Not only is this more gentle than soda ash, it works with a wider variety of dyestuffs. In ancient times it was mined and constituted a strategic mineral resource. A kingdom which could control the alum trade could wield considerable power. Not all colors are equally available. Many substances will dye wool various shades of brown, tan, and buff.
Fewer dyes are available for reds and yellows and still fewer for blues and greens.
Purple was the rarest of all. Tyrian Purple was extracted by the Tyrians from a local shellfish. 8500 shellfish were needed to produce 1 gram of dye, making this a color to be afforded only by kings and princes. Some dyes tend to fade over time while others remain vibrant for a long period. A dye that does not fade is called lightfast .