The coloured oil is rubbed onto a reference surface, and the workpiece is then rubbed against the coloured reference; the transfer (by contact) of the pigment indicates the position of high spots on the workpiece or conversely highlight low points.
Prussian blue is widely used by tool makers when the core and cavity of a mould is matched during final assembly.
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.
Prussian blue is considered as an integral part of precision tool making.