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Re: platinum/palladium coating
Eric Neilsen wrote:
> [I had written:]
> > Another important consideration is to apply the coating mixture
> > immediatly after mixing. Do not mix a coating solution and let it sit
> > around before coating as several things could go wrong.
> >
> I don't agree with this point. There are some mixtures that can sit for
> several hours and produce better prints.
Yes there are. However, some considerations are:
+ If the metal salts must be heated to keep them in solution, once
mixed with the other coating ingredients, they could (sometimes and more
likely if at higher concentrations) precipitate back out in a matter of
minutes.
+ I have noticed a few times the if a coating mixture sits for a while
(over an hour) sometimes crystals form, showing up in the mixture and as
particles while applying the mixture.
+ Contrast agents can adversely impact the metals salts and the
sensitizer, especially over time. It is recommended to not add contrast
agents to the stock sensitizer solutions nor the metal salt solutions.
+ I have found it to make a difference WHEN (the order of) the contrast
agent is added to the coating mixture. To avoid possible problems, the
order of mixing should be sensitizer, metal salts, contrast agent.
+ I have not studied changes in prints as the coating has sat for
various times, but I do suspect that with the "better prints" Eric
refers to, the coating solution may not have reached a steady state
condition. Therefore control of ambient conditions and timing of the
sitting (resting, maturing) of the coating solution may be needed to
provide for consistency of the results. Also covering the mixture to
prevent contamination or evaporation is a good idea. And when maturing
a coating mixture, one may consider adding in the contrast agent
immediatly prior to using. But then, it could even be some effect of
the contrast agent that is at work.
--
Jeffrey D. Mathias
http://home.att.net/~jeffrey.d.mathias/