Quote Originally Posted by mitomac View Post
Many thanks to Dennis, Flying Camera, Ian, Doug and Jeffrey!

The mottling is more of a touch of gray that is not quite uniform in the highlights (I apologize for any confusion with the term). I just wanted to make sure I wasn't missing something with the metal mixture when using dichromate in the developer.

Stray light

It appears that Pt/Pd is much more sensitive to stray UV then Pd with NaPt for contrast control. Tonight I will try coating a sheet of cot320 in very dim room and will be militant about keeping the sensitized paper in the dark until dry. In the past with Pd (NaPt), I was able to coat inside during daylight hours - I tried that once with Pt/Pd and got a dark gray mess.

Humidity

Usually not a problem in NC, but out of habit I typically steam the paper over a kettle before exposure. Starting to think this may also be contributing to the problem and some of the variability. Will try without steaming.

Solutions

I am biochemistry professor by day - so fairly confident my solutions are OK (although I have been known to make boneheaded mistakes - which of course I blame on my students).

I will make up fresh FO (cheap enough to mix on demand). Given that Pt/Pd appears to be more sensitive to stray UV light than straight Pd, it may also be more sensitive to the age of the reagents .

Development

Almost always done between 20C and 37C

Many thanks for all the help - despite the cost of (Pt) and my problems with the highlights - the overall tone of the prints are fantastic. My Pd prints were always a touch too warm for my taste and these are perfect. Once I dial in the 1:1 ratio I will see how low I can I dilute the Pt for the same effect.

Cheers,

mitomac
From what you've said here, my suspicions fall on your kettle or developer temperature.

Platinum is actually less active than palladium (on many papers it is significantly less active) so there's no reason why it should fog more easily than palladium. However, the level of paper moisture has a significant impact on both platinum and palladium (Dmax and contrast range). A patch of paper that has more moisture than another may well fog more quickly. if you've got a fairly normal room humidity then you should be able to use paper without having to humidify it first.

When platinum is developed in a cold developer (e.g. room temperature) it can get a gritty look. You may want to heat up your developer a bit to see if this changes things. At 65C you should see greater Dmax and greater contrast (and if developer temperature is the problem then perhaps the mottling will go). Beware though that higher developer temperature will boost the palladium more than the platinum.

Good luck!

Ian.