The Cyanotype Print
The cyanotype or blueprint process, a method for making prints using iron salts, was invented by Sir John Herschel in 1842. The process involves mixing solutions of ferric ammonium citrate and potassium ferricyanide. This is then brushed onto paper or fabric and allowed to dry.
The print is made by placing objects or a negative in contact with the sensitized material and exposed to UV light, such as the sun or skin tanning unit. The material is then developed by washing in running water and dried to reveal an image in Prussian Blue. (illustrated step by step method)
The Gum Bichromate Print
Chromates, used in several photographic processes, were known to be light sensitive as early as 1832 by Gustav Suckow, but is wasn't until 1855 that Alphonse Louis Poitevin realized the photographic potential by adding carbon as a pigment to dichromated gelatine.
The gum bichromate process involves coating a substrate such as paper with a mixture of gum arabic, potassium or ammonium dichromate and a pigment. The paper is then exposed to UV light through a negative. The dichromate causes the pigmented gum to harden proportionally to the amount of UV light received. Development is then done by placing the print in water, washing away any unhardened pigmented gum. Most gum prints require more than one printing to increase contrast and depth, to add selective colour or to make full colour prints.