Re: Rethinking pigment stain

On Dec 14, 2005, at 7:58 AM, Yves Gauvreau wrote:
>
> I know this doesn't give a satisfactory explanation as to what causes
> dichromate stains. Katherine as refered us to a PVA dichromate study
> from
> which she seems to infer that overexposure is the cause of these stain
> because an excessive amount of activated dichromate molecules is not
> bounded
> to colloid molecules (in this study all colloids where made insoluble
> thus
> where bounded with dichro).

Yves, maybe you didn't get the message where I clarified that in the
particular case of the dichromate stain that I showed you, I
deliberately caused the dichromate stain by overexposure in the sun,
because I know from practical experience that gross overexposure is the
best way I know to reliably create dichromate stain. But I certainly
wasn't suggesting that overexposure is the only way to create
dichromate stain, and in fact there have been many lively discussions
here with many people reporting their own observations about dichromate
stain and many theories advanced. As far as I'm concerned, there may be
many possible different causes for many different cases of dichromate
stain.

I wouldn't quite agree with your characterisation of Duncalf & Dunn,
but that's irrelevant to this discussion). What happens to the excess
reduced chromium is a question both beyond the scope of Duncalf &
Dunn's research and the state of current knowledge about the process,
as far as I know, so I wouldn't want to be misinterpreted as having an
opinion about that finer theoretical point one way or the other; all
I'm saying is that in my practical experience, for whatever reason,
excess exposure simply produces dichromate stain and then more
dichromate stain, without adding any particular benefit, and Duncalf &
Dunn's observation that dichromate continues to be reduced after the
colloid has become insoluble seems to be in concert with my personal
observation. But I wouldn't make any more of it than that, because I
don't think we know enough about it to make any more of it than that.
Katharine

