Alum

Alum is both a specific chemical compound and a class of chemical compounds.

1. It means potassium aluminium sulfate, a chemical compound.
2. It also means a double sulfate salt.

Examples:

• Potash alum is potassium aluminum sulfate
• Sodia alum is sodium aluminum sulfate
• Ammonium alum is ammonium aluminum sulfate

Words like "alum" and "potash" were used before modern chemistry developed. The word 'alumen' occurs in Pliny's Natural History (23–79 AD). In the 52nd chapter of his 35th book, he gives a detailed description. Different substances were distinguished by the name of 'alumen', but they were all astringents,^[1] and were all used in dyeing and medicine.

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