Along with the tooth fairy and Santa Claus, a beautiful myth held dear by some new distillers is the Magic Boiling Myth, in which the sorcerer's apprentice, er, stiller, can take a mixture of volatile liquids like water, ethanol, methanol, ethyl acetate, acetone and others (which describes the wash in a still's boiler pretty closely), and cause one component to boil (by some clever temperature control), while the others do not boil.

By this means, the magician can take a simple mixture of water, which boils at 100C, and ethanol, which boils at 78.4C, and by holding the boiler temperature at, say, 79C, he can cause the ethanol to boil off (or just evaporate - there are some variations to the myth) and be collected at the condenser as pure ethanol, while the water does not boil (or evaporate - see above) to dilute the collected distillate.

A common corollary to the myth is that the distiller can "take control" of the internal temperature of a still while boiling. This part of the myth is guaranteed to make the new distiller crazy on his first still run, because, like the rest of the myth, it's purest bullshit.

Where Does the Myth Come From?

Although the Magic Boiling Myth seems to come out of nowhere, while seeming like it's everywhere, there may appear to be a tiny bit of support for it when reflux-column distillers speak of having some control over the temperature at the top of their packed or plated separating reflux columns. In this case the new distiller does not understand that an operating reflux still has many different temperatures inside, while the reflux distiller concerns himself most with only one of those temperatures, and it's not the temperature of the boiling wash.

Understand that the Myth is only about wash boiling temperature, and in a theoretical potstill, everything inside the boiler, up to the condenser is at the wash boiling temperature.

So what's the truth? The truth is that all liquids are volatile, and at any temperature vapor is lost from the surface of the liquid. When a liquid is raised to its boiling point, any additional energy input causes a specific amount of the liquid to be vaporized, but its boiling point is not changed by that additional energy input. The flip side of that is that if you have some evaporation at the boiling point without adding that energy, the liquid will cool a bit, and boiling will stop; think of blowing on a spoonful of hot soup to cool it. The boiling point of any mixture of liquids is a single fixed number (at least for a given atmospheric pressure) and you can't increase that number by turning up the heat.

If your liquid is a pure liquid like pure water or pure ethanol, and not some mixture, we can say for sure that water will boil at 100.00C and that ethanol will boil at 78.37C. No matter how hard the Magic Boiler tries to turn up the heat, those two liquids will (separately) boil at those two numbers. Period. More heat just makes more vapor.

When you mix the liquids, however, like, say, our water and ethanol, that previous simplicity goes out the window, and the boiling point of the mixture can be anywhere between the 100C of water, and the 78.37 of ethanol, depending on their relative concentrations. Without getting into (yet) the physics of how that works, take a look at this graph, which relates the ethanol concentration of an ethanol-water mixture to the boiling point of that mixture. Just to make it interesting, it also relates the boiling point to the ethanol concentration in the vapor produced.

Wash and distillate temperatures vs %ABV

If your mixture is 10% ethanol by volume, and if you put that mixture in a still, and if you bring it to a boil, it will boil at 93C. To reassure yourself of that, find the 10% ABV number on the bottom of the graph, and follow that 10% line up until it intersects the blue curve. Look at the horizontal line that passes through where the 10% line and the blue curve intersect. If you follow that horizontal line to the left, you'll find that it's the 93C line, so that 10% ethanol water mixture boils at 93C. Less than 93C, nothing boils in that mixture. It doesn't matter a bit whether you're heating with a match or a thermonuclear explosion; if it's boiling, it boils at the boiling point, determined only by the contents of the liquid.

For those who need to see the physics and the math, here's a link to information about Raoult's law, which determines the boiling point, again irrespective of applied heat. The example is for a mixture of only two liquids, and in the still pot we are dealing with many more, but the principle is the same in all cases.

A Related Myth

A myth often given voice by the Magic Boilers, is the "Easy Boiling Myth", whereby some attribute of the still, often a greater surface area of the wash, is believed to make the vaporization of the wash "easier". While it's never stated clearly, the impression given is that if vaporization is 'easier', it will take less energy to vaporize a given amount of wash.

Sad to say, the Easy Boiling Myth smells of the same animal that gave us the Magic Boiling Myth. Some very strict physical laws predict exactly how much energy it takes to vaporize a given amount of any liquid that is already at its boiling point, and fiddling with still dimensions do not change those laws.