Possible causes of Selenium precipitation?

[Smudger]

It is winter here in Aotearoa, and I've recently taken to warming my RST in the microwave. My latest batch has developed a black powdery precipitate,unlike the usual reddish plating on the container walls that users will be familiar with.
Would anyone care to comment on the possibility that the microwave is a factor?
Also,Paul Butzi has claimed that the use of acid stop causes Se precipitation..any comments?
I use Dektol,very dilute citric acid stop,and Agfa FX Univeral Fixer (pH around 7.5.
Thanks all,
Ian.

 

[Michel Hardy-Vallée]

Precipitate in Se toner is just a natural thing that happens from usage. I think it is a byproduct of the selenium-silver reaction, but I don't have data on hand about that. Just filter it. I don't see how an acid stop would have any impact; you should wash your prints properly before toning.

 

[gainer]

The selenium toning reaction replaces silver in the emulsion with selenium from the bath, IIRC. The silver is precipitated.

 

[Dave Miller]

This has recently been discussed in another thread so it would be worth searching that one. One point on your post though is that by the time you get to use the toner your print should be so well washed that there shouldn't be any stop / fixer left to react with the toner.

 

[dancqu]

The selenium toning reaction replaces silver
in the emulsion with selenium from the bath, IIRC.
The silver is precipitated.

Not quite the case as I understand it. The in solution
selenium combines with the image silver forming silver
selenide.

Working strength solutions of KRST have very low
levels of ammonium thiosulfate. The in emulsion and
paper thiosulfate complexed silver carried into the toner
MAY by accumulation eventually produce a precipitate of
silver selenide. To test tone some very clean prints and see
what happens. Dan