Looks like the process ain't dead yet...
..
>I use 2% dichromate (either ammonium or potassium) for gum and other
>colloidal printing (except carbon: change of dichromate strength changes
>contrast for carbon), including photo-silkscreen, and have always wondered
>why everyone else seems to use "saturated" solutions, which come to 13 to
>15%. My printing times are quite short, and see no reason to increase the
>strength of the sensitizer for speed (2 to 4 minutes with the NUARC).
This is a very good point, which has taken me quite sometime to understand.
I just assumed that everybody else had managed to figure it out for
themselves. My assumption was wrong. I will go at length about this in an
upcoming book, but for now, Here's the Reader's Digest version:
In the early days of photography, speed was everything. Early processes
took hours to produce a faint image of anything. Early daguerreotypes
required exposures of 20 to 30 minutes. Optical improvements, introduced by
Lerebours, in May 1840, reduced exposures to two minutes. Chemical
improvements by Claudet and others shortened exposures in bright sunlight
considerably. By 1859, nearly "instantaneous" photographs were produced. It
is not before the introduction of the gelatin dry plates in 1878 that
exposures of one tenth of a second or less became common.
At that point of course, people were using the negative-positive process,
where commercial production depended mostly on the amount of time required
to obtain a printing-out print (aka P.O.P.) by the then popular albumen
process. In London, thanks to the smog, there were days were no significant
commercial production of positive prints was possible. For this reason,
(among others, e.g., permanence) dichromated processes enjoyed a certain
popularity as you could produce prints on cloudy days, *if you used
saturated solutions of dichromate.*
This is why all early books on carbon printing recommended the use of
highly concentrated solutions.
BTW, some commercial printing establishments took advantage of the
"continuous effect" (not to be confused with "dark effect") where prints
were given very short exposures say, on Monday, and developed on Tuesday or
Wednesday. This had the same effect as giving them a long exposure with
immediate development.
Also, some organizations specialized in making *enlargements* on carbon paper.
>
>This relates to the thread a while back on the disposal of dichromate
>solutions - since the less you use, the less gets dumped down the sewer
>(and the less you have to pay for it). If someone has done controlled
>testing and found that more concentration of dichromate contributes to the
>process, I'd be very pleased to know.
>
>Would Luis be willing to share with us when he would use "a 1% sol. of
>ammonium dichromate, and a 4% sol. of potassium dichromate..." for gum, and
>the reason for the difference between the two? Thanks.
My comments referred to the previous comment which said that ammonium was
more concentrated than potassium. What was meant, I presumed, was that
ammonium *can be* more concentrated since it is more soluble. It does not
have to be.
In practice, the cheapest grade of potassium dichromate has always worked
for me. Speed is not an issue with modern platemakers. Of course, a
saturated solution of ammonium dichromate will give you less contrast than
a saturated solution of potassium dichromate, with carbon transfer, other
factors being equal.
Luis Nadeau
NADEAUL@NBNET.NB.CA
Fredericton, New Brunswick, Canada