stain

Stain refers to a discoloration on a surface caused by the absorption or adherence of a pigment, dye, or chemical. It can describe marks on textiles, walls, wood, skin, or other materials, as well as substances used to impart color for analysis. Stains can be intrinsic (part of the material) or extrinsic (introduced from external sources).

Common everyday stains result from spills such as coffee, wine, or ink; from oxidation and rust; mineral deposits; or natural tannins in wood. Some stains are difficult to remove because the staining agent bonds with the substrate or penetrates porous materials. Cleaning approaches vary with the substrate and stain type and may involve blotting, solvents, detergents, oxidizers, or gentle heat. Prevention includes prompt absorption of spills, using protective coatings, and choosing stain-resistant materials when possible.

In science, staining is a technique that uses dyes or colored chemicals to increase contrast and reveal structures under a microscope. Biological staining highlights different cell components; common examples include hematoxylin and eosin (H&E), Gram stain for bacteria, and specialized stains such as acid-fast or PAS. Staining can be simple or differential and is often combined with fixes and mounting media. While staining aids diagnosis and research, it requires careful handling and interpretation.

Stains also occur in archaeology, art, and wood finishing, where opaque or transparent stains alter appearance and protect the material.