Cask filling is a noisy, repetitive business that requires precision, strength, concentration, and a few handy bits of kit. In the filling store, an empty cask is rolled over to the filling pump station and the warehouseman picks up the bung extractor. Its corkscrew end is centred over the closure in the bunghole and forcibly pumped up and down with a circular twist until the old bung is yanked out. On filling day, the distillery's tank will contain new make spirit that has been brought down to a strength of 63.5% ABV. This is an industry standard filling strength that facilitates easier reciprocal trading of casks between whisky companies. There are exceptions; if you are an independent company with no requirements for blending or any need to fill casks for others, then you can fill casks with the spirit at the strength that it comes off the stills (approximately 70% ABV). This saves money on casks in the long run, as well as being an ingenious way to have cask strength aged whiskies with surprisingly high ABVs to bottle later on.
Many distilleries use the Cask Master Filling System manufactured by J E Cockayne of Glasgow, kit they have been installing and developing for the whisky industry for over 30 years. The business end looks like a high performance straightened forecourt petrol pump. The steel nozzle is lowered through the bunghole and spirit is pumped in pneumatically until the cask is 'bung full'. Older systems made a satisfyingly loud, rhythmic sound that reverberated around the typically draughty filling store. There is a sensor to protect from overfilling, though it's not unusual for a final dribble to splash over the adjacent staves as the nozzle is removed. Modern pumping systems can deliver spirit at an impressive 400L/min and will adjust the filling rate as the level rises, enabling an American oak hogshead to be filled in under a minute.
For HM Customs and Excise purposes, it is vital that the volume of spirit is carefully recorded. The older systems incorporate a flowmeter with a row of white numbers on black dials, a little like an old-fashioned electricity meter, which is zeroed with each filling and records the volume inserted in litres. Additionally, it tracks the grand total filled by the system. The Flowlog calculates the litres of alcohol using the sprit strength and the final flowmeter volume readings. In the latest systems, Flowlog software keeps track of everything on the distillery computer system, but there are still plenty of classic industrial-looking models around if you know where to look.
Finally, a suitable bung is selected to match the size of bunghole. Poplar and alder are common woods for the job, circular cut with slightly conical walls. The warehouseman drops it into the hole, and with three sharp taps of the bung hammer, he ensures the cask is sealed tight. The filled cask is shifted to one side ready for transportation into the maturation warehouse, and the next empty cask is rolled into position.