The distillation rate is controlled by the degree of heat applied to the still, and this is readily adjustable. Steam heating is usual, with a boiler (oil or gas-fired) heating water to produce steam conducted through pipes coiled in the base of the still. Increasing the flow rate of steam accelerates the distillation rate.
“Steam flow is manually controlled using a small wheel on the console, similar to adjusting the volume on a radio, and is very controllable from 0-100 per cent. When adjusting the flow rate the new temperature is established within about five minutes,” says Stuart Robertson, Dalmore’s distillery manager.
Meanwhile, “At Knockdhu the flow of steam is manually controlled using the equivalent of a steering wheel, to open and close the valve. At Speyburn the flow rate is operated and adjusted using a computer program, which is in turn monitored by an operator,” says Derek Sinclair, distilleries general manager, Inver House Distillers.
The more traditional heating method is direct-firing, using a naked flame beneath the still. The few distilleries using this method include Glenfarclas and Springbank.
“You need to be trained in a different way to operate a direct-fired still, it’s a different challenge to steam-heating. The size of the flame can be adjusted by turning a knob, but direct-firing takes longer to cool down than a steam coil, and can take up to 10 minutes for any change to be established, so you must think ahead all the time,” says Callum Fraser, distillery manager, Glenfarclas.
The first and second distillations require different approaches. When heating the charge (i.e. liquid being distilled) for the first distillation, the foam that forms needs to be addressed.
“Initially the charge is heated using maximum steam, with the still man monitoring the build up of foam. Once foam starts to rise up, which is visible through the sight window on the still, the steam is turned off completely. This allows time for the foam to settle and subside, otherwise maintaining the heat poses a risk of foam rising up the neck and reaching the condenser, which would foul the equipment,” says Derek Sinclair.
Once the foam has settled in the wash still the heat can be turned up again. Meanwhile, dealing with foam at Glenfarclas means monitoring the temperature within the still. This is measured by a probe and displayed on the exterior of the still.
“At a certain temperature we know when to turn down the flame from the maximum to about 0 per cent, and it takes about 25-30 minutes for the wash to settle. Then we gradually increase the heat to about 50 per cent and maintain this level for the remainder of the run,” says Callum Fraser.
The second distillation divides into four stages: heating the charge, the heads and tails (the initial and concluding distillate, which has an inappropriate character and quality), and the spirit cut, collected separately as new make spirit for ageing.
“When heating the charge we gradually increase the steam to maximum, which takes up to an hour. As soon as spirit starts to come through, the steam is turned right down to about one-eighth of the total to collect the heads.
This flow rate of steam continues for the spirit cut, in order to maintain the traditional distillation rate,” says Findlay Ross, Springbank’s director of production.
“We have a slow distillation rate for the spirit cut, with the flame at about 4 per cent of the maximum, then we gradually increase this over 10-15 minutes up to the maximum to drive off the tails,” says Callum Fraser.
Pot stills require a particular type of accommodation when using direct-firing. At Glenfarclas the pot is positioned on a circular brick wall (a flue) that encloses the underlying area. This brick wall includes an inner layer of ‘fire bricks,’ which heat up and reflect heat back into the area under the pot, intensifying the effect of the flame, which is about a meter below the base of the still.
At Springbank the wash still is heated externally using direct-firing, but is also heated internally using steam.
“Oil fires the boiler to produce steam, but oil is also used to create a broad flame that covers the entire base of the still and is in contact with the copper. The size of the flame can be adjusted, from low to high, but remains on one setting for the entire distillation run. It’s the flow rate of steam which is adjusted as this provides most of the heat, though direct-firing creates particular conditions that contribute to the character of the new make spirit,” says Findlay Ross.