>On Mon, 6 Jan 1997, Luis Nadeau wrote:
>> Printing
>> When after drying? If you hot dry a print and then wait hours to
>> expose your print you can assume that the paper fibers are now back
>> to normal RH.
>
>
>Luis, I'd expect the *paper* would be back to "normal relative humidity,"
>but the *emulsion* is not necessarily returned to the status quo ante. At
>least I've always assumed that changes induced in the emulsion by heat
>were probably permanent...
Certainly not. I don't know of a single photographic process that is
perfectly stable before exposure and development. Photographic
manufacturers have spent hundreds of millions to come up with "stable"
plain vanilla silver gelatin emulsions for over 100 years. From accounts I
have d, treahis became apparent in the 1930s and 40s when three-color
separation processes became relatively popular. Speed and contrast
variations, back in the monochrome days, went largely unnoticed. When
tricolor carbro became serious business, workers found it necessary,
especially in the hot summer months, to have their Rochester emulsions
shipped in ice cream trucks to New York and other centers. They had to be
used asap for consistent, predictable results. Both the film and the ice
cream;-)
For years manufacturers have recommended to have their "pro" emulsions kept
in the refrigerator prior to exposure, and processing had to follow asap.
With black and white papers, manufacturers have always had a helluva time
manufacturing very high contrast paper, e.g., Agfa Grade#6, with a decent
shelf life. Both Agfa and Ilford were fairly successful, but Kodak was not,
at least with that kind of product.
Now back to Alt-photo processes: the most unstable are the dichromated
processes. Their sensitivity changes drastically over a period of a few
hours. There is a fair amount of scientific and patent literature
recommending a variety of additives to make the emulsions more stable.
Inevitably, they also make the emulsions slower. Over the years I have run
into research papers, patents, etc., that cover various improvements to
make blueprint papers more stable before processing.
One should always keep in mind that any temperature above Absolute zero, is
"heat". Therefore, normal room temperature, ca. 20C, is extremely hot,
relatively speaking. Chemical reactions accelerate with heat, so if you
want to keep an emulsion stable, cool it... In the conservation field, we
teach that for each increase of 10 degrees the rate of deterioration
doubles.
>and I have, rightly or wrongly, assumed it was
>effects on emulsion that changed "speed" etc. of the paper.
>
>For testing purposes, BTW, I do try to standardize drying time (1/2 hour
>for gum strips), but I use "available" RH, since I have a studio, not a
>NASA lab. In making an actual gum print, however, such variables are
>easily dealt with by varying temperature and/or time of development. ( Eat
>your heart out, platinumists!)
and carbonists...
With gum, you can get away with murder. The secret to success as a rule,
goes like this.
1. Make notes of everything.
2. Try to change only one variable at a time.
3. Practice a lot, i.e., make lots of small prints on a regular basis,
until you feel very comfortable with all variables.
In practice, most people find it difficult to follow No 2. This is
especially true with the more difficult processes, e.g., carbon transfer.
Luis Nadeau
nadeaul@nbnet.nb.ca
Fredericton, New Brunswick, Canada
http://www.micronet.fr/~deriencg/nadeau.html
http://www.primenet.com/~dbarto/lnadeau.html
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