Basic dye is a class of dyes, usually synthetic, that act as bases, and which are actually aniline dyes. Their color base is not water soluble but can be made so by converting the base into a salt. The basic aniline dyes, while possessing great tinctorial strength and brightness, are not generally light-fast; therefore their use in the dyeing of archival materials is largely restricted to those materials not requiring this characteristic. Basic aniline dyes were at one time used extensively in dyeing leather, mainly because they are capable of combining directly with vegetable-tanned leather without the use of a mordant. Basic dyes show virtually no migration in acrylic fibers under normal dyeing conditions, compatibility is of major importance in selecting dye combination with optimum level dyeing behavior.