Denatured alcohol
Also known as: methylated spirits, surgical spirit, denaturat, Brennspiritus
The same ethanol as in rectified spirit, only deliberately contaminated so that it cannot be drunk. Denaturing exempts it from excise duty, and that is where the price difference comes from.
Regulations set out which denaturants may be used. It may be isopropyl alcohol, methyl ethyl ketone, a dye, and denatonium benzoate — a substance so extremely bitter that it is detected at concentrations measured in parts per billion. Methanol was once used too, which ended in fatal poisonings.
The question comes back: could it not be distilled out? It could not — and this is not a matter of equipment. The denaturants are chosen precisely so that their boiling points sit close to ethanol. Distillation separates substances by the difference in those boiling points, so where there is practically no difference, the method stops working. Some denaturants additionally form azeotropes with ethanol, which closes the matter.
The methods circulating online — activated carbon, milk, freezing, bread — remove the dye and some of the smell. They do not touch the denaturants. The result is worse than no result: the liquid looks clean and is still poisonous.
Denatured alcohol is for lighting fires, degreasing, cleaning glass and spirit burners. For nothing that goes near your mouth.