A single batch of dye, together with the yarn colored with it, all of which takes the same shade. Natural dyes are made in small artisanal batches, and when one runs out the replacement cannot be matched exactly, so yarn from different lots differs slightly in tone. In a finished rug the difference shows as the color shifts known as abrash, which is read as a sign of hand dyeing, whereas a perfectly flat, uniform color suggests synthetic dyes and machine making.
Colorants manufactured chemically rather than extracted from plants or animals. They dominate machine-made and hand-tufted rugs, where they give flat, uniform color, and are sometimes used for a few shades in otherwise naturally dyed hand-knotted rugs. They tend to bleed more readily than natural dyes under a damp cloth and to fade or age less gracefully. Experts read a pile that is lighter at the tips than at the base as synthetic dye faded by light, which points to a rug made in the late 19th century or later. Weavers choose between the two according to the colors they want.
The green outer covering of the walnut, a traditional plant dye for brown wool. Rug dyers boil it to obtain deep brown to umber shades, and it belongs to the same vegetable palette as madder root (terracotta reds), indigo (blues) and pomegranate rind (yellows). Plant dyes of this kind are what separate traditionally dyed rugs from those colored with synthetic dyes.
Various techniques for coloring carpet yarns, including beck dyeing (immersion in dye baths), continuous dyeing (application after tufting), space dyeing (multiple colors on a single yarn), and solution dyeing (color added to liquid polymer before extrusion). Each method offers different benefits for colorfastness, pattern capabilities, and environmental impact. In hand-knotted rugs the wool is dyed as spun yarn before knotting, with natural dyes from plants, roots, and flowers, which tend to mellow with age, or with synthetic dyes; differences between dye batches are a main source of abrash.