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A mordant is a substance used to set dyes. A mordant is either inherently colloidal or produces colloids and can be either acidic or basic.
Mordants include tannic acid, alum, chrome alum, and certain salts of aluminum, chromium, copper, iron, iodine, potassium, sodium chloride (ordinary table salt) and tin.
Iodine is used as a mordant to set the first dye in Gram stains.
See also
"Mordant"