This is a very old printing process that uses paper coated with potassium ferrocyanide and ferric ammonium citrate. A negative (usually 4x5 or 8x10 inches in my examples) are placed on the paper, which is then exposed to UV light for around 15 minutes. UV light produces ferric ferrocyanide, or Prussian blue salts, which can give deep, rich shadows all the way to luminous highlights. The print is developed using water. The creamy highlights in some images were made by immersing the print into black tea.