Keep it secret, keep it safe.
In most distilleries there exists a piece of equipment that doesn't get enough credit, the Spirit Safe, also know as the tail box. This singular piece of equipment has a very rich history within the distillery operation. The Spirit Safe was invented by Septimus Fox to allow a Customs & Excise Officer to tax a distillery a fixed license fee of £10 plus a payment of two shillings and five pence per gallon. These tax men were to make sure that whisky was not illegally being siphoned off and sold (without first being taxed). Only in the last 36 years has a distiller been allowed to have the key to open the safe. However, the safe is so much more than just a way for the government to enforce the Excise Act of 1823. It allows the distiller to see the first amount of liquid to emerge from the spirit condenser. As the spirit remains heated at this stage, the undesirable foreshots (the first vapors to boil off during distillation) are directed from the spirit safe to the low wines and feints receiver for re-distillation. Gradually the heads will start to emerge and will be sent to the low wines and feints receiver for re-distillation as well; these are made up of Acetone, Acetaldehyde, and Acetate which have a very harsh and foul smell. "The hearts," made up primarily of ethanol and congeners that are responsible for most of the taste and aroma of distilled alcoholic beverages, will end up as whisky.
The distiller has to judge the right moment to redirect the flow of product to the spirit receiver instead of the low wines receiver (part of the "art" of distillation). This is also when the alcohol content is measured by a hydrometer in the Spirit Safe. If it falls to 75%, or when the emerging spirit no longer turns the water cloudy, it is conducted remotely inside the spirit safe. Once the alcohol content of the emerging liquid starts going to 70% to 60% alcohol by volume and continues gradually to fall during the run, it will become "the tails" - made up of propanol, butanol, and amyl alcohols. The distiller, again within the Spirit Safe, switches the flow from the spirit receiver back to the low wines and feints receiver for re-distillation. The precise point at when this switch takes place depends on the character of the whisky being produced. These Safes keep more than their fair share of distillers secrets, sturdy locks, and taxed products. Yet they also give us a small but vital window into the history and evolution of the spirits themselves! Sláinte!