Three cuts of one run
The run on the chart is the spirit still from the front page, worked out to the end. The gold line is the strength of the spirit as it runs: high at first, then sliding down as the pot loses its alcohol. The two dashed lines show how much of the unwanted compounds the stream carries compared with the charge as a whole.
Heads, or foreshots
The first few minutes bring the most volatile substances: acetaldehyde (green apple, raw and pungent) and ethyl acetate (pear drops and nail varnish), together with whatever the still has rinsed out of its own pipework. They are not poisonous in these amounts, but they make a spirit hot and solventy, so the stillman lets them run to a separate receiver.
Heart, or middle cut
Then the spirit is clean: fruity, sweet and strong. In a malt whisky distillery the heart typically runs from about 75% down to somewhere around 60–65%. Where exactly to stop is house style: cut early for a light, delicate spirit; cut late for an oily, heavy one with more character and more yield.
Tails, or feints
As the strength falls, the heavier compounds grow: fusel alcohols such as isoamyl alcohol, fatty acids and their esters, giving oily, cardboard, sweaty notes. The stillman swings the spout back to the feints receiver and runs the still on until almost no alcohol is left.
Nothing is wasted. Heads and tails go back into the next charge with the next low wines, so the alcohol in them gets another chance to come over in a heart.
Boiling point is not the whole story
It is tempting to read the run as a simple ladder of boiling points, and for the lightest compounds that works. But look at the ladder below: ethyl acetate boils at 77 °C, practically with ethanol, yet it rushes out with the heads; isoamyl alcohol boils at 131 °C, far above water, yet it does not wait for the end of the run. How eagerly a molecule leaves a liquid depends on its company. Oily molecules are pushed out of water strongly, so in a strong, early spirit they stay put, and in the weak, watery pot at the end of a run they come out easily. That is why fusel oils arrive in the tails, not after them.
Boiling points at sea level of some compounds found in a spirit run. Where they come over in a still depends on more than this scale.
The truth about methanol
Folklore says the first spirit from a still is methanol, the poisonous wood alcohol, and that throwing away the first cupful makes moonshine safe. Both halves are wrong. Methanol mixes completely with water and ethanol, so it comes over through the whole run, somewhat richer at the start and the end but never cleanly in the heads. And a still cannot make methanol: it can only pass on what the wash already holds.
Methanol comes mostly from pectin, the gel of fruit skins and pulp, which releases it as the fruit ferments. Grain washes make very little; fruit brandies make the most, which is why the law caps it: in the EU at most 1,000 grams per hectolitre of pure alcohol for most fruit spirits (a little more for plum, pear and quince), 200 g/hl for wine spirit, 10 g/hl for vodka. The mass poisonings that make the news nearly always come from industrial methanol added to bootleg drinks, not from distilling itself.
Not a recipeThis site explains how distilling works; it does not teach anyone to do it. In many countries making spirits without a licence is illegal, even for your own use. And it is genuinely dangerous: alcohol vapour catches fire easily (pure ethanol can be ignited at about 13 °C, a 40% spirit at about 26 °C), and a leaking still beside a flame is how home distillers get burned. Distilleries are built around fire and explosion rules for good reason.