Re: printing gum on glass

[Katharine Thayer <kthayer@pacifier.com>]

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