The light-sensitive silver halide in calotype paper was silver iodide, created by the reaction of silver nitrate with potassium iodide. First, "iodised paper" was made by brushing one side of a sheet of high-quality writing paper with a solution of silver nitrate, drying it, dipping it in a solution of potassium iodide, then drying it again. At this stage, the balance of the chemicals was such that the paper was practically insensitive to light and could be stored indefinitely. When wanted for use, the side initially brushed with silver nitrate was now brushed with a "gallo-nitrate of silver" solution consisting of silver nitrate, acetic acid and gallic acid, then lightly blotted and exposed in the camera. Development was effected by brushing on more of the "gallo-nitrate of silver" solution while gently warming the paper. When development was complete, the calotype was rinsed, blotted, then either stabilized by washing it in a solution of potassium bromide, which converted the remaining silver iodide into silver bromide in a condition such that it would only slightly discolour when exposed to light, or "fixed" in a hot solution of sodium thiosulphate, then known as "hyposulphite of soda" and commonly called "hypo", which dissolved the silver iodide and allowed it to be entirely washed out, leaving only the silver particles of the developed image and making the calotype completely insensitive to light.