While generally associated with the backwoods whiskey still, the thump keg, or “doubler”, is a very old design element that probably arrived with the early settlers and was incorporated into the stills they built on arriving in North America (Fig. 1). Indeed, some older European stills (fig. 2) made use of what appears to be chambers that functioned as thump kegs, so the principle was surely well known to colonists from the British Isles and the continent. The thump keg is one of the most clever and iconic design elements of the traditional hillbilly still whose purpose, briefly stated, is to distill the output of the pot still a second time, without actually having to run the distillate through the still twice.
Figure 1, Typical backwoods whiskey still. Smaller copper pot in center is thump keg.
An ordinary pot still, without a thump keg, is capable of distilling a wash to only a “low wine”, which will be about 40-50% alcohol. A second, or even a third, distillation is needed to achieve the high alcohol content necessary to make high-proof whiskey or other spirit. Most European distillers still use swan-neck pot stills, and will have both a “beer stripper” to distill the wash to the low-wine state, and a second “spirit still” to rectify the low wine to a high-proof spirit. In the hillbilly still, the thump keg serves the same purpose as this second, spirit still.
Figure 2, Old Ukrainian vodka still using what are apparently thump barrels
The thump keg, moreover, does this in a very clever manner, utilizing waste heat from the still pot for its function. Many shiners in fact prefer to use a wooden barrel for the thump keg, precisely because it loses less of this useful heat than would a metal one. As the hot vapor comes out of the still (Fig. 3), it exits the arm into the low wine that condenses in the bottom of the thump keg – indeed, it’s the thumping sound of the the vapor and condensed low wine (and not, as some sources assert, “bits of mash”) periodically erupting out of this pipe that creates the characteristic bumping noise giving this piece of equipment its name. This hot vapor continuously heats the low wine to the boiling point of alcohol, thus distilling it a second time, and producing a much higher-proof product than could otherwise be obtained in a single run through a pot still.
Figure 3, Diagram showing flow of vapor through still and thump keg
While offering obvious advantages over a simple pot still, how does the the thump keg compare to a more sophisticated reflux column still? For the backwoods moonshiner, of course, there’s the obvious advantage of material-on-hand. It’s much easier – and cheaper – to find a discarded wooden barrel than to purchase enough Raschig rings or copper scouring pads to fill a large fractionating column. On the other hand, a well-constructed column is capable of yielding near the theoretical maximum alcohol content, in excess of 95%, while also giving better separation between the ethyl alcohol and the esters and ketones in the heads or the heavy fusel alcohols in the tails.
Some, including traditional moonshiners and the connoisseurs of single-malt, pot-distilled Scotch whiskeys, would probably argue that this separation is a bit too good, and that the column strips out too many of these tasty (in moderation!) cogeners, producing a bland, albeit strong, whiskey. They would maintain that properly-managed pot still set up with a thump keg can deliver just the right amount of these compounds to create a full-flavored, robust whiskey that indeed tastes like whiskey, rather than like an aged vodka. To what extent this is true and how much is folklore is debatable, but it is inarguably the case that the flavor-changes between cuts will less sharply pronounced and that more of these “whiskey” flavors will be present in the final product. In the Discovery Channel series, “Moonshiners”, former shiner Tim Smith maintains that his old family recipe, made in a modern distillery and distilled using a column still just doesn’t taste the same, and even invests several thousand dollars to install a high-tech thump keg to replace the fractionating column.
However, if getting these traditional flavors in one’s own distillate is a concern to the user of an efficient fractionating-column still, by simply reducing the amount of reflux in his column, he can obtain results similar to those one would get using the pot still/thump keg setup. Finally, it should be added that a thump keg can be used to impart flavor in another manner as well: it can be used just like a gin basket. Fruit, herbs, or spices can be placed in the thumper where their essences are, in effect, steam-extracted along with the alcohol during this virtual second distillation. This will provide a fresher, fuller flavor than will simply adding these materials to the mash, since more of the volatile flavoring compounds will be carried over into the final distillate.
Here’s a video I found on Youtube that demonstrates a traditional copper pot still, thumper and worm set up.