Engineer's blue

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Engineer's blue is a non-drying paint used to assist in the mating of two or more components.

Joseph Whitworth popularized the first practical method of making accurate flat surfaces, during the 1830s, by using engineer's blue and scraping techniques on three trial surfaces. Prior to his scraping technique, the same three plate method was employed using polishing techniques, giving less accurate results. This improvement led to an explosion of development of precision instruments using these flat surface generation techniques as a basis for further construction of precise shapes.

Engineer's blue is prepared by mixing Prussian blue with an oily material. The colored 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. This method has been used to test the flatness of surfaces and the trueness of a bearing assembly.

When Prussian blue is mixed with methylated spirits it forms a quick drying stain which is known as marking blue. This stain is used in the marking out operation in metalworking. Both the "marking out" blue, and the "scraping blue" may be referred to as engineer's blue, which can lead to substantial confusion.