Molly Callahan, age 15, of Columbus, Ohio, for her question:

WHERE DO WE GET INDIGO?

Indigo is a deep blue dye used to color cotton and wool. In the past, this dye was taken from the indigo plant, a member of the pea family that grows chiefly in India. The name "indigo" is a Spanish form of the English word "India."

Today, synthetic indigo is made from aniline, a coal tar product. The process, discovered in England by Sir William Henry Perkin in 1856, was first applied in the dye industry by German chemists in 1897.

Indigo is a vat color, so called because it does not dissolve in water. Indigo paste must be treated with an alkaline reducing agent before it can be used as a dye.

A chemical reaction turns the paste yellow and makes a substance that will dissolve in water. After a cotton or woollen fabric has been dyed, it is removed from the vat and exposed to air to oxidize it a deep blue which is resistant to removal by water.