Re: Autoclaving gum and gelatin

On Sat, 5 Feb 2005, Sandy King wrote:

> Well, to be perfectly fair the only reason I "corrected" you was because you
> were simply way wrong about the temperature. The plain fact of the matter is
> that gelatin can be heated to a much higher temperature than 140º F and still
> retain its setting properties. In mixing gelatin solutions for making carbon
> tissue some of us are heating the gelatin to close to 200º F, for a short
> period of time, without experiencing any problems with setting properties

I'm glad, Sandy, that you're TRYING to be "perfectly fair," but...

Did I say anything about setting properties? I usually say 140 degrees
because first, it's quite hot to the touch, but not a bad scalding, so you
can test it by hand -- and second, it's safe. That is, there's a margin of
error. Also, third, it's the figure in "the literature" so less confusing.

I daresay you can go higher and blah blah blah blah, but I know from
experience -- from having seen dozens of students suddenly have speckles
all over their prints, -- that it can be lethal for gum printing, which
was the process at issue, remember?

Tracing the print history back, there was (in the case of speckles) just
about every time, the finding that they had developed the print by hosing
with scalding water or overheated the gelatin in melting it, or some other
excess application of heat.

On the bright side, one student who'd done this recoated with "normal"
gelatin and was able to rescue several sheets of paper. Still,

1. Why waste electricity to heat gelatin to the danger point (for gum!)?

and

2. Of course what the books say is no proof of anything, since as we know
there are many shibboleths in the books, like our favorite, the great
gum-pigment-ratio test, that are wrong. Also the one about step on a crack
you break your mother's back. But the part about 140 degrees getting into
the danger range is quite universally stated in "the literature." And,
like I said above, in this case I think it's a good rule of, so to speak,
thumb.

> Granted, there must be some temperature so hot that it will break down the
> gelatin within a short period of time, but 140º F is not even a ballpark
> figure for that happening.

Sandy -- don't extrapolate from carbon printing. The gelatin function is
NOT the same.

Judy
Received on Sun Feb 6 00:52:25 2005