In response to a reader's question, the editors describe research on lightfastness of natural dyes, undertaken by Patricia Crews (USA). Natural dyes are more vulnerable to light than synthetics, but ultraviolet absorbers added to the dybath work well with some dyes. Dyes and mordants have been classified according to their lightfastness; a table of yellow dyes is included. The use to which a textile is put, the kind of light it is exposed to, and the relative humidity all affect colour stability.