There are two methods of heating the stills. Direct firing means burning gas or coal to create flames that heat the exterior of the pot (ie. base of the still), while indirect heating means conducting steam through tubes arranged within the pot.
Choosing either method involves practical considerations, such as differing maintenance costs, and the heating method can also influence the character of the new make spirit.
Direct firing was the traditional method, though most malt whisky distilleries in Scotland switched to indirect heating between the early 1960s and mid 1980s. This requires an oil or gas fired boiler to produce steam, which is conducted along tubular copper or stainless steel coils. A set of four or five coils fixed around 30cm from the interior surface of the pot is typical, though coils can also be tailored to the shape of the still. Another option is fitting stainless steel cylinders known as ‘kettles’ (also termed ‘pans’ and ‘percolators’). Again, four or five are typically used to heat a still, with small pipes branching off a central coil conveying steam to each kettle. Whichever option is used the heating elements must remain ‘submerged’ below the surface of the liquid being distilled.
Steam provides the gentlest, most uniform build-up of heat, and opening or closing a valve (from the convenience of a control panel) is all that’s required to adjust the flow of steam. Consequently, steam is considered easier to control, and provides a more immediate change of temperature than direct firing using gas or coal.
“Controlling gas means using a dial to adjust the gas supply to the fire, which varies the size of the flames issuing from a circular ring under the pot,” says Ian McWilliam, formerly assistant distillery manager and now marketing executive at Glenfarclas. “The pot ‘sits’ on a circular brick wall which follows the outer perimeter of the pot, and so encloses the area beneath the pot. This brick wall also includes an inner layer of very dense ‘fire bricks,’ which heat up and reflect heat back into the area under the pot, intensifying the effect of the flames. Consequently, even when turning down the gas, the fire bricks continue providing residual heat. So, to reduce the temperature more rapidly we can use a ‘cold blast’ fan to blow in air drawn from outside the brick wall.
Coal is considered even more challenging to control than gas. The options include increasing or decreasing the rate at which coal is fed into the furnace, or opening the doors of the furnace, allowing more air in to cool the fire.
Using direct firing also entails a higher risk of solids within the wash, such as yeast cells, burning onto the wash still (ie. during the first distillation). As these solids can caramelise they can also affect the character of the new make spirit. This is prevented by installing rummagers, essentially lengths of copper chain mail around 30cm wide. Attached to motorised arms within the still, rummagers are dragged across the base to prevent any solids from adhering.
Consequently, different heating methods pose varying challenges that require different skills and experience to control them. Additionally, direct firing and indirect heating have a reputation for producing different styles of spirit, but this doesn’t have to be the case. At Glenfiddich for example, one stillhouse uses steam heating while the other uses gas.
“We carried out various tests looking at different rates of distillation, as the rate of distillation has more impact on the character of the new make spirit than the heating method,” says Stuart Watts, Dufftown site leader, Wm Grant & Sons. “The gentler the heat applied to the still the slower the rate of distillation, and the lighter the resulting spirit. Similarly, the more intense the heat the faster the rate of distillation, which produces a richer spirit. So you can adjust the distillation rate to produce the style of new make spirit required, and both of Glenfiddich’s still houses produce the same spirit.”
There are also practical factors to consider. The ‘tech spec,’ and the maintenance of a pot using direct firing is significantly different to a steam heated pot.
“Direct firing can be very hard on the copper, which needs to be 16-20 mm thick compared to a steam heated pot which only needs to be around six mm thick,”says Richard Forsyth, chairman of Forsyths. “This additional copper makes the cost of a pot for direct firing significantly higher, and there’s also the cost of rummagers to add on. The heating method also determines the life cycle of a pot. This could be around 10 years when using direct firing, depending on how busy the distillery is, compared to 15 years with indirect heating.”