Obtain a piece of 40 mm pipe (1 1/2" PVC building grade pipe, a 1 1/2" ABS water pipe, a stainless steel tube, a copper tube for example), with a length of 1.5 metres. Using a stainless steel jubilee clip to close one end with 2 filter papers or 2 linen coffee filters. This end is the base. At the top, place a funnel. The seal between the funnel and the pipe must not leak. The funnel should preferably overlap the pipe. We now have a filter unit. The pipe should be filled with activated carbon through the funnel. The filter paper or linen at the base prevents the carbon from running out. The entire 1.5 metre length of the pipe should be filled. Fill carefully. Some types of activated carbon have such small particles that it cannot be settled very much. If the carbon is settled too tightly the tube will not pass a flow. Normally one can tap the pipe lightly to settle the carbon a tiny amount. If one taps too much the carbon must be removed and the filling sequence repeated. Usually the pipe remains unblocked with a grain size of 0.25-1 mm or 0.5-1 mm. If larger grain sizes are used the pipe must be tapped a great deal in order to settle the grains as much as possible. Despite this the spirit will almost run straight through without being properly purified (i.e. with grain sizes of 1-3 mm, 35 mm etc). If pulverised carbon is used the pipe will be immediately blocked.
Before the spirