How was whisky tested for proof strength?

Spirit of proof strength was the technical standard by which strength was measured until 1st January, 1980. Hundreds of years ago, spirit of this strength was proved when whisky and gunpowder were mixed and ignited. If the gunpowder flashed, then there was enough whisky in the mixture to permit ignition. Such whisky was held to have been proved. If the spirit was weaker than this proof strength ignition did not take place.

In the 1740's. the Customs and Excise and the London distillers began to use Clark's hydrometer, an instrument devised to measure spirit strength. A more accurate version by Bartholomew Sikes was universally adopted under the Hydrometer Act,1818, and remained in standard use until 1980.

© SWA 1995