Re: printing gum on glass
To address Judy's suggestion earlier about Christopher James' first
editiion: no, there's not a chapter on printing gum on glass per se
in the first edition. There's a section on printing gum on
"alternative surfaces, such as metal plates, stone, chalkboard slate,
glass, mica, vinyl or plastic" which addresses different ways to
introduce tooth to hard surfaces, including abrading the surface or
(more relevant to glass) subbing the surface with material that will
help the gum adhere. He doesn't (by my reading) repeat or deny any
myth about midtones and gum on glass, which by the way is a myth I've
never encountered. There is also a separate section about printing
on glass, not geared to gum especially, which gives Galina's method
of subbing the glass with gelatin.
It's no particular myth that gum doesn't adhere well to glass because
of the slick surface of the glass which offers no tooth for the gum
to hang onto, and as James indicates, the glass usually needs to be
treated in some way to increase the adhesion of the gum to the glass.
Some people use silane very effectively for a sub on glass for gum
printing. I have used a light dusting of fine pumice in acrylic
medium, for a sub to provide tooth for printing on glass.
The glass must be scrupulously clean; I find a final swash of 200-
proof alcohol works very well to remove every trace of whatever
cleaning material may be clinging to the surface after complete
rinsing; when it sheets cleanly off the glass without beading or
catching anywhere, you know the glass is clean. Then a sub, of
whatever works for you, and then expose the heck out of it (someone
told me this once a long long time ago and I didn't believe him, but
indeed, an extra-long exposure does increase the likelihood of
retaining the hardened gum on the glass) and then develop very very
gently, face up, with fingers crossed.
Or, as Marek suggests, expose from the back.
Katharine
On Jul 19, 2007, at 7:34 AM, Katharine Thayer wrote:
Hi Marek,
Hope it stops raining soon.
I was pretty sure I had reported that experiment to the list at the
time, so I went to the archives and searched for the post to
refresh my memory about the particulars. but couldn't find it.
Anyway, I don't remember the details exactly, and I must not have
kept that experiment since it wasn't among the gum prints on glass
that I unpacked the other day; I must have scraped the gum off the
glass and re-used it. It may be that I did that print inside
under the photoflood bulb rather than outside under the sun; I just
don't remember. I think there's nothing for it but I'll have to do
it again to be sure. It's been raining here this week too,
although we had a couple of 100-degree days last week (that would
have been the time to do this experiment).
Katharine
On Jul 19, 2007, at 6:50 AM, Marek Matusz wrote:
Katharine,
My typical dichromate concentrations are usuallu lower, for 1
volume of 14 baume gum/pigment I use 1/2 to 1/3 volume of
saturated ammonium dichromate solution. The dichromate
concentration definitely changes exposure, but I would say you
ball park estimate of yours and mine of about 1 minute sun
exposure would be a good starting point. It has been raining in
Houston forever and I am keeping my fingers crossed for this
weekend to get some sun.
I was surprized by your comment that you lost sharpness with back
exposure through the glass. DIrect sun creates such a sharp shadow
edge that I thought there should be no sharpness loss over a thin
piece of glass. I guess the experimentation will show.
Thanks for your comments.
Marek
From: Katharine Thayer <kthayer@pacifier.com>
Reply-To: alt-photo-process-l@usask.ca
To: alt-photo-process-l@usask.ca
Subject: Re: printing gum on glass
Date: Wed, 18 Jul 2007 00:17:59 -0700
>Marek, what dichromate concentration are you using? that would make
> a difference, of course, to the exposure . Also, the variation in
> intensity from place to place. If I remember right; you're in
>Houston? Your sun is probably more intense than mine in the Pacific
> Northwest. I lived on the coast when I last did exposures in the
>sun for gum on glass; as I recall they were a minute or less with a
> fairly heavily pigmented mix of lamp black; that's with saturated
>ammonium dichromate.
>
>I tried exposing from the back on regular picture glass, after our
>thread about back-exposing on plastic a year ago or more, and found
>that while the exposure worked well (the gum adhered well to the
>glass with back-exposure) the thickness of the glass between the
>negative and the gum resulted in a loss of sharpness and detail,
>which didn't work very well with the image I chose. I still think
>that's the best way to go for printing on glass, as you say, but it
>needs to be the right kind of image that won't suffer too much from
>not having direct contact between the negative and the emulsion;
>perhaps a composition depending on abstract shapes rather than fine
>detail.
>Katharine
>
>
>
>
>
>
>
>On Jul 17, 2007, at 11:59 AM, Marek Matusz wrote:
>
>>Chris
>>
>>Good to hear the list is alive. I will miss the APIS activities
>>this year. I really wanted to go this year, but something came up
>>last moment. As far as the gum on glass I have tried it last
>>summer. I made a couple of very thin blue layers for the
>>preparation for tricolour gums. With very thin layers my exposures
>>were short, and I remember long development times as well. I do not
>> think I optimised it. The project was never finished as one day
>>my stack of plates crashed and I never started again. I was very
>>tempted to do some gum on glass with the back exposure. This should
>> give a nice continuous gum layer sticking to the glass. If you
>>have a colimated UV light source that would be the ideal way to
>>make gum on glass. Direct sun exposure is another possibility,
>>which I might try this weekend. What is the typical direct sun
>>exposure (not in the shadow) if anybody is using this method. A
>>good guess would save some calibration tests.
>>
>>Marek Matusz
>>
>>
>>
More photos, more messages, more storage—get 2GB with Windows Live
Hotmail.
|