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During distillation different alcohols are boiled off at different temperatures, the impure alcohols before ethyl alcohol (drinking alcohol) are often referred to as ‘Heads’ and the impurities after are often referred to as ‘Tails”. The temperature drinking alcohol boils off at is 78C-82C, which is why we always try to run our still between these temperatures. Impurities like acetone and methanol have a boiling temperature which is a lot lower, and is why they are the first impurities to come out of your still, hence why you should always throw the first 150ml of alcohol (impure alcohols) that comes out of your still.
You then collect the ethyl alcohol that comes out between 78C-82C, switch off your still, and water down your alcohol to 40%. Now although you have kept you still at the desired temperature, impure alcohols do not stop and start directly at the temperatures they boil off at, there is a degree of ‘Tail’ impurities (eg: Propanol -1) that will be present in your alcohol. That’s where carbon comes in!
Any ‘Tails’ that have come through with your alcohol can now be removed by carbon treating. Carbon works like a sponge with many millions of tiny crevasse/holes that the impurities get stuck in, then once you remove the carbon, the alcohol is free from these undesirables. For a detailed scientific explanation on how carbon works see: “Carbon – An explantion on the how carbon works!“