Roughly this is 100 proof; that is more or less what 100 proof means. They would pour some of the brew over a small pile of gunpowder, then set it alight. If it burned down till it ignited the powder it was "proved" ie passed the test.  The concentration was established to be 49% ethanol,  but nowadays this has been legally rounded up to 50% in most countries, I believe.

In practice the concentration varies with conditions, such as the ambient temperature and humidity, and how hot the liquid is, but 50% is a good enough figure for most purposes.

Go well,

Jon

It depends on the temperature.

The flash point of a liquid is the temperature at which it will ignite when a source of ignition is provided. For pure ethanol the flash point is 55 degrees F. At concentrations of 50% and 40% (around the strength of spirits like vodka, cognac and whisky) the flashpoint is still in the room temperature range - 75 and 79 degrees F respectively.

You can however get much lower percentage alcohol solutions to ignite simply by heating them - for example when cooking with wine, it will catch fire when added to a hot pan. The flashpoint for wine-level concentrations of alcohol (about 10%) is 120 degrees F.

Your specific question is about sustained flame - so rather than the flash point you are probably looking for the fire point - defined as the temperature that a liquid needs to be at to sustain a flame for 5 seconds after a source of ignition is supplied. These will be slightly higher than the flash points.