Marking blue, layout stain or Prussian blue

Prussian blue

Prussian blue is a dark blue pigment with the idealized formula Fe718. Another name for the color Prussian blue is Berlin blue or, in painting, Parisian blue. Turnbull's blue is the same substance but is made from different reagents....

is a dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

 used in metalworking

Metalworking

Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewelry. It therefore includes a correspondingly wide range of skills,...

 to aid in marking out rough parts for further machining. It is sometimes called Dykem (after a popular brand). 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

Scriber

A scriber is a hand tool used in metalworking to mark lines on workpieces, prior to machining. The process of using a scriber is called scribing and is just part of the process of marking out. It is used instead of pencils or ink lines, because the marks are hard to see, easily erased, and...

 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 blue is made by mixing methylated spirit

Methylated spirit

Denatured alcohol or methylated spirits is ethanol that has additives to make it more poisonous or unpalatable, and thus, undrinkable. In some cases it is also dyed....

s with shellac

Shellac

Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes , which are dissolved in ethyl alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish...

 and gentian violet. This is not to be confused with engineer's blue

Engineer's blue

Engineer's blue is a highly pigmented paste used to assist in the mating of two or more components.Joseph Whitworth popularized the first practical method of making accurate flat surfaces, during the 1830s, by using engineer's blue and scraping techniques on three trial surfaces...

, made by mixing Prussian Blue with oil.

Alternatives

A felt tip marker 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. On rough structures, such as casting

Casting

In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...

s or forging

Forging

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons...

s, whitewash

Whitewash

Whitewash, or calcimine, kalsomine, calsomine, or lime paint is a very low-cost type of paint made from slaked lime and chalk . Various other additives are also used...

 or a mixture of chalk

Chalk

Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....

 and water can be used. A solution of copper sulfate, distilled water

Distilled water

Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container.-History:...

, and a few drops of sulfuric acid

Sulfuric acid

Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 can be used on machined surfaces. This thin copper coating is more resistant to rough handling and the action of cutting fluid

Cutting fluid

Cutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols , and air or other gases. They may be made from petroleum distillates, animal...

.