From: Sandy King (sanking@CLEMSON.EDU)
Date: 11/11/02-01:35:11 PM Z
By analysis of formulas from different sources it is certain that the
actual thickness of carbon tissue varied considerably, from perhaps
as little as .2mm to as much as slightly over 1mm. In my own work I
have settled on a thickness of approximately 1mm, which in my
experience represents a good compromise between the easier working
properties of thin tissue and the greater relief of thicker ones.
However, with regard to the latter question it needs to be noted that
the surface qualities of the final support have as much, if not more,
impact on the appearance of relief than on the actual dry thickness
of the image.
With regard to the difficulties of working with thick tissues I have
found that significant mechanical difficulties are introduced merely
by doubling tissue thickness form 1mm to 2mm. The nature of some of
these problems, i.e. greater drying times and the difficulty of
working with a heavier tissue, would appear obvious. Others are less
obvious and rather unexpected, for example, the problem of early
spontaneous hardening of sensitized tissue. I can certainly imagine
from my difficulties in working with 2mm thick tissues that going to
4mm would impose a very rigid system of work to make this all come
out as it should.
>Some remarks about the real difficulty to make a Woodburytype, just to
>complete the explanations given by Sandy:
>1) The "WET or DRY thickness".
>Nowhere in the technical books I have, the thickness of the resist is given.
>Why? Because it can vary, as said by Sandy, for many reasons. At the end of
>nineteen century, each person in charge of making the final printing on
>definitive support had several hot gelatine containers with various pigment
>percentage, just to accomodate this difference of mold depth. That's why
>because it was difficult to control the thickness of the initial resist.
>The carbon resist used to make the mold seems similar to the carbon print
>resist but is significantly THICKER:
>* let's assume a carbon print has a max thickness about .1 mm when DRY.
>* We can admit that the Wooburytype has a similar thickness
>* Therefore the original carbon print must be .1 mm thick when DRY, that is
>to say must be 1 mm thick when WET (I suppose the case of a soft gelatin
>capable to swell ten times in water)
>Maybe these figures are not totally exact but the range of magnitude is
>right: just to underline that the initial resist must be VERY THICK which is
>not usual for the carbon print. This technics is rather in the phototypie
>Why a such thickness is necessary? Just to limit the effects of the flatness
>of the various supports used (steel intermediate support and final paper
>support), each flaw being exactly printed. Less thick the mould, more
>pigmented the gelatine, more apparent the flaws.
>To obtain this thickness, it is necessary to:
>* put the dichromate directly in the hot gelatine bath (as for phototypie)
>* determine the right quantity of pigment to control the right hardening
>depth during exposure. Not easy. Requiring experiments.
>* get this very thick gelatine layer (4mm when WET are recommanded in my
>books!). Paper (or collodion) is not suitable as support, due to huge
>curling problems. The solution I use is to replace paper by silk screen to
>support this gelatine layer: able to symmetrically dry on the two faces,
>this choice limits curling.
>* dry successfully the layer and keep it in the adequate atmosphere.
>* determine the right exposure time
>* transfer on a perfectly flat and polished hard metal board and develop in
>* let the resist evenly drying. The complete drying time is very long and
>variable (depending on the temperature and environmental humidity). During
>this drying operation, complex dark reaction phenomenums begin to occur,
>making very variable the sensitivity of the gelatine. So the initial resist
>thickness can vary and therefore the mold depth. That is why it was (or is)
>necessary to use several pigments ratio cantainers for the final printing.
>2) After this preliminary operation, the 'molding'operation may be
>considered from several points of view:
>* If one wants to be in accordance with the Woodbury spirit, use lead and
>hydraulic press to make the mold.
>* If not, use a soft molding process as suggested in the mails: an adequate
>silicone or epoxy compound will work well. Maybe could it be possible to do
>this 'soft' molding operation when the resist is still WET, so limiting the
>thickness necessary for the initial resist!
>3) Once the mold made, the printing can take place under a low pressure
>device but you will have to use a perfectly flat paper as an image support
>because all flaws of flatness will be visible on the print.
>My conclusion on this process
>Due to a not perfect molding process and a not perfect printing, the
>Woodburytype is less precise than the carbon process and dedicated only for
>low cost (high) production purpose. So if one wants to make a woodburytype,
>it is only to perpetuate an historical process: interesting! But if one
>wants to be very faithfull to history, one has also to use the nineteen
>century available materials which today is a challenge as far as the
>perfecly flat materials (metal or paper) are concerned.
>I agree with Sandy: for a limited edition, it is better and less uncertain
>to use the classical carbon process; the 3D effect is only the result of a
>big resist thickness + a low pigment ratio in the gelatine. Other advantage:
>the process is not sensitive to the paper flaws.
>About precision or imprecision of a Woodburytype, many causes can degrade
>* The operation under hydraulic press is very delicate, due also to the
>problem of air bubbles likely to be trapped in the details of the image.
>* During the final printing, it depends also on the time between
>'thickening' and mold opening time. An other problem: the mold has to be
>greased between two printings: the grease has a tendancy to fill the
>'details' of the print, 'blurring' the print. Anyway, as written by Dick, it
>is visible only under a magnifier. Maybe the epoxy molding process could
>give better results than the historical process. Who knows?
>For 5 years, I have had a small hydraulic press in my garage. My dream is to
>be able to devote one year to experiment the Woodburytype process, but...
>life is short and I have so many things to do before! I must admit that my
>actual experience is very short: I just began to carefully study all my old
>books and to make gelatine layer. Perhaps some errors!?
>Sorry for this long mail: difficult for me "de faire court dans mon mauvais
>(Today, 11th november 1918, date of the end of First World War, a moment to
>think to all people killed during this war, and specially to American
>Soldiers, a lot of Sammies sleeping in battle field cemeteries in the north
>2 allée des Pervenches
>33 1 69 41 09 48
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