> I suppose one could say just increase the number
> of available colloid molecules such that you don't have a surplus of
> dichromate. I'd be curious to here ear from those who suggest to use a
> 2:1
> gum / dichro ratio, about their dichromate stain??? Assuming this
> simple
> adjustment could resolve the dichromate stain problem, then the
> exposure
> scale could be largely extented without causing those infamous stains
> and
> leave us with just beautiful clear gum and some pigment of course but I
> don't know if this is true so lets forget about it here.
>
>
>
> ----- Original Message -----
> From: "Tom Sobota" <tsobota@teleline.es>
> To: <alt-photo-process-l@sask.usask.ca>
> Sent: Wednesday, December 14, 2005 5:55 AM
> Subject: Re: Rethinking pigment stain
>
>
>> It all depends on how you define 'fog'. To me, fog is a global
>> hardening or insolubilization of the gum-dichromate layer which
>> does not contribute to the formation of an image.
>>
>> It can be produced by any (or a sum of) several causes, among which
>> accidental exposure to light too, of course. Others are heat, exposure
>> to chemicals, and the simple passing of time. Surely others.
>>
>> The normal exposure to light of a layer of dichromated gum under
>> a negative produces a hardening which is not physically different
>> from a fog-producing exposure to light, but it produces an usable
>> image
>> since it is not global but rather differential. So we don't call it
>> fog.
>>
>> This loose definition of fog could be applied to silver-gelatin
>> emulsions
>> too, actually.
>>
>> Pigment stain is different, and perhaps unique to gum, carbon and
>> other processes using finely ground particles of pigment.
>>
>> But dichromate stain has a sort-of analogue in silver-gelatin
>> emulsions
>> when you develop with pyrogallol or pyrocatechol, for example, which
>> also produce stain. This stain contributes to the image, however,
>> because
>> it is in the negative and not in the positive.
>>
>> If you want to see fog in gum, coat a paper and keep it in the dark
>> for a
>> week or two in a warm place. Then develop side by side with a
>> just-prepared emulsion. The relative darkening of the older coat
>> should
>> be evident.
>>
>> Tom Sobota
>> Madrid, Spain
>>
>>
>> At 03:38 14/12/2005, you wrote:
>>> I don't know all the details regarding that past discussion, but I
>>> think
>>> "fog" is a misleading term. I consider fog to be accidental
>>> exposure,
> e.g.
>>> someone opens the back of a 35mm camera and the film gets "fogged".
> Perhaps
>>> gum paper can get fogged from ambient light levels being too high in
>>> the
>>> darkroom, but I've never had that problem.
>>>
>>> Dave Rose
>>> Powell, Wyoming
>>>
>>> ----- Original Message -----
>>> From: "Katharine Thayer" <kthayer@pacifier.com>
>>> To: "alt photo" <alt-photo-process-l@sask.usask.ca>
>>> Sent: Tuesday, December 13, 2005 12:16 PM
>>> Subject: Rethinking pigment stain
>>>
>>>
>>>> Hi gum printers,
>>>> Because for most of my gum printing career I've printed on unsized
>>>> paper, I developed a definition of pigment stain that I realize now
>>>> probably applies mainly, if not exclusively, to printing on unsized
>>>> paper. My definition required that in order for something to be
> called
>>>> pigment stain, the pigment had to penetrate the paper and be
> indelible.
>>>> And in my experience, when pigment stain occurred, it occurred
>>>> immediately when the coating was applied; if the pigment was
> excessive
>>>> in relation to the gum, it would stain the paper immediately on
>>>> application.
>>>>
>>>> The tonal inversion thing made me rethink that idea, as I said a
> couple
>>>> of days ago, and after doing some experiments with sized and unsized
>>>> paper, I've decided I need a more inclusive definition that
>>>> incorporates what happens on sized paper, or maybe two different
> terms;
>>>> I haven't decided yet for sure.
>>>>
>>>> A more inclusive definition for "pigment stain" would say that
>>>> pigment
>>>> stain is whenever you get pigment in places where it shouldn't be,
>>>> such as in unexposed areas of an image or step print. Whether or
>>>> not
>>>> that out-of-place pigment forms an indelible *stain* will be a
> function
>>>> of how well the paper is sized. On sized paper, this "stain" will
> wipe
>>>> off easily, whereas on unsized paper it will be indelible, but in
>>>> either case, you've got pigment you don't want in areas that should
>>>> be
>>>> very light or paper white, hence: stain.
>>>>
>>>> One problem with this more inclusive definition is that it doesn't
>>>> distinguish between stain and fog. Someone referred recently to a
>>>> discussion from last summer where Mark showed a gum test print where
>>>> there was color on areas where the print should have been paper
>>>> white.
>>>> I called that stain, and was told that it was fog. I conceded the
>>>> point; when told that it could be wiped off the paper I assumed
>>>> (given
>>>> my then understanding of stain) that it couldn't possibly be stain
>>>> and
>>>> must be fog, although I didn't have a clear understanding of what
> could
>>>> have caused the fog. And when that was brought up recently, I
>>>> acknowledged I'd been wrong when I'd called it stain. But now that
>>>> I
>>>> have seen for myself that pigment stain can also be easily wiped off
>>>> sized paper, (while still wet, of course) I'm not sure I know how to
>>>> tell the difference between stain and fog on sized paper.
>>>>
>>>> They are of course different in substance, because what I would call
>>>> "pigment stain" is just pigment, since it occurs in areas where no
>>>> exposure, and therefore no formation of crosslinked gum, has
>>>> occurred,
>>>> whereas fog, in my opinion, would involve the formation of
>>>> crosslinked
>>>> gum.
>>>>
>>>> On unsized paper, excess pigment impregnates the paper as stain,
>>>> and
>>>> that's why it stays with the paper rather than dissolving away with
> the
>>>> dichromate and soluble gum from unexposed areas. But on sized paper,
>>>> even though the pigment isn't held in the paper as stain, or in
>>>> crosslinked gum as "tone" it still remains on the paper in
>>>> unexposed
>>>> areas, as seen in the examples of "tonal inversion." This is
>>>> interesting, but puzzling, to me. At any rate, I've satisfied
>>>> myself,
>>>> by cutting coated papers in half and exposing one side and putting
>>>> the
>>>> other side directly into water, that the "pigment stain" is the
>>>> same
>>>> on unexposed areas of exposed coatings as it is on completely
> unexposed
>>>> paper, whether sized or unsized, which makes me even more confident
>>>> that the effect has nothing to do with exposure, heat or anything
> else
>>>> related to the exposure itself, but is simply pigment stain.
>>>>
>>>> Thoughts, anyone? I will soon be revising my page on stain, lord
>>>> willing and the creek don't rise, to reflect the evolution of my
>>>> thinking on this topic.
>>>> Katharine
>>>>
>>>>
>>
>
Received on Wed Dec 14 12:04:52 2005