According to wikipedia:

http://en.wikipedia.org/wiki/Mordant

Unlike cotton, wool is highly receptive toward mordants. Due to its amphoteric [a substance that can react as either an acid or base] nature wool can absorb acids and bases equally effectively. When wool is treated with a metallic salt it hydrolyses [ water molecules split into hydrogen and hydroxide ions] the salt into an acidic and basic component. The basic component is absorbed at –COOH group and the acidic component is removed during washing. Wool also has a tendency to absorb fine precipitates from solutions; these cling to the surface of fibres and dye particles attached to these contaminants result in poor rubbing fastness.
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Oh and I just had to comment on Betty Hiner's sig line with this.

http://archives.chicagotribune.com/2...tchicago-salmo