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In ancient Russia the way to measure "proper" strong spirits was to burn it - if the volume decreased by half than it was considered right one. How can I convert this into modern days alcohol percentage? Also, as a side note, how accurate this conversion would be? I mean which factors besides actual alcohol percentage would affect the process - ambient temperature? humidity? atmospheric pressure? And to which degree: +-1%? 5%?

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There are a lot of different factors that provide the alcohols percentage,

It's not really the liquid burning. It's the vapors that catch fire. Higher proof equals more vapor, depending on temperature. source

So burning alcohol is not a guaranteed way of finding out the percentage as whilst cooking wine will 'flame on' despite being low percentage and this is all to do with the vapor of the alcohol. Sure a High proof alcohol will catch fire easier but that will only tell you that it is a high proof alcohol not the percentage in it. A low proof vodka can ignite if a flame is held to it as the change in temperature will make it produce more vapor and it will ignite

(Please don't try and set fire to alcohol if you don't need to it can course spillage that will then cause an out of control fire and we don't want any of you guys to start burning up on us now )

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You don't define "burn" and I don't find a reference.

Is burn distill or literally burn.

Fire requires heat, oxygen, and fuel.

Flash point is the temp it will burn with an external ignition source. The flash point is a function of temp. Not much effected by humidity.

The flash points of ethanol wt % concentrations[102]
wt %    Temperature
10%     49 °C (120 °F)
20%     36 °C (97 °F)
30%     29 °C (84 °F)
40%     26 °C (79 °F)
50%     24 °C (75 °F)
60%     22 °C (72 °F)
70%     21 °C (70 °F)
80%     20 °C (68 °F)
90%     17 °C (63 °F)
96%     17 °C (63 °F)

Even at the flash point it will not generate enough heat to continue burning once the external ignition source is removed.

Burn off would be hard to gauge and I don't think any alcohol (for human consumption) would burn 1/2 off at room temperature.

Give 1/2 is the threshold it is more like they just boiled the alcohol off in a pot. The boiling point of alcohol is 173 F. Just stop when the temp get to 173F. 50% would be 100 proof. Yes some water boils off even at 173 and some alcohol is left behind. But it is about a wash. I would say the margin of error would be like 5%. If you did a true batch distillation would be within 1%.

  • Thanks for the "flash point" explanation - didn't know the term. Yepp, I've meant literal burning (with fire) - so I'm looking for smth opposite to flash point - the percentage at which already burning alcohol is incapable to support fire anymore. – god Jun 25 '16 at 13:27
  • @god Opposite? Can't help you there. Answer is only as good as the question. As I stated in the question no alcohol for human consumption is going to burn 1/2 off (100 proof). The flash point of 50% is room temperature. – paparazzo Jun 25 '16 at 15:14

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