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. This method has been used to test the flatness of surfaces and the trueness of a bearing assembly.

The fitter may be told to "blue it up" when using this piece of equipment.

Use in toolmaking

Prussian blue is widely used by tool makers

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