Re: (Gum) Tonal scale

Mark,

your are right about missing posts, something funny is happening with the server(s) I suppose.

At first I thought that in Joe experiments the stain could be caused by the dichromate "reacting" with the gelatine used in sizing but I tried the same test without sizing the paper. I though that whatever color I would see immediatly after the exposure would dissapear in the water but surprise, this dam dichro stain had a mind of its own and just stayed there on the paper. Now, I'm thinking there must be something in the paper that "reacts" with the dichro??? When I received Katharine message and went to see her own experiment made with gum and dichro I kind of said to myself what if there was only dichro on mylar or some other material that would most likely not "react" with the dichro??? So I tried a bit of dichro on glass and guess what??? If you think the dichro stayed on the glass, well it didn't. I also tried a couple more things like putting some dichro on paper, let it dry after and try to wash it off, yes it worked fine and as far as I can see I got a clean paper back. I even tried to force out the stains out of previously exposed dichro+paper using only water but contrary to unexposed dichro+paper I was left with what I call a 'ghost image' showing clearly the exposed area but without significant color, something like Katharine blue grey but definitively not paper white.

Why does it stay on some material and not on others??? I don't know for sure, I don't know either if it's a chemical or a physical "reaction" though I'm 100% sure there is a portion of what happen that is physical. I suspect the UV light causes a physical change in the structure of the dichromate, the details of which I'll leave to more knowledgable people. These changes must do at least 2 things at the same time if not more. One of these can be associated with the staining effect and the other can be associated with the conversion of soluble gum into clear insoluble gum. By the way, this 'insolubilisation' can happen with many different materials.

>From the experiment I did I would suspect some part of the dichromate (colored) byproduct after exposure must have some mean to stick on porous materials but it seems this stuff remain soluble in hot water and washes off relatively fast and the other near clear part either become insoluble and leave this 'ghost image' I saw or it as a stronger link with the paper that water alone can not break.

Regards
Yves

  ----- Original Message -----
  From: Ender100@aol.com
  To: alt-photo-process-l@sask.usask.ca
  Sent: Tuesday, December 06, 2005 1:39 AM
  Subject: Re: (Gum) Tonal scale

  Katharine,

  I think I missed a post..... so what caused the Dichromate Stain?

  Mark Nelson
  www.PrecisioinDigitalNegatives.com

  In a message dated 12/5/05 5:02:33 PM, kthayer@pacifier.com writes:

    I accidently sent this to myself instead of to the list, (still
    figuring out my new mail program) so I'm sending it again, to you.

    On Dec 3, 2005, at 1:00 PM, Katharine Thayer wrote:

>
> After I wrote that about dichromate stain, I remembered that I have a
> page showing the color of unstained gum, of dichromate-stained, and of
> cleared stained gum, on my site; here it is if anyone is interested:
>
> http://www.pacifier.com/~kthayer/html/gumcolors.html
>
> The samples don't show different tones, because it was gum that was
> exposed without a step tablet or any kind of negative, just hardened
> gum. that's when I was working on that project of making gum for
> chemical analysis.
> kt
>
Received on Tue Dec 6 20:25:11 2005