Engineers Blue

Also known as marking blue, layout stain or Prussian blue this dye is used in metalworking to help make scribed lines easier to see. A thin layer is applied to a metal object that can be scratched off using a scriber to reveal a thin, bright 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. Engineers blue can be applied with a brush but also comes in pen form. A permanent marker can be used as a quite effective alternative.