Q:4

A  X is a reproduction of a technical drawing, an architectural plan, or an engineering design, using a contact print process on light-sensitive sheets. Introduced by Sir John Hershelin 1842,the process allowed rapid, and accurate, production of an unlimited number of copies.
processing of X is based on a photosensitive ferric compound. The best known is a process using ammonium ferric citrate and potassium ferricyanide.The paper is impregnated with a solution of ammonium ferric citrate and dried. When the paper is illuminated, a photoreaction turns the trivalent ferric iron into divalent ferrous iron. The image is then developed using a solution of potassium ferricyanide forming insoluble ferroferricyanide prussian blue in color with the divalent iron. This process is known as cyanotype.
A image in cyanotype:

ID X