The Thump Keg What It Does and How It Does It!

Mid 1700's Caribbean Double Thump Keg Pot Still

      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. Indeed, some older European stills 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 having to run the distillate through the still twice.

 

Flow Diagram of Double Thump Keg Pot Still

    An ordinary pot still, without a thump keg, is capable of distilling a wash to 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, rum 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 “refining 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, refining still.

A Glass Double Thump Keg shows How Gases Heat and Boil Contents
 of the Keg, Distilling the  Contents 

     The thump keg 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, it exits the arm into the low wine that placed at the bottom of the thump keg.  The low wine vapors condense as the bubble into the keg making a thumping sound 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.  Many of the distillers will add another thump keg to the system allowing the vapor to distill a third time before entering the condenser to turn it back to liquid.

     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 cogeners, producing a bland, albeit strong, spirit. 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 spirit that indeed has tastes rather than being a neutral spirit like 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 that are less sharply pronounced and more of these 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.