blåprint
A blåprint is a historical type of copy or reproduction of architectural and engineering drawings. The term "blåprint" itself is a misspelling of blueprint, a process that produces a white line on a blue background. These prints were created using a chemical process involving ammonium citrate and potassium ferricyanide, which would react to ultraviolet light. When a drawing was placed on sensitized paper and exposed to light, the areas covered by the drawing remained white, while the exposed areas turned blue.
This method was widely used for its ability to create inexpensive, durable, and accurate reproductions of technical