Re: Two methods of sensitizing

From: Katharine Thayer ^lt;>
Date: 04/04/04-06:38:40 AM Z
Message-id: <>

Hello everyone,
Several weeks ago I noticed this discussion, and out of curiosity I ran
a test and put the results up on a web page and wrote a post about it,
and then I didn't send it to the list because I didn't want to get
pulled into the back and forth that can be so time-consuming. But later
I got pulled into it anyway on another issue, and since this is a rather
important issue it seems like I might as well go ahead and say something
about it, since the page is out there and there's no point in wasting

On Sat, 13 Mar 2004 wrote:

> For Direct Carbon which resembles the gum process the sensitizer
> is mixed in with the colloid on coating. Am I correct in assuming that a
> pure gum + pigment coat cannot be sensitized after drying....the gum
> would be washed away by the dichromate? (Judy ??)

Judy replied:

"They say that John, just like they say a lot of other things. But not
You could make the dichromate solution very cold, so that by the time it
started to dissolve the gum it would be done -- before exposure anyway
it's not so serious if you have a little blurring; it dries solid

I wonder if this is one of those "theoretical" arguments or if you've
actually tried this. I did and it didn't work very well for me in spite
of a truly honest effort; see URL below.
and Christina also responded:

"Gum could be precoated with gum/pigment and then sensitized with
by floating or by brushing (brushing with methyl alcohol in place of
the water, too). This was talked about even until the late 30's."

The topic of the solubility of dried unhardened gum has been on my mind
a lot lately, what with the recent discussion about dissolving
unhardened gelatin and dissolving unhardened gum, and also given that a
couple weeks ago I made a visual for my web page on the mechanism of
the gum process (still in progess) demonstrating the solubility of
unhardened gum (I include this visual on the page refeenced below) and
what with my recent experience, chronicled here, of attempting to harden
gum for painting.

So I've thought and observed quite a lot regarding the solubility of
dried gum recently, and my answer to John's question above is an
unequivocal yes, John, you're exactly right, dried unhardened gum and
pigment would be washed away, or at least significantly eroded, by the
dichromate if you attempted to either brush dichromate solution over
dried gum or float the dried unhardened gum on dichromate solution.

To suggest otherwise seems a very curious suggestion to me, since the
solubility of dried gum is one of the fundamental principles underlying
the gum printing process itself, after all.

Some of those early guys tended to print with a great lot of pigment; I
wonder if perhaps their papers were so pigment-stained they didn't even
notice that most of the gum dissolved when they followed this practice.
The early guys said a lot of things that aren't terribly useful to
current practice, and at any rate, it's the gum itself, as I keep
saying, that teaches me what I need to know about how gum works, and the
gum tells me that no matter what early workers said, unhardened gum
dissolves when it encounters liquid.

And as far as that goes, am I the only one who has ever accidentally
flicked a drop of water onto a coated and dried sheet of paper? It makes
an instant hole in the coating; there's nothing to be done but to put
the sheet into water (unexposed) to dissolve the coating off and start

Here's the URL for my page on the question:

Katharine Thayer
Received on Sun Apr 4 13:35:02 2004

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