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Re: platinum/palladium coating - exposing POP
Istvan Bibo wrote:
> I use ammonium ferric oxalate and lithium palladium chloride. I have the
> problem of having only one deep black contrasty print (that i really like)in
> one printing session, the others are browner and less contrasty. I was
> trying to figure out, what made the difference,because the conditions were
> kept the same.
> Now I am almost sure that it was the freshness of the solution that made the
> difference, because the unrepeatable black prints were the first ones in the
> sessions. ...
Perhaps. However do not be too sure that the conditions were the same.
Did you monitor the temperature of your contact printing frame?
I have found that after a couple prints the printing frame warms enough
to alter the color (toward warm) of the print usually in a non uniform
manner. This is likely due to a change in the RH (relative humidity of
the coating) from the higher temperature). The result would be as you
describe cooler color shifting toward warmer (and lighter blacks).
A solution is to let the contact printing frame cool between exposures.
If one has a lot of prints to make, they should consider getting a
second or third printing frame. They can be rotated so they have time
to cool. The temperature can be measured with a flat thermometer
(liquid crystal type) placed in the frame. I have noticed the glass to
be warmed to the touch. Another solution might be to build a
temperature controlled frame (water cooled or such).
Using the sun might produce more of a challenge as it will heat the
frame much more. (I have not used the sun for POP, but have done plenty
DOP with that light source.) Using UV lamps will produce enough heat to
cause this problem. This is dependent on the exposure times, number of
previous exposures, and rest between exposures. I found differences on
the third or fourth prints with exposures of 6 to 8 minutes. Keep in
mind that an exposure longer than 20 minutes may not be able to maintain
and control temperature or the proper RH.
I have found that in order to maintain consistency and get the coolest
color, the temperature should be kept below 70F and the RH at 70%.
Lower RH will warm the color. At higher RH, results will become
inconsistent. Higher temperature can be inconsistent and may produce
some other problems (especially over 85F, definitely over 140F which can
occur when using the sun). Lower temperature is OK.
--
Jeffrey D. Mathias
http://home.att.net/~jeffrey.d.mathias/