From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 11/03/01-02:24:03 PM Z
david distefano wrote:
> ... totally dry
> the coating and then re humidify it over a humidifier
> for 2 to 3 min. If so, is the humidity in the room as critical.
The key is to have a stable and known humidity in the paper. If a piece
of humidified paper (5 inch by 5 inch size) is placed on a scale
(accurate to .01 gram) in a dry room or visa versa, a change in weight
in the paper can be observed as the humidity of the paper changes to
that of the room.
One may be able to set the humidity of the paper and place into a vapor
proof contact frame quick enough, but my experience has shown the
transfer of humidity to be rather rapid and a more reliable strategy to
set and control the RH of the room. The paper being in the room a while
and the coating being dried without heat and resting will assure the
paper is humidified according to the amount of humidity in the room.
Do be cautious with precoating Pt/Pd as the coating does start to go
"bad" in about an hour and definitely "bad" in 4 hours. Also be
cautious about too much humidity (or too high a room RH) as this can
result in coating sticking to the gelatin of the negative. A sheet of
thin Mylar film can help here.
Keep in mind that the temperature is very important with Pt/Pd coatings
and exposure, especially when working with high RH (60%). I have found
it best (with most consistent results) if the temperature is around 60F
and definitely below 70F. Inconsistencies are definite above 80F and
quality deteriorates above 90F. This is the temperature alone and can
effect exposing in low RH as well.
The temperature can also influence the RH. This is especially noticable
when the printing frame heats after several exposures resulting in brown
splotches throughout the print. The brown areas are where the RH has
been lowered by the heat. The lowering does not occure uniformily
unless enough time has passed for a steady state condition of the RH.
Also coating can be influenced by the RH and temperature and may even
require an adjustment in the amount of coating mixture or technique.
With the Pt/Pd process, it may be desirable to have a separate room for
coating and exposing which has accurate and adjustable temperature and
humidity control.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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