Stain

A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. They are caused by the chemical or physical interaction of two dissimilar materials. Staining is used for biochemical research, metal staining, and art (e.g., wood staining, stained glass).

There can be intentional stains (such as wood stains or paint), indicative stains (such as food coloring or adding a substance to make bacteria visible under a microscope), natural stains (such as rust on iron or a patina on bronze), and accidental stains such as ketchup on a shirt.

Different types of material can be stained by different substances, and stain resistance is an important characteristic in modern textile engineering.

The primary method of stain formation is surface stains, where the staining substance is spilled out onto the surface or material and is trapped in the fibers, pores, indentations, or other capillary structures on the surface. The material that is trapped coats the underlying material, and the stain reflects back light according to its own color. Applying paint, spilled food, and wood stains are of this nature.

A secondary method of stain involves a chemical or molecular reaction between the material and the staining material. Many types of natural stains fall into this category.

Finally, there can also be molecular attraction between the material and the staining material, involving being held in a covalent bond and showing the color of the bound substance.

In many cases, stains are affected by heat and may become reactive enough to bond with the underlying material. Extreme heat, such as from ironing or dry cleaning, can cause a chemical reaction on an otherwise removable stain, turning it into a chemical compound that is impossible to remove.

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Wikipedia