There is enough misinformation about the thump keg to need some explanation to help moonshiners know what their best options are as well as to clarify exactly what a thump keg is.

The “Thump Keg,” or “Thumper Keg,” or “doubler” is a second chamber or distiller in the classic hillbilly still that essentially allows the distiller to double distill their output a second time, without having to rerun it through another still.

They are used to produce higher alcohol content moonshine. Where a single still is capable of producing 40 – 50% alcohol. By running that still a second time, you can increase that percentage.

The “thumper” is an additional piece of equipment positioned between the kettle and condenser. As part of a moonshine still, it is used in place of a column. While it might be considered “old-fashioned,” the thump keg is going through a resurgence.

Modern thumpers do not make the characteristic noise and are nicknamed doublers, although they do not actually double the alcohol content. They simply perform as a double distiller producing the same purity and increased proof as if you had separately put the batch through a second distillation.

How a Thump Keg Got It’s Name

The thump keg got it’s name from the sound made while you’re distilling. Most people thought the sound came from lumps or clumps of mash from your first chamber that enter the distilling column along with the vapor, make their way through, and land in the second keg, making a “thump” sound.

In actuality, it is the hot vapor erupting out of the pipe that periodically makes this noise. The low wine mixture is continuously heated by this hot vapor until it reaches the boiling point of alcohol. In the thump keg, this constitutes the second distillation, which produces the higher-proof alcohol than that which could otherwise be produced in a single run through the pot still.

Generally associated with the backwoods hillbilly still, the thump keg represents one of the cleverest design elements of a traditional still. Iconic, perhaps, the thump keg is a very old aspect of still design having arrived in North America along with European settlers. It is clear the use of a doubler, or thump keg, was well-known among the early colonists.

How a Thump Keg Works

The main purpose of the thump keg is to compound the distillation process, which is the process of evaporation that occurs when alcohol turns to vapor as the liquid mash is boiled. Alcohol has a lower boiling point than water, so it will turn to steam and get collected before condensing back into liquid form.

The thump keg usually holds both water and mash, or tails. As the pot still is heated, it gives off ethanol vapors, which travel through the pipe into the thump keg where it bubbles up through the contents already contained therein.

This is where it gets interesting: The vapor condenses where it heats the cold liquid that is already in the thumper. Simultaneously, a heat exchange occurs in the thump keg where the vapor cools.

There will not be any distillate until the temperature gets high enough for the compounds to vaporize. This has everything to do with the bubble size, the depth of immersion and the volume of alcohol in both the vapor and the thumper liquid. If your thumper is wide at the base, you may find it more difficult to produce vapor. This is why you tend to see more cylindrical thumpers to better facilitate this heating process.

While the kettle and metal piping used in the still are typically made of copper, the thump keg can be made of copper, stainless steel or wood. It is advisable to avoid glass, however, as this can be catastrophically dangerous should the glass break. If the thump keg is too small, the contents will heat up quickly enough, however the yield will produce a low percentage of alcohol. This is why the thump keg is usually charged with a high-alcohol content liquid, such as the tails from the last run.

Using the Thump Keg to Infuse Flavor

The thump keg can serve a dual purpose in your moonshine production. Not only does it serve well to increase the pure alcohol percentage, but it can also add a purer infusion of flavor, if you desire to flavor your alcohol. While most shiners add fruit to the mash or add the flavor to the finished product, adding it to the thump keg will steam extract the essences along with the alcohol during this second distillation process.

For some shiners, this is a mountain secret. Fruit infusion is a way to impart maximum fruit flavor without compromising potency. It could be fruit, herbs or even spices that provide a fresher, full-flavored robust whiskey. A well-managed pot still including a thump keg is capable of delivering the right volume of compounds that will carry over into the final product.

To get the most out of your thump keg, you should have:

Plenty of vapor to liquid contact, this mean a lot of small bubbles whether you use a screen or have enough pinholes at the end of the inlet tube.
A reasonable percentage of alcohol charged in the thumper liquid whether that is a small volume of mash or, better yet, some tails from the last run. Some shiners do not even add any water and, instead, only use the tails or mash.
A larger thumper, rather than smaller. A good rule of thumb is a 2:1 ratio – make the thump keg volume twice as big as the pot still. For example, to create 1 liter of spirits, make the thumper contents equal to 2 liters.

Sometimes, shiners want to know if a thump keg will work with their moonshine still’s column. These two components are incongruous. You would use a thumper instead of a column. This can be confusing when a complete moonshine still includes both, but you just need to keep in mind that it is a one or the other situation with these two pieces of equipment.

Another common misconception is that the thump keg is the same as a slobber box, considering their similar appearance in some cases. First off, the inlet pipe on the slobber box does not extend into the liquid, so there would be no way for it to bubble meaning there would be no distillation occurring. The slobber box simply provides a place for any foam or mash to be deposited so as to avoid it reaching the condenser. Sometimes, there will be a drain cock to facilitate emptying while the still is operating. The whole aim of a slobber box is to manage the waste by avoiding it reaching and contaminating the main spirits that are being collected. On a well-managed still, a slobber box is not even needed.

Hopefully, this information has been helpful in giving you the knowledge to understand what is a thump keg. Remember, the best liquids to use in your thump keg first are the tails from a previous run for the additional alcohol. Second, you can use some of the liquid you are about to distill. While not giving you as much alcohol volume as the tails would, it is still better than water. Water should be your last resort only because you are not going to get the “double-distillation” as you will with the other two options.