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. Accidental staining may make materials appear used, degraded or permanently unclean. Intentional staining is used in biochemical research, and for artistic effect, such as in wood staining, rust staining and stained glass.
There can be intentional stains (such as wood stains or paint),[1] indicative stains (such as food coloring dye, and staining, the use of one or more substances to enhance visibility of samples in a microscope or other imaging device.[2] Numerous naturally-occurring stains exist, such as rust on iron and a patina on bronze,[3] as do accidental stains such as from ketchup and oil on fabrics and other materials.
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 backlight according to its own color. Applied paint, spilled food, and wood stains are of this nature.[4]
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.[5]
In many cases, stains are affected by heat and may become reactive enough to bond with the underlying material. Applied heat, such as from ironing, dry cleaning or sunlight, can cause a chemical reaction on an otherwise removable stain, turning it into a chemical.
Various laundry techniques exist to attempt to remove or reduce existing stains. Stain removal is frequently a highly desired characteristic in laundry detergents and other types of cleaning agent. Some agents are designed to be applied directly onto stains. The removal of some types of stains requires other chemicals or special techniques. Use of an inappropriate technique could make permanent an otherwise removable stain or cause unwanted discoloration of clothing.[6]