Marking blue

Marking blue or layout stain (sometimes called Dykem after trademark erosion of a popular brand, or Prussian blue after the blue pigment) 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.

Not to be confused with engineer's blue.

Prussian blue and Dykem or layout fluid are not the same. Prussian blue is non drying and used to mark place of pressure contact between two surfaces. Prussian blue is transferred to the surface. Dykem, layout fluid dries and is mostly used to provide bright lines when using a scribe to mark hard surfaces or point of contact between two surfaces. Dykem is removed from the surface.

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