Dear List,I am looking for some alt-process listmembers from the 20th century :-) In the archive i found the thread below; but i do not have a valid e-mail address. If someone can give some info i would be very grateful. The one i am looking for is Don Stepka, but maybe Richard Knoppow could help me out. Thanks in advance. Henk
gelatin flexibility
Wayde Allen (wallen@boulder.nist.gov)
Thu, 03 Jun 1999 11:26:37 -0600 (MDT)
On Thu, 3 Jun 1999, Don Stepka wrote:
> One thing to note is that pigment tissue (at least, mine) has a gelatin
> layer an order of magnitude thicker than camera film, and the tendency of a
> gelatin layer to crack instead of bend seems to be strongly correlated with
> thickness.
Yes, I've noticed that too. I believe that this is due to the buildup of
internal stress in the film.
> Also, layers dried as gels are significantly harder to crack than layers
> dried as sols.
If you don't mind, I'd like a bit of clarification here. A Sol as I
understand it is a colloidal dispersion in a liquid. I presume that this
is in fact the correct description of a gelatin solution? Accordingly, my
dictionary says that a gel is: a jellylike substance formed by the
coagulation of a colloidal solution into a solid phase. What isn't clear
to me is how you could dry a gelatin Sol without it forming a gel?
> I'm curious why you are interested in greater plasticity -- if one were
> marketing rolls of tissue, it might be a virtue. However, for homemade
> tissue it seems irrelevant. Ever since I demonstrated to my own
> satisfaction that freshly-made tissue with dichromate incorporated works
> much better than any tissue I made or bought (in the good old days) and
> then sensitized, I've never been interested in making more than I'd use the
> next day (i.e., enough to wish that I could roll it up).
For relatively small sheets of carbon tissue this is fine. However, large
sheets tend to curl badly, and it becomes more difficult to unroll a large
section (say 50x61 cm) without cracking. I also seem to have some
problems with cracking if I cut a piece of my tissue. Humidity
is part of the problem since the brittleness seems to be very
dependent on the amount of water retained in the emulsion. Here in
Colorado the humidity is very low.
> Over the years I've used a number of additives in both tissue and
> silver-gelatin emulsions. Sugar seems to hold moisture (note that most
> 19th century photographers were using sugar that is more like our light
> brown sugar than our white granulated sugar, and much more hygroscopic than
> the latter). Honey does the same, only better.
Yes, and I believe that the primary reason for the addition of sugar is to
help the gelatin retain moisture. I simply have gotten curious though,
since it seems that too much sugar could cause problems as well.
> I've found (again, to my
> own satisfaction) that drying tissue in a controlled humidity (70-75% seems
> to work best),
That is amazingly high humidity for around here! The relative humidity
right now is 43 %, and we had a few rain showers last night!
then using it immediately, obviates the need for sugar or
> honey. The old literature mentions several types of gum, agar, water
> glass, milk, and albumen (that I can recall) to strengthen and/or
> plasticize tissue.
>
> Mees & James (3rd, 1966 p. 54) say that glycerine seems to work not just as
> a hygroscopic additive. They also say that "[p]lasticizers recently
> mentioned in the literature have generally been materials of higher
> molecular weight than glycerine. Many of them consist of short alkyl or
> alkylene groups, combined with polar linkages such as ether, ester, or
> amide." I've used PVA in silver-gelatin emulsions, but have not tried it
> in tissue.
This seems to be consistent with some of what I've recently been reading.
- Wayde
(wallen@boulder.nist.gov)
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