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Prussian blue

Prussian blue is a blue pigment used in paints and formerly in blueprints. It has several different chemical names, these being iron (III) ferrocyanide, ferric ferrocyanide, iron (III) hexacyanoferrate (II), and ferric hexacyanoferrate.

The chemical formula is Fe4[Fe(CN)6]3.

Prussian blue can also act as a chelating agent, and is used as a treatment for some kinds of poisoning.

As engineer's blue it is mixed with an oily material, and rubbed onto a metalic surface. This is then rubbed with another surface, and the removal of the pigment indicates the position of high-spots. Thus it can be used to indicate the flatness of a surface or the trueness of a bearing assembly.

Joseph Whitworth invented the first practical method of making and polishing accurate flat surfaces in 1830. This used engineer's blue and three trial surfaces. This led to an explosion of development of precision instruments[?] using his flat surfaces as a basis for further construction of precise shapes.

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