Re: Chrysotypes and humidity

["Jeffrey D. Mathias" <jeffrey.d.mathias@worldnet.att.net>]

Bor Breukel wrote:
> ... It seems that Chrysotypes are very sensitive to humidity ...
> ... Do people have experience with humidity chambers? ...
> I am wondering: how long would it take for a coated Chrysotype to
> reach this rel. hum. from say 80 to 35%?

The rate of change of RH seems to be determined mostly by the paper
used.  I have observed the weight change in a rather short period of
time as moisture is absorbed or released due to a differential in RH.

This rather quick change seemed to necessitate the control of the room
ambient rather than placing into a chamber and later removing to place
into the contact frame.  I have had good success with keeping a fairly
consistent RH in the paper by controlling the room temperature and RH. 
A smaller room for just coating and exposing would also help.

Temperature is very important and should ideally be kept below 20C and
definitely below 25C.  The RH can be controlled with dehumidifiers (also
produce heat so an air conditioner, cooling may be required) and sonic
misters.  Running both (as well as the AC) usually gives a more constant
result for mid range RH, however do expect the RH to be harder to
control than the temperature.  If a separt room is used for coating and
exposing, ventalation and exhaust of air may be reduced helping to
achieve more stable conditions.  However, I have still been able to get
good control in a larger dark room with constant venting, although I do
use a heat exchanger to help prevent the change of temperature.  For RH
I run two (when needed) dehumidifiers, and a sonic mister as the RH
cannot be recycled as the temperature.

And again, an increase in temperature is to be avoided as much as
practical, such as may occur during exposure.  Also, the contact frame
should have a vapor barrier to help prevent a change of RH.

-- 
Jeffrey D. Mathias
http://home.att.net/~jeffrey.d.mathias/