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10:57am on Friday, 5th August, 2005:

Alum

Weird

Alum is a salt used to fix dye to cloth. It's old — the Romans used it — and was traditionally very expensive. In the Middle Ages, England was a huge producer of wool but had to import the alum it needed to dye it (from the Middle East and — until the Reformation — the Papal states). It wasn't until the turn of the 17^th Century that England managed to create its own alum works (at Carlton, near Whitby).

Here's how alum was made:

• Find a vast seam of shale, break it up into pieces and make a pile of it 100 feet high.
• Set fire to the shale, and let it burn for a year.
• Dissolve the burnt (calcined) shale in water, until the water won't hold any more of it.
• Let the water (raw alum liquor) settle.
• Boil the water while adding human urine to it.
• Take the crystals that form as a result and purify them by washing and recrystalising them several times.
• You now have alum.

What I want to know is how, in ancient times, people figured out that if they did this they'd get some stuff that would fix dye to cloth.

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Copyright © 2005 Richard Bartle (richard@mud.co.uk).