Prussian blue
Prussian blue is widely used by tool makers when the core and cavity of a mould is matched during final assembly. It is also used in other tooling applications—especially during assembly—such as stamping tools and pressure die casting tools. A thin coating of Prussian blue is applied (usually with a paint brush) on the "insert"—regardless of the shape or contour—of the mould or tool before the matching is done with the mating part. If the Prussian blue (generally called just "blue") appears evenly on the mating area, it is considered—by the tool makers—as "good matching", indicating a good final product from the tool. Usually no tool would be transferred to testing or production without "blue matching", (a term generally used by tool makers in Asia). Prussian blue is considered as an integral part of precision tool making.