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.

Marking out a metal bar


Marking blue is made by mixing methylated spirits with shellac and gentian violet.[1]


A felt tip marker can be used as they are convenient and tend not to dry up as quickly. On rough structures, such as castings or forgings, whitewash or a mixture of chalk and water can be used. A solution of copper sulfate, distilled water, and a few drops of sulfuric acid can be used on machined surfaces.[1]


Marking blue is considered non-toxic. If heated above 250 degrees Celsius it will give off toxic Cyanogen gas. Metal cutting commonly heats material above this level.


  1. ^ a b Brink, C.; McNamara, B. (2008), Engineering Fabrication & Boilermaking, Pearson South Africa, p. 44, ISBN 978-1-77025-374-2.