In England, a unit of capacity and mass used for soap. By a law¹ of 1531, the empty barrel could weigh no more than 26 pounds, and must have a capacity of 32 ale gallons or more (the same as an ale barrel). Similarly the empty half barrel was limited to no more than 13 pounds and no less than 16 gallons, and the empty soap firkin to no more than 6½ pounds and no less than 8 gallons.
In 1711 the soap barrel was legally redefined² as 256 pounds of soap, net weight, which is more or less equivalent to the earlier definition.
Soap is made by reacting fat with an alkali. If the alkali is lye (sodium hydroxide), the result is usually a solid — bar soap. In the middle ages the alkali was often potassium hydroxide, potash, leached from wood ashes. Potassium hydroxide, reacting with fat, usually yields a liquid soap, which is what was being packed in soap barrels.
1. 23 Henry VIII chapter 4.
2. 10 Anne chapter 18 section 9.
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Last revised: 6 June 2009.