Re: Gum, newbie questions

Hi Yves,
I feel your frustration, but iIt might be easier to help if you could
simply describe the results you're getting, in as much detail as
possible, then we could address the problem more specifically.

I've intended, since I put up my website three (?) years ago, to add a
troubleshooting page showing visual illustrations of specific kinds of
failures and the solutions most likely, IME, to cure the problem, but I
haven't had time to do this yet.

In the meantime, let's address the problem of the emulsion floating
off, which is one specific problem you raised. If the emulsion
"floats off" as a film, in other words if there in fact is a hardened
gum layer that leaves the paper and floats off, either as a lovely
intact veil of colored film wafting hearbreakingly through the water,
or as a lot of shards or flakes of colored film, then the problem could
be too much sizing (although from your sizing question earlier, I'm not
sure you're sizing at all, in which case this wouldn't be your
problem) or too slick a surface (this happens for me a lot with yupo or
glass)--- or too thick a coating. But if the emulsion washes away as
a liquid, in other words if you see color going into the water like dye
rather than like a veil of film, leaving no image on the paper, then
the problem is almost certainly simply underexposure: the exposure
hasn't been sufficient to create a layer of hardened gum at all.

As to whether the upper portion of the emulsion hardens first and can
be washed away by the insoluble gum underneath, this is one of the many
controversial issues in gum. One alt photo personage has a theory that
dichromate ions gravitate to the paper, and therefore hardening
actually takes place mostly at the paper surface and below, rather than
higher in the emulsion. I have some questions about that theory, but
won't go into them here, and besides, without some kind of microscopic
analysis, it's impossible to know which is right, so it's one of those
discussions that don't yield any practical help for the beginning gum
printer.

While I think your reasoning is sound, theoretically, about the role of
the thickness of the emulsion interacting with the opacity or
transparency of the pigment, I'm not sure that's a really fruitful path
to take in terms of getting quickly to good gum prints; at any rate it
hasn't been my experience that practically it makes enough difference
to worry about. I guess it depends on what you want. I think the
answer to this question would be useful to gum theoreticians, if there
indeed be such folks, but I'm not sure it would be the fastest way to
get yourself to printing gum, if that's what you want to do most.

Not sure you'll find any of this helpful, but it's what comes to mind.
At the moment, I'm very tired after spending the day out shooting, so
might have some fresher thoughts in the morning.
Katharine

On Nov 17, 2005, at 9:59 AM, Yves Gauvreau wrote:

> Hi everyone,
>  
> I read a lot of stuff on gum printing, here and elsewhere and
> yesterday I tried a few things (my first steps) to see how it goes. As
> I expected the results where so so, well very so so if you see what I
> mean.
>  
> I understand pretty well the principles that a dichromate when expose
> to light (UV) in the presence of a coloid as the effect of rendering
> the exposed area more or less insoluble to water and thus any pigments
> or whatever else is mix with the coloids get's trapped inside those
> insoluble areas. In other words, there is no magic in this phenomenon.
>  
> Now, that's very nice but intuitively I'm quite certain there are many
> if not gazillions little details that can make a huge difference. For
> exemple lets consider the following 3 of those little details, the
> tickness of the emultion, the pigment density or pigment/gum ratio and
> the transparency or lack of it of the pigments.
>  
> The tickness of the emultion which as at least 2 properties that we
> must consider. First its physical dimension (tickness) implies that
> the exposing light as some distance to travel until it gets all the
> way to the paper. If the light doesn't get to the paper it will leave
> a space between the paper and any exposed emultion above it and at
> development time, water will get into this space and the insolubles
> above it will be carried out in the flow thus leaving us with an area
> with no pigment where there should be some.
>  
> In a way the second property as basically the same effect as the above
> with the following difference, the relative opacity or tranparency,
> whichever you prefer as what is called an inverse effect. A tick and
> very transparent emultion may produce similar results to a thin and
> opaque one.
>  
> Basically the pigment density and their relative transparency directly
> affect the tranparency of the emultion and what I've said above apply
> for these as well. But something tells me that this thickness and
> transparency stuff as bonderies beyond which the results will be
> useless as in my first try.
>  
> I think I saw this in one of Katherine text about her
> approach. Basically one must learn all this through experimentation
> with the material he/she is using and one must remember that each
> combination will produce varying results.
>  
> All this can take a considerable amount of time to figure out and I
> wonder if at least intuitively or by experience someone or some of
> you could guide me and others to the kind of knowledge that could help
> us kind of troubleshoot our own results. If I go back to what I said
> above about the emultion flotting away in some areas. I assume this
> could also be caused by insufficient exposure and possibly by a
> combination of exposure and "relative thickness" of the emultion. Say
> I see what most would call staining I think, what could be the cause
> of this. I'm sure you can understand what I mean but in the event
> you'd prefer to give your opinion, solution, etc. to specific question
> I or others may have, please let us know.
>  
> Maybe finding a proper working emultion could be arrived at by some
> systematic testing approach. Any hints on how to do this testing would
> be excellent as well and less demanding to you.
>  
>  
> Thanks in advance to all
> Yves
>  
>  
Received on Thu Nov 17 21:23:17 2005