Whilst going through Penn's notebooks on platinum printing that can be found online at the Ryerson & Burnham Archives.
I came acorss his notes on 'Pigment and Porcelain' http://digital-libraries.saic.edu/cd...n/mqc/id/15080
It says 'This notebook contains tests and details concerning Irving Penn's experiments in printing photographs on porcelainized plates using a pigment process'
Have yet to make head nor tail of it yet but will post more when i can. My initial thoughts are that he was making a colour image with platinum and palladium and colour pigments.