Re: printing gum on glass
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.
From: Katharine Thayer <firstname.lastname@example.org>
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
heavily pigmented mix of lamp black; that's with saturated
>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
>perhaps a composition depending on abstract shapes rather than fine
>On Jul 17, 2007, at 11:59 AM, Marek Matusz wrote:
>>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.
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