Tithe Barn, England
The platinum process, like many of those alternative processes from the late 19th and early 20th centuries, requires a negative the same size as the final image. This is because the platinum sensitiser is only sensitive to ultraviolet light. These large negatives are output using Pictorico film and an inkjet printer.
The platinum print is made by hand-coating a sheet of paper, by brush, with light sensitive chemicals containing platinum and palladium metal solutions. The ratio of the two metals can be slightly adjusted to control the colour of the image – a higher proportion of platinum will result in a cooler image, while more palladium will give a warmer image.
Once the sensitised paper is dry, the negative is registered on top, placed in a vacuum frame to ensure complete contact and exposed to ultraviolet light. After exposure the negative is removed from the paper and the print placed in the potassium oxalate developer where the image appears instantaneously. The print is rinsed, placed in several successive clearing baths of EDTA to remove unwanted chemicals and metals and finally washed and dried. Just as the ratio of platinum to palladium can control the image colour, the type of developer and its temperature can also have a dramatic effect on the colour of a print. When potassium oxalate is used as a developer it will result in a warmer tone, especially when heated, than either ammonium or sodium citrate developers.
Assuming the paper used is of archival quality and that the processing workflow is carried out correctly, the resulting print will have archival properties far in excess of any other photographic medium, making it desirable to collectors and galleries.