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 improved by it. The red dyes, kermes and madder, and the yellow dye weld, are especially mordant or adjective dyes: they are all dyed on an aluminous basis. To put the matter plainly, the goods are worked in a solution of alum (usually with a little acid added), and after an interval of a day or two (ageing) are dyed in a bath of the dissolved dye-stuff.

A lake is thus formed on the fibre which is in most cases very durable. The effect of this "mordanting" of the fibre is clearest seen in the maddering of printed cotton goods, which are first printed with aluminous mordants of various degrees of strength (or with iron if black is needed, or a mixture of iron with alumina for purple), and then dyed wholesale in the madder-beck: the result being that the parts which have been mordanted come out various shades of red, etc., according to the strength or 207