Proof Spirits

   Lemons are lemons, more or less, and sugar is sugar. There might be some incidental differences between what was available along those lines before Prohibition and what we can get now (for example, their limes were the small, round, and seeded Key or Mexican limes, not the larger, oval, and seedless Persian or Tahitian type, which was introduced in 1895 and didn’t catch on until the 1920s), but they won’t be truly decisive. Spirits, however, are entirely products of art, and though art is long and life is short, it’s still subject to the game of telegraph that is the transmission of information over time.


   Whatever spirits you use, with some exceptions they should ideally be at what would have been considered “proof” at the time: a proof spirit was one that was 50 percent alcohol. Strengths were recorded as a percentage of that 50 percent. Thus an 80 (percent of) proof spirit contains 80 percent of 50 percent, or 40 percent alcohol.


   Easy enough, except for the fact that there were two different systems in use for measuring the percentage of alcohol: the Sykes system, which was by weight, and the Gay-Lussac system, by volume. Anything from Great Britain or its colonies used the first, while anything from France, Continental Europe, or the United States went by the second, which is now the modern standard.


   Because alcohol is lighter than water and it therefore takes a greater volume of alcohol than water to make the same weight, 50 percent Sykes works out to a little more than 57 percent Gay-Lussac, while (for example) 33 percent Sykes, the maximum strength British law allowed until the 1860s for “compound spirits” such as gin, works out to 47 percent Gay-Lussac—a figure you might recognize from your Tanqueray or Beefeater label.  


   In the nineteenth century, as now, not everything hewed to the ideal. In practice, there was a great deal of variation in the strengths of spirits sold. Bars bought many of their spirits in the cask and bottled them themselves, reducing them to proof in the process. Some bars seemed to have wider water taps than others. Nonetheless, the best-quality spirits generally fell into a range of between 43 and 58 percent alcohol by volume (abv). Nowadays, many spirits are again being bottled at these higher proofs rather than the legal minimum of 40 percent that was increasingly becoming the industry standard. 


Wondrich, David. Imbibe! Updated and Revised Edition (pp. 68-69). Penguin Publishing Group.