Dyes differ from pigments, which are insoluble materials that impart color by being spread over a surface, or by being mixed in the form of an ingredient, as in the coloring of some papers and leathers. Natural dyes, such as indigo, madder, fustic, butternut, orchil, logwood, catechu, tumeric, etc., are seldom used today. The discovery in the mid-19th century that dyes could be produced artificially from a constituent of coal tar was the first step in the decline of the use of natural dyestuffs. In large part this was because the quality and effectiveness of natural dyestuffs depended on a variety of factors, over which the user might or might not have control. These included: 1) the problem of storage; 2) the time involved in extracting color from the raw materials; 3) dependence upon a growing season; and 4) impurities. On the other hand, dyes made in the laboratory: 1) do not depend on growing seasons; 2) do not have to be ground or chipped to be made usable; 3) are, in many cases (e.g., indigo), chemically the same as the natural dyes; and 4) since they are manufactured in pure form, are unaffected by the impurities that reduce the quality or effectiveness of their natural counterparts.
However, unlike synthetic dyestuffs, natural dyes produce what can be described as unique colors. They can never be duplicated exactly, and this undoubtedly adds to their appeal. No two natural dye lots are identical for the simple reason that each is going to contain impurities peculiar to the plant material from which the dye is produced; therefore, the very characteristics of natural dyes which made them obsolete also make them appealing to many craftsmen of today.
An enormous range of dyestuffs can be obtained from the manufacturers of chemical dyestuffs. They are still often referred to as "coal tar dyes" or, perhaps more commonly, "aniline dyes," because the early materials were prepared from aniline and many of the intermediates required in their manufacture are obtained by the distillation of coal tar. See also: ACID DYES ; BASIC DYES ; DIRECT DYES ; LAKE ; PIGMENT . (4 , 72 , 235 )