Re: Image formation in gum

From: Katharine Thayer ^lt;>
Date: 12/20/05-01:30:01 PM Z
Message-id: <>

On Dec 19, 2005, at 1:25 PM, Yves Gauvreau wrote:

> Katharine,
> would you be so kind to tell if I read your message correctly???

Yes. :--)

> I think you said that you coated a piece of glass with a mixture of
> lamp
> black, gum and (I presume) some dichromate.


> You exposed this mixture under a
> step tablet.


> When this was done you coated the other half of the glass, let
> it dry and then you proceeded to develop the whole thing.

I did this once, and once I coated the other half of the glass after
developing and drying the exposed dichromated gum; I got the same
results either way. But I should add that I never "let" a coating dry
by itself; in my humid climate I would certainly get a fogged emulsion
  if I left the coating for very long before exposing. I dry my
coatings, always, immediately and very quickly, with a hair dryer. And
to show you how complicated gum printing can be, there are others who
invariably get fog if they use a hair dryer, while I never have, and
I'm on my third hair dryer now, so it couldn't be just something about
one particular hair dryer.

And to perhaps overemphasize, this pigment stain is not a result I get
printing lamp black, or any other color, on glass or on paper if the
pigment doesn't overload the gum's capability to hold the pigment in
suspension. When the pigment is held easily within the gum, it goes
off in the water along with the gum, leaving the glass (or paper)
completely clean. Its only when the coating is over-pigmented that
these results happen, IME.

> If I understand, you did this to see if some "pigment stain" would be
> left
> on the glass even if it wasn't exposed at all (one half at least) just
> like
> it did on paper.

Yes, my point here was that using an overpigmented emulsion, I found
that I got the same what I would call "pigment stain" that is, pigment
left behind after the gum had left, on unexposed glass or paper, that I
got on the unexposed areas of exposed glass or sized paper. And on
unsized paper, I got the same overall pigment stain (in the traditional
sense) with the same overpigmented emulsion ) whether the paper was
exposed or not exposed. In other words, the same overpigmented emulsion
gave me overall permanent indelible pigment stain on unsized paper,
and "tonal inversion" on sized paper and on glass. Like I said in an
earlier post on the topic, it might look like different things, but to
my mind, these are just different cases of the more general phenomenon:
excess pigment that overloads gum's ability to hold it.

But a "normal" amount of pigment (an amount of pigment that prints as
dark as the pigment will print without overloading the gum) doesn't
have that effect; it clears every time; since it is held suspended
within the gum, it leaves with the gum whether the gum is hardened and
floats or flakes off, or is unhardened and simply dissolves in the
water. A coating that is not over-pigmented will not leave any pigment
residue behind.

> Is that basically it???
Yes, I think so.

> ----- Original Message -----
> From: "Katharine Thayer" <>
> To: <>
> Sent: Friday, December 16, 2005 4:59 PM
> Subject: Re: Image formation in gum
>> On Dec 16, 2005, at 8:32 AM, Katharine Thayer wrote:
>>> To my eyes, this shows even more clearly the pigment "stain" in the
>>> form of excess pigment unevenly deposited across the glass, lighter
>>> in some places and darker in others in an apparently
>>> non-image-related
>>> way. It also shows how the hardened gum ends neatly at the edges of
>>> the letters (you can see this especially on the T, U, and second F,
>>> you can clearly see the hardened gum pulling off along the edge of
>>> the
>>> letters. This is as one would expect, since the gum shouldn't have
>>> hardened under the letters, which are black on the negative, and the
>>> unhardened gum would have simply dissolved away from the letters
>>> themselves, leaving clear glass, letters spelled out in negative
>>> space, letters made of non-gum. The question of course is why the
>>> pigment deposits itself so nicely on the non-image areas. I surely
>>> don't know why that is, although I have a sort of theory, but in this
>>> case I think this image shows pretty clearly that the gum is not
>>> involved.
>> I've just done a series of tests with concentrated lamp black on
>> glass,
>> to answer a question I was going to ask Tom but decided, taking Judy's
>> advice, that I could just as well discover the answer for myself.
>> From my own experience, I know that an image printed in gum on
>> glass ,
>> once dried, is not easy to remove; it requires a sharp razor blade
>> applied with effort behind it, and sometimes more than one blade per
>> image. So it seemed to me that this would be a perfect way to check to
>> see whether this tonal reversal is made of hardened gum or just
>> pigment. Because if this reverse image is really just pigment, as I
>> suspect, it should wipe off easily from glass; it would be like
>> wiping
>> soot off glass, because that's essentially what lamp black or carbon
>> black is, just fine charcoal or soot.
>> I did several prints and watched the gum float off, watching very
>> carefully to make sure the gum was floating off the entire glass
>> including the image areas. In each case, there was a step tablet
>> print
>> left behind, after the gum was completely gone, much like Tom's,
>> with
>> the heading printed in black (reverse) but the step numbers also
>> printed in black, (which is the right way, so the inversion wasn't
>> complete) also a general pigment tone over the entire tablet rather
>> than actual steps, and also a general pigment tone (what I would call
>> "pigment stain") over the entire area where the gum had been coated. I
>> also coated the other half of the glass, after making the exposure for
>> the "treatment condition" dried it and washed both at the same time,
>> to see if I got the same pigment tone left from the unexposed coating
>> as I did from the exposed coating, just as I had with paper. I did.
>> What's more, in each case, the image that stayed after the gum left
>> was
>> indeed simply made of pigment rather than gum, because after being
>> dried it wiped easily off the glass with a tissue. So that was the
>> answer to my question, and now I am quite satisfied that this tonal
>> reversal is indeed just pigment, no matter what fancy theories you
>> guys
>> come up with for it. But I also think it's not just an either-or
>> thing, it's probably a continuum, so there are probably intermediary
>> steps where there is some gum involved. I don't know, I'm just
>> guessing.
>> Unfortunately these tests are all wiped off so I can't scan them to
>> show them to you, and you'll just have to take my word for it, unless
>> I
>> have the energy to make some more to scan later.
>> Katharine
Received on Tue Dec 20 13:41:02 2005

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