Vodka production: Pot still finishing
Pot stills are credited with having a discernible rounding effect and adding character, so it is no coincidence that many 'boutique' vodkas such as Ketel One include pot still finishing as part of their production process.
Distilling wash (beer) in a pot still produces a liquid with an alcohol content of around 21% alcohol by volume. Subsequent pot still distillations can be used to increase the strength of the distillate to 70% alcohol by volume and this is exactly how malt whisky is made to this day. Vodka was also once made in this way but 70% alcohol means there are 30% impurities and while that is OK, even desirable in a spirit that will be mellowed by ageing for a minimum of four years in oak casks, this process would produce pretty rough vodka.
Modern fractional distillation in a column still can produce alcohol up to 95.6% alcohol and in an age where vodkas are praised for their 'cleanness' and 'purity', so column stills of one type or another are usually employed in vodka production. In early vodka production, prior to the invention of the column still, various methods of filtration were employed in an attempt to further purify the distillate.
Thankfully, column distillation has not completely displaced the pot still in modern vodka distillation and some of the most premium vodkas, most notably Ketel One, use pot stills in their production process. Rather than start with a pot still, these vodkas use column still made spirit which is redistilled in a pot still.
The wash is distilled in column stills using fractional distillation to produce a very pure 'neutral spirit' at 95.6% alc./vol. This is diluted back to around 50% alcohol by volume using purified or spring water and then redistilled in the pot still. Sometimes very small quantities of botanicals are added to the pot still to add a slight flavouring to the vodka.
Finishing an already very pure distillate in a copper pot still helps produce a softer, some say 'smoother' distillate. The copper the stills are made from plays a chemical part in this process as it acts as a catalyst to promote the formation of esters which impart desirable fruity notes to the spirit. Copper also reacts sacrificially to remove unwanted sulphur compounds - those are the ones that smell of struck matches, drains, rotten eggs, farts and cabbage - none of which make for an appealing vodka.
Copper helps turn these nasty smelling sulphur compounds into easily removed Copper Sulphate (U.S. Copper Sulfate) a copper salt (CuSO4•5H2O). Copper Sulphate (when in its pentahydrate form) is bright blue and when visiting a distillery blue deposits can often be seen on the flow pipe from the condenser (or inside spirit safe). The stills are cleaned with mild caustic solutions every now and then to remove the sulphates.
Pot still finishing will produce a discernibly heavier spirit so vodkas such as Ketel One which employ pot still finishing tend only to a small proportion of this spirit which they blend with more neutral alcohol so that the pot still affect is not too heavy.