thank you again for all your efforts. I am by no means asking you to do
a certain test. It is simply a suggestion from me, a lurker now. :-)

Dave

-----Original Message-----
From: Katharine Thayer [mailto:kthayer@pacifier.com]
Sent: Tuesday, April 11, 2006 11:22 AM
To: alt-photo-process-l@usask.ca
Subject: 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 09:52:30 2006