Re: PVA vs. PVA

Anh.... I think the guy was just confused. That's why I don't bother
reading those old guys, because most of them didn't know what the heck
they were talking about.

Have you ever seen gum arabic clot when dichromate is added to it? I
sure haven't. So the idea that gum becomes insoluble when dichromate is
added to it and then becomes soluble and then insoluble when the light
hits it is, well, gibberish. Gum mixes easily with dichromate; therefore
it cannot be said that it's insoluble when first mixed.

Katharine

Christina Z. Anderson wrote:
>
> David, Katharine, et al,.
>
> Something clicked in my head with this thread, from a BJP article from
> 1918-19, where a Mr. Starnes presented his research on gum based on his
> premise that when gum and dichromate first came together, the gum becomes
> immediately insoluble, and then light made it soluble at first and then
> increasingly insoluble after (I know, I know, pretty outrageous).
>
> Starnes developed this whole idea based on the opinion that he would get a
> reversal of image at times. I know both Judy and I and probably others have
> noticed this in step wedges. I've seen it strongly in a gum with added
> lemon juice to make it more acid when I was testing how acid relates to
> speed of gum. It resembled more stain to me than exposure, though, because
> there would be clear areas in the middle of the highlights of the step wedge
> and then increasing pigment the closer to step 21 the steps became.
>
> Thus when I first read Starnes' explanation and process, I dismissed it as a
> guy having problem with stain with a too acid gum and trying to explain it
> away with the wrong excuses. I filed it in my "weird gum story" file.
>
> However, now when I read that both Katharine and David have seen a lump of
> PVA happen in dichromate and not water, it makes me rethink my doubts :).
>
> Because Starnes was such a "flash in the pan", though--his advice having
> appeared and died, I'm not convinced it is to be trusted; however, who
> knows--maybe there is some truth lurking in there.
>
> He advocated using gum senegal instead of arabic, and his final formula
> included added hydrochloric acid and alum to his mix. No wonder he was
> confused.
> Chris
> ----- Original Message -----
> From: <davidhatton@superonline.com>
> In my eternal quest to distill art from the depths of space between my ears
> and apply it ( the art) to paper I have determined the following:
> 1)
> PVA - white but dries clear - soluble in water - used as a waterproofing
> agent and plasticizer in cement etc..Does indeed work as a size applied once
> diluted 1 - 12 with bottled water. When dry it is insoluble in water.
> 2)
> PVA - trans/lucent/parent and sold as 'gum' - mixed 1 - 3 with bottled
> water - initially rejects Potted dick solution, clotting with the pigment.
> After leaving to stand for a couple of days at room temperature, PD solution
> is not rejected and mixes well with pigment. It is very fast exposure wise
> and is soluble in water unless hardened. It also renders nice tonal images
> from diginegs and shows superb detail (if required of course).
Received on Fri Aug 5 12:28:43 2005