Distillation Basics: Boiling Points, Vapor, and Condensation

Distillation can sound mysterious until you strip it down. At its core, it’s a managed separation process: you heat a liquid mixture, create vapor, and then cool that vapor back into a stronger liquid.

Quick summary

Distillation in one sentence

Distilling is separating two liquids by boiling points—then managing that separation so you get the result you want.

So let's start with a little bit of distilling. We have two topics after this on still design that will dive very deep into how stills are designed and how you can take benefits from certain designs and what are the positives and negatives of certain designs. But I just want to start out with a really simple sketch on distilling.

Distilling is basically the separation of two liquids by boiling points. Doesn't sound very sexy, especially if you understand that you now, as the distiller, are the manager of the separation process of two liquids via their different boiling points. Let's just call you a distiller and let's call this machine a still or a prototype still.

The simplest still model

Start with a pot of wine (for example, ~11% alcohol). Add a heat source to boil it, a vapor path (riser/bridge) to guide the vapor away from the heat, and a condenser to cool vapor back into liquid.

For example this is a pot and we put wine in there. We're not going to mash, we're not going to ferment, we're just going to go to the shop, buy a few bottles of wine, pour them in a pot and we're going to create gases. Because if we create gases, we can actually take advantage of different boiling points, create a stronger wine, fortify the wine, make a brandy.

But in order to create gases, the first thing that we need is a heat source. We need to basically bring the wine to a boil so that all the energy that we put in is translated into gas formation, basically like boiling potatoes or spaghetti or macaroni. Bring the water to a boil, create a lot of gases.

Why vapor ABV is higher

Even if the boiler is only ~11% alcohol, the vapor can be much stronger (for example, ~40%) because the lower-boiling compounds transition into vapor more readily. You get an overrepresentation of alcohol in the vapor compared to the liquid you started with.

Now if you put wine in there, now if you put water in there, the only molecule if we put water in there is H2O. And the only gas we're going to create is water vapor, water gas. And as you are well aware from boiling potatoes or spaghetti in a pot, those vapors basically go anywhere.

So what you do in the kitchen is basically put a lid on it so that the amount of evaporation and the number, the amount of steam in your kitchen is manageable under control. Or maybe you have one of those vents on top of your stove that actually pulls the vapors out and makes sure that your kitchen isn't looking like you're driving into a dense fog. For example that wine is 11% strong in alcohol.

Why proof drops during a run

As you distill, alcohol leaves the boiler faster than water. That means the boiler drops from ~11% toward lower numbers—and the vapor strength drops with it. Early distillate might look like ~40%, while later distillate may be closer to ~30% or lower.

That still means there is 89% of water and there's 11% of alcohol. Now what I want you to understand is that the alcohol boiling point is around 78 degrees Celsius. And the water boiling point is around 100 degrees Celsius.

That basically means that the molecules, the alcohol molecules in the boiling, boil earlier than the water molecules. But since it's a mixture, everything will boil at the same time. Water would boil at 100 degrees, pure alcohol at 78, but since the wine is a mixture of both, the boiling temperature is going to be somewhat under 100 degrees, for example 95.

Why many spirits are distilled twice

A single distillation pass from a ~10%–11% wash can land you around ~30% overall—often not enough for traditional barrel entry strengths. So you strip first (collect low wines), then re-distill to reach higher strength (for example, ~70%–75%), and dilute to a target like ~60%–65% for aging.

If you want the process-map version of stripping vs finishing inside the full workflow, see stripping in the five-step process.

In the process of evaporation, because I want you to envision those alcohol molecules with their very low boiling point as lighter molecules, chemically that is not a correct definition, but please look at them like that. The water molecules are the heavy, the fat dudes. The alcohol ones are the tiny bits, the small ones, the lean characters.

What this translates to is the following. And even if we have only 11% of those alcohols in the wine that we are bringing to a boil, in the vapors, since they boil at a lower point, since it takes less energy to heat them up, to bring them to gas phase, in the gases we have an over-representation of those alcohols. If we have 11% in the boiler, the gases may well be 40%.