[Alt-photo] gum 'solarisation' issue SOLVED!

johnbrewerphotography at gmail.com johnbrewerphotography at gmail.com
Wed Aug 2 00:00:20 UTC 2017


Hi Edward et al

Edward, what you're describing, unless I'm mistaken, is solarisation in the shadows of the print. Mine is in the highlights. I always use a Stouffer half on/half off the negative media to get Dmax for minimum time but er on the side of caution so drop the exposure just a little. 

The Souffer test what threw me the most. 

Anyway, I've found the answer to my issue. I, and am sure most gum printers, size large pieces of paper then cut to size. The large sheet of paper I used for my tests wasn't that well sized after looking carefully. I tried another sheet (unsized) from the same batch, sized and problem solved. Same pigment amount/gum/dichromate. Repeated the test several times and always good. 

Thanks David, Marek, Joe and Edward for your input. My sanity has returned. 

John



Sent from my iPhone

> On 1 Aug 2017, at 6:32 am, Edward Draper via Alt-photo-process-list <alt-photo-process-list at lists.altphotolist.org> wrote:
> 
> Last Year I did a lot of work on pigment levels with gum to try to find the maximum pigment loading with gum
> 
> When you overload the pigment, especially lamp black, you get an interesting solarised effect. With regions of the image completely reversed. This is most noticeably when you only just overload the pigment. I use a quantity of pigment that only just avoids solarisation - I believe it to be the maximum I can get away with
> 
> The effect I think is something to do with the light only penetrating the very topmost layer leaving the lowermost layer next to the paper essentially unexposed and so it washes away
> 
> I've not got the necessary microscope equipment to study this phenomenon in more depth. And did consider trying a "gum transfer", which in theory, if I'm right, would allow me even greater levels of pigment.
> 
> Hope this helps,
> 
> 
> Edward
> 
> Working on the go on a screen keyboard - forgive minor errors, please
> 
> The big errors you should challenge
> 
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