Ammonia process paper is a type of paper that is commonly used in blueprinting, engineering, and architectural applications. The paper is specially coated with a mixture of ammonium and iron salts that react to create blue lines when exposed to light. The process involves placing a light-sensitive paper, such as ammonia process paper, over a pre-drawn original copy or blueprint and then exposing it to light. The light causes a chemical reaction in the coating, resulting in the formation of blue lines where the light has passed through the original. The blue lines on the ammonia process paper provide a detailed copy of the original, which can be used for reproduction, modification, or storage. Ammonia process paper is available in various sizes and thicknesses and can be used with a range of printing and copying techniques.