From: Ryuji Suzuki (RSuzuki@MIT.EDU)
Date: 02/22/03-03:37:22 AM Z
From: John Glen <microcrystals2002@yahoo.com>
Subject: Re: Glass plates setting temp and health points... (rather long/2short)
Date: Sat, 22 Feb 2003 04:18:52 -0800 (PST)
> What are you talking about here?
>
> Maybe I am miss reading something, but what is it with
> this NOT "gelated" at a low temperature." typically,
> Gelatin will "gelate" (gel or set) at room
> temperature, or upon slight cooling.
Gelatin in dispersion form at typical coating temperature is in sol
form. Sol form and gel form differ in physical arrangement of gelatine
constituents. Gel form is more regular and gives much better physical
strength, as well as better hardening response when hardener is
used. Sol form is softer and does not respond to hardener as nicely.
In order to get the gelain to become gel form (gelation) you need to
bring the material to a temperature lower than a gelating temperature,
which is determined by viscosity, Bloom, etc.
Above this temperature, coated material will dry without adequate
gelating. Physical strength will be inferior, though it may be enough
if you handle very carefully. It is for this reason I'm considering to
have a refrigerator - to gel the emulsion before drying. Gel and sol
are not necessarily black and white. If you dry at a marginal
temperature, you may get intermediate result.
Thus gelation is not synonymous to setting or drying.
> Commercially, with large amounts to coat, the coating
> speed is rather fast and cooling (quick setting) is
> essential... in such cases the exact temperature mmay
> indeed affect dry "Scratch Strength" or what have you,
> but IMHO the small scale experimenter has nothing to
> fear... the results will be OK.
One partial reason modern films are said to be "harder" is because
coating is done at higher viscosity and gelated at a lower temperature
than old films. I'm simply trying to get the benefit of good gelation.
(Of course, modern films use hardener to allow processing at higher
temperature, but hardener itself doesn't make emulsion hard when it's
dry. Note again hardener does not significantly influence the
mechanical strength of gelatin layer when it is dry.)
> In any case the gelatin solution will "gel" without a
> specfic low temperature "setting" period. In fact I
> doubt that a gelatin solution can be dried without it
> setting...
You can dry gelatin in mostly sol form, which is easier to get damaged
compared to geled material. This is rather easy to experiment - coat
pieces of glass with plain gelatin and gelate one and don't gelate
another. Then dry them.
> (well I guess if you drove off all the
> moisture you could... but that in itself would be
> abnormal! not something you are likely to choose to
> do.)
If you dry the emulsion well above gelating temperature, this is what
you'll get.
> On the otherhand, if you are talking about the special
> non-gelling gelatins, well... I hope you are not.
I'm talking about inert, demineralized, type B photographic gelatin.
> I understand you correctly... you just won't be doing
> it at a particulary low temperature... most likely,
> you will be doing it at night (wild guess)when the
> temperature will be a bit cooler anyway.
Even at night, the temperature in summertime can be as high as 30C.
In this season, my darkroom is heated to 20C. Either way, I need lower
temp to gelate my test gelatin dispersion. If you are coating less
viscous emulsion, you'll need a lower temperature to gelate compared
to more viscous emulsion.
All these things are found in literature, for example, gelatin chapter
of Mees and James, and also in Keller. They don't give actual numbers
for temperature of gelation, because it varies a lot with gelatin and
dispersion viscosity. On this part, I'm supplementing with info from a
guy at Eastman Gelatine and also some testing with a gelatin sample
from them.
> Oh, and while I am here, s.o. asked if you or some one could use
> denatured alcohol, for glass plates... well, it depends on the
> reason, but in general, yes, denatured alcohol is usually ok, I
> belive it is usully denatured with methanol or perhaps phenol, and
> in either of these two cases it should be fine. (Don't Drink / Don't
> Breathe);-)
It's not me. I don't know why you need alcohol to make emulsion for
plates... Or maybe you're talking about degreasing?
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