[Alt-photo] Black specks in Pt/Pd?

Mike Ware mike at mikeware.co.uk
Thu Dec 7 16:54:19 UTC 2017

Thanks, Chris, for your kind words concerning the Platinotome. If it impresses such a prolific author as yourself, it must have something going for it!

In part answer to your question: "Maybe it’ll include, finally, definitive information on black specks in pt/pd?” I’m not aware of any specific information on that in the text - but I haven’t read it all yet!

The most annoying species of black speck appears only on development. These may be due to particles of iron(II) oxalate having been deposited in the coated layer from reduction of the iron(III) oxalate sensitizer by impurities. Iron(II) oxalate forms insoluble yellow microcrystals which would be invisible in the yellow sensitized layer, and their presence would only become apparent when solubilised by the developer to reduce the Pt/Pd salt to noble metal. It may be significant that black specks have never been observed (AFAIK) with the Malde-Ware print-out platinum-palladium version, which uses ammonium iron(III) oxalate: this substance, if accidentally reduced, would disperse as a soluble iron(II) product, instead of forming particles of insoluble iron(II) oxalate.


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