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