Assignment, Platinum Prints.

The Process for Platinum Prints was invented by Williams Willis(1841-1923) in 1872 who continually refined it until 1878. It is made up of a dried peice of paper sensitized with a solution of potassium chloro-platinate and ferric oxolate an iron salt. Which is contact printed under a negative in dalylight until a faint image was produced by the reaction of the light source with the iron salt. This process also depends on the light source of iron salts which is crucial to making it work. The paper was developed by immersion in a solution of potassium oxolate that dissolved out the iron salts and reduced the chloroplatinate salt to platinum in those areas where the exposed irons salts had been. The paper is washed in a series of baths to remove excess iron salts and yellow stains and final the print is washed in water.
With the price of Platinum Prints rising steeply they where made right up to the 1920's. They where replaced by palladium prints a process which is similar but employed a compund of the metal rather that platinum. These processes were valued for there great tonal range, usualy silvery greys.
Recently Platinum prints have enjoyed a revival.

"Looking at Photography" A guide to technical terms by Gordon Baldwin