Recently I’ve been reading some of the original sources for platinum printing. It’s fascinating to compare what we know and do today with what photographers knew and did back in the nineteenth century.
This is an excerpt from William Willis’ first patent for the platinum printing process (apparently he didn’t believe in paragraphs!):
“This invention has for its object improvements in the chemical treatment of the surfaces of paper, wood and other suitable materials employed for receiving images from photographic negatives or from any other object that may be interposed between the light and such prepared paper or other surface. For this purpose according to my Invention I apply to surfaces of paper, wood, and other suitable materials (by either one or more coatings) solutions of simple or compound salts of platinum, iridium, or gold, or a mixture of such salts. After this has dried I sometimes apply another coating of a solution of a salt or salts of other metals. I then coat the material again, using a solution of ferric oxalate, or tartrate, or a combination of these salts with others, and dry again. I then expose the coated surface to light under a photographic negative or other suitable object until a faint brown image appears; after this I apply to the coated surface a solution of the neutral oxalate of potassium or other suitable oxalate, which speedily changes the brown tints to black ones. I then wash the surface thus treated for a short time in a dilute acid, and finally wash in water; but I sometimes immerse the surface in a solution of chloride of sodium, or hyposulphite of sodium, or other suitable salt before giving it a final wash in water.”
If you’re unfamiliar with the name, it was William Willis who invented the platinum printing process, and his Platinotype Company was one of the major manufacturers of early platinum papers and developers. Without his devotion to platinotypes we’d all be using silver gelatin today.