Marking blue

Marking blue, layout stain or Prussian blue is a dye used in metalworking to aid in marking out rough parts for further machining. It is used to stain or paint a metal object with a very thin layer of dye that can be scratched off using a scriber or other sharp instrument to reveal a bright, yet very narrow line in the metal underneath. The advantages are that any existing scratches are covered with the dye and the new lines have a contrasting background.

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Marking blue is made by mixing Prussian blue with alcohol to form a quick drying stain. Alternatively, felt tipped marker pens can be used as they are convenient and tend not to dry up as quickly; marking blue in the form of dye or ink has a tendency to dry in the containers and become unusable quickly. Other alternatives are chalk (used on rough castings) or a copper sulfate solution that leaves a deposit of copper on the clean metal. This thin copper coating is more resistant to rough handling and the action of coolants or cutting compounds.

This is not to be confused with engineer's blue, made by mixing Prussian Blue with oil.

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