Blueprinting was one of the first reprographic processes used by cartographers and engineers to quickly reproduce maps and drawings without the use of engravers and lithographers. Blueprinting is based on the process of the cyanotype invented by John William Herschel (1792-1871) described in his communication “On the action of the rays of the solar spectrum on vegetable colors and on some new photographic processes”, presented to the Royal Society of London in 1842. The general principle of the cyanotype process is the photochemical reduction of iron (III) to iron (II) salts forming the inorganic complex commonly known as Prussian blue, in areas not obscured by lines in the original document which had been placed in contact with paper pre-washed with a mixture of iron (III) salts to make it light sensitive. Special machines were invented around 1895 to automatically produce full document printing (exposure to light, development, washing and drying). Blueprinting and similar photo-reproduction processes like Pellet and Vandyke prints were used extensively between 1870 and mid-twentieth century.
In this study the various photo-reproduction processes and related chemical processes will be discussed with an emphasis on the analysis of maps and drawings kept in Portuguese historical collections.