Re: Streaming with pure Pt on Cot 320

Dan wrote:
> Wanted: suggestions as to why a 3:1 ratio of pd:pt makes a great print
> but the same neg printed with pure pt suffers from either weak blacks
> (when single coated) or streaming (migration of metal into highlight
> areas) when double coated. ...

1) If the paper works with one metal ratio, it is unlikely that it would
be a problem with full strength or another ratio.

2) The weak print is from not having enough Pt metal present.
What are the solution strengths? For Pd, Pt and FO solutions?
Note that B&S powder is not 100%, but neither are others. A old test of
B&S powder checked out at 89.2%, however each batch could vary. An FO
strength of 25% should be OK for typical printing, however it might be a
good idea to use the traditional 27% as this would account for FO
impurity (not being at 100%).
If FO is truly 25%, you should have a 21.4% Pt solution to have a match
of enough Pt for the sensitizer. Note that this is a minimum required
for some papers like Crane's cover stock and greater than the
traditional amount. Note also that it is unlikely that this Pt will
stay in solution at room temperature and will require some heating prior
to use (enough to put into solution, but less than 140F). Not having
enough metal (the minimum) will result in a weak print. Having too much
will only waste the metal, not cause "streaming" (except as c) below).

3) The "streaming" to which you refer could be:
a) Impurities such as K2PtCl6 (not soluble in water) in your Pt which
should be pure K2PtCl4. This could be from bad supply, temperatures
above 140F, or cross contamination.
b) The use of agents such as EDTA. I only recommend the use of CAS
60-00-4 EDTA in concentrations of .04% in the sensitizer solution as I
have found more (over .1%) to result in "streaming" at times. This will
vary with the paper used.
c) Too much sensitizer and metal fully exposed might produce enough
metal that it cannot be held in the paper fibers and thus "stream".
However this should be considered rare and more of a defect of the paper
to not hold the metal.

There could be other things, but I do not have experience with this paper.

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