Marking blue

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.

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.