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We recommend reading Making Moonshine - The Dummies' Guide prior to this article, as it provides a good overview of the topic of distillation. If you're already familiar with the basics, read on!
The Boiling Temperature of Ethanol
We get a lot of questions about distillation and temperature. If you've read anything about distilling you might have come across information stating that the boiling temperature of ethanol is 174 degrees Fahrenheit. However, this is only partially true. The boiling temperature of pure ethanol is 174 degrees F. The boiling temperature of ethanol in a wash, that is to say, ethanol mixed with water, is completely dependent on the ratio of ethanol to water. The more water there is in the solution, the higher the boiling temperature.
If you measure liquid temperature while distilling, you may have noticed that liquid temp can often be much higher than 174 before a still starts producing. Again, the boiling temperature of a solution that is 100% ethanol is 174 degrees. However, if you have a solution that is 50% ethanol and 50% water, the boiling temperature will be significantly higher. In fact, in a 50/50 solution of ethanol and water, the boiling (liquid) temperature of ethanol will be about 180 degrees.
Should a Still Start Producing Alcohol At 174 Degrees Fahrenheit?
One question we hear a lot is "Should I see alcohol coming out of my still once the temperature reaches 174 degrees F?" The answer to this question is "No, you should not." Why? As we mentioned, the boiling point of pure ethanol is 174 degrees F. Wash in a still is not pure ethanol. If it was, why would you be distilling it? A first run wash is generally never any stronger than 20% ethanol. The rest (80%) is water. More likely it's something like 10% alcohol (ethanol) and 90% water.
A solution that is 10% ethanol and 90% water is going to have a boiling temperature of about 197 degrees Fahrenheit. Did you catch that? A wash that only has a starting alcohol of 10% ethanol won't boil anywhere near 174. The temp will have to be much higher than that before you see alcohol coming out of the still. If you don't know how to determine the alcohol % of your wash, read our How to Use a Hydrometer article, because it'll tell you how to do it.
To determine the exact temperature at which ethanol will start boiling in your wash, check out this chart (Source: Craft of Whiskey Distilling by the American Distilling Institute). It shows both the liquid boiling temp of ethanol based on the concentration of ethanol in your solution. For example, in a wash with a starting alcohol of 20%, ethanol won't begin to boil until liquid temp reaches 190 degrees F. Also, keep in mind, the data on the chart above only applies if you're distilling at sea level! As you move up in elevation, boiling temperature decreases.
Should a Still Maintain a Constant Temperature During Distillation?
Another question we hear is "should I keep my still temp exactly 174 F during a distillation run?" The answer to that is "no, absolutely not." Why? Well the answer has a lot to do with the chart above. For example, if you start with a strong wash that has a starting alcohol of 20%, you can expect that ethanol will begin to boil out of the solution once liquid temp reaches about 190 F. As you let your still run and you extract alcohol from the wash, the wash alcohol % drops accordingly. Lets say that half way through the run you've pulled half of the alcohol out of the wash and have reduced the wash to 10% abv. Well, at that point you can expect liquid temp to have increased to about 198 F. The bottom line is that as your still runs, the temp constantly increases.
Where Should a Thermometer Be Installed on a Still?
We prefer to install a temperature probe in the boiler, at the very least. Always make sure you use a 100% copper adapter and a stainless steel thermometer. Adding secondary thermometer at the top of the column to measure vapor temperature is helpful too. I like having one in both locations as it makes the distilling a lot easier. The two thermometers read different temperatures during the run, but they are also measuring two different things. The boiler temperature is measuring the temperature of the liquid inside the still, while the vapor thermometer is measuring the temperature of the vapor inside the column.
Keep in mind that the temperature reading you get is highly dependent on where you are taking the reading. First of all, never use an infrared thermometer to try and measure still temp. You know, the laser pointer types. Don't use them. They aren't accurate. They measure surface temp of your still, not of the liquid or vapor inside. Also, if your still is polished and has a shiny surface, the signal could be bouncing off and giving you reading of ambient surfaces.
Should Vapor Temperature and Wash Temperature be the Same?
Vapor temp and wash temperature should be very different. Once vapor begins to form in the pot and it migrates up the column, the vapor temperature probe at the top of the column (if you have one installed there) will rise from ambient temperature to 175 in a matter of a minute. Hypothetically speakingm at this point the boiler thermometer may read something like 195 F (again, depending on your starting alcohol) and the vapor probe could read as low as 175 F. There will always be a big difference between the two temperatures (boiler and vapor) which is not a big deal.
How to Use Temperature During Distilling
Temperature is mostly helpful determining when to seal the still, to know when it's about to start producing, and to know when it's about done producing. As far as making good product goes, we still think the most reliable method is to adjust heat based on the amount of product coming out of the still. You're looking for steady, fast dripping, not a stream. Also, keep an eye on proof. If your proof is super low at the beginning of a run, you either have very low starting alcohol, or your running the still too hot. We use our parrot kit to constantly monitor the proof of the product coming out of the still.
We also like to make notes in our logbook so we can reference them then during the next batch. We make notes on taste, smell, temperature, how fast the still is producing, how it feels (is it oily or not), and when we make our cuts. This all helps when we try to repeat a batch that was outstanding... It also helps us figure out what we did wrong, if the batch wasn't up to our standards.