 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.