In the 17th century, distilled spirits production grew as consumption soared. Governments needed reliable instruments to gauge alcoholic strength, as liquor taxes became a major source of treasury revenue.
Since 1350, imported wine to England had been taxed by the cask, disproportionately rewarding larger-capacity tuns and pipes over smaller hogsheads or tierces. By the late 16th century, Royal gaugers measured the liquid volume of wine and beer in different casks using complicated mathematical formulas predicated on their cylindrical shapes, capacities, and fluid contents. As the demand and trade expanded, better diagnostic methods to measure the alcoholic strength of spirits, notably brandy, became revenue prerequisites. Distilled spirits can contain more than 10 times the alcohol of beer and four times that of wine. Measuring proof was either by alcohol’s liquid volume or density – ethanol’s specific gravity, being less dense than water.
The Dutch were European leaders in distillation and pioneers in the early French brandy industry, founding businesses in the Loire Valley and Charente-Maritime region. They introduced the term alcoholic proof, ‘preuve de Holland’ or ‘Holland proof’, in the 16th century; in Germany, ‘die Hollandische Probe’, and ‘Holandas’ in Spain. Holland proof equated to roughly 50% ABV. By 1583, all states in Holland taxed ‘all fired wines’ by Holland proof for korenwijn made from grain and brandywijn, formally grape based, remaining a trade term into the 19th century.
Shaking a glass phial or flask by the Holland proof method caused bubbles to form, where their size and duration indicated strength. Bubbles are caused when ethanol molecules pack closer than water with surfactants such as fusel oils, esters, aldehydes, and proteins. It was called the bubble or bead method from the mid-16th century in Britain. The Distiller of London, written in 1638 by Theodore de Mayerne, was the first reference in English to ‘strong proofe spirit’; 58 years later, ‘phyal proof’ became British law. Holland proof was unreliable as additives such as sugar and acid obstructed the bubble effect, distorting alcoholic strength. Additives also affected modern hydrometer readings of specific gravity, misrepresenting the alcoholic strength and falsifying data for customs duty. In the 20th century, obscuration required laboratories to test imported whisky for caramel content added for imitation colouring.
In Italy, philosopher-technicians followed Galileo Galilei’s 1603 equilibrium of fluids experiments based on the Archimedes principle. Dropping hollow glass spheres, calibrated with specific weights, into a sealed glass tube allowed them to float to their density level. When England’s first duties on distilled spirits passed in 1643, gaugers needed superior instruments for measurement. Enter Robert Boyle, who manufactured mathematical balls in the 1660s, steering Britain towards specific gravity. This was followed by John Clarke’s improved hydrometer, adopted by government act in 1787 and replaced with Bartholomew Sikes’s instrument in 1818. It was not until 1980, after Britain had joined the European Union (then the European Economic Community), that % alcohol by volume become the standard measure. The Scots broke rank, instead using calibrated spirit beads similar to the Italian and Boyle spheres. Alexander Wilson manufactured glass eggs or philosophical beads in 1757. An Italian immigrant to Glasgow, Angelo Lovi, later manufactured Lovi beads, as did other Glasgow instrument makers including John Brown (gravity beads), John Corte (hydrostatic beads), and Anthony Galletti (spirit beads).
The Spanish dropped oil into liquor to observe its buoyancy. However, oil proved as unreliable and fallacious as gunpowder due to black powder often having chemical disparities in the mixtures and moisture absorbency making combustion variable. Not all spirits were distilled or casked to flammable proof, making the ignition test impractical. The Royal Navy never employed gunpowder proofing; instead, it sufficed in modern quixotic fiction for pirates and smugglers.