A purifier is essentially a cylindrical copper vessel, typically around 1 metre in diameter and 1.5 metres long, located mid-way along the lye pipe (as it's generally known, though technically the lyne arm). The lye pipe conveys alcohol vapours from the top of the pot still to the condenser.
Vapours typically enter the purifier at the mid-way point (rather than the top or base). The temperature in the purifier is relatively cooler than in the lye pipe leading to it, and significantly cooler than the neck of the still. A 'matching accessory' makes the most of this lower temperature.
"The purifiers at Scapa and Tormore are fitted with a baffle, a copper plate occupying about two-thirds of the surface area. Vapours entering the purifier meet the plate which forces the vapours to descend to the base of the purifier, where it is cooler, before ascending to the top and leaving through the lye pipe. Using a baffle exposes the vapours to the full range of temperatures in the purifier," says Alan Winchester, The Glenlivet master distiller at Chivas Brothers.
Purifiers can also include an element of 'temperature control.'
At Glen Grant, for example, cold water (ie. at the ambient temperature) is conducted through a sealed 'dome' on top of the purifier. This cools the exterior surface, and so simultaneously cools the corresponding interior surface of the purifier, which the alcohol vapours come into contact with.
Any change of temperature has a significant influence on the vapours within the purifier, just as any temperature change does throughout the distillation process.
For example, at the beginning of distillation when the charge (liquid being distilled) is heated, lighter flavour compounds such as certain esters (fruity notes) evaporate at lower temperatures because they are physically lighter. As distillation continues and the temperature rises, richer flavour compounds (including certain cereal notes) which are physically heavier, begin to evaporate.
As vapours ascend the neck of the still and continue along the lye pipe, they experience progressively cooler temperatures (the longer the neck and lye pipe the cooler it becomes). This results in some richer flavour compounds condensing, because the temperature isn't hot enough for them to remain in a vapour form, and they trickle back down the neck of the still. Consequently, vapours reaching the purifier (or condenser in the absence of a purifier) contain a higher proportion of lighter flavour compounds.
Similarly, when vapours experience cooler temperatures in a purifier, it's still warm enough to keep lighter flavour compounds in a vapour form, and these vapours exit and continue to the condenser. Meanwhile, cooler temperatures cause some richer flavour compounds to condense, and trickle down to a drain at the base. This leads to the 'return pipe' which returns the condensate to the pot still.
"The return pipe is connected to the shoulder of the pot still (ie. between the neck and the boil pot, which forms the base). A corresponding pipe inside the still subsequently conducts this condensate to the boil pot where it joins the remaining liquid and is redistilled," says Richard Forsyth, chairman of Forsyths, which provides distillery design, installation and maintenance.
The essential question is, what influence do purifiers have?
"When the purifiers were installed at Glen Grant in the late 1870s by James Grant, the son of the founder, there weren't of course any labs that could quantify the difference this made. But the purifier does make an integral difference, making the character of Glen Grant's new-make spirit more elegant and more focused on fruity notes," says Dennis Malcolm, Glen Grant's master distiller.
Meanwhile, it seems purifiers will remain a rarity.
"Clients who commission Forsyths to design a new distillery do ask: should we have a purifier or not? But few do," says Richard Forsyth.
The Question of Copper
The purifier also provides another opportunity for the alcohol vapours to interact with copper, and typically the greater the contact with copper the more elegant the resulting spirit, while less contact promotes a fuller-bodied spirit. This is because various reactions occur each time vapours are in contact with copper. For example, copper absorbs sulphur compounds from the vapours. Sulphur compounds include
rubbery, meaty and vegetal notes, which are only present in tiny quantities, but are very assertive and 'mask' other characteristics. Consequently, lowering the level of sulphur compounds 'reveals' lighter notes such as esters (fruityness) and sweetness.
Meanwhile, purifiers are subject to regular checks: "We test the thickness of the copper annually, which gradually decreases as a natural consequence of the distillation process. The original thickness is 4mm and once it reaches 1.5mm it's time to replace it," says Richard Forsyth, chairman, Forsyths, which provides distilleries with comprehensive maintenance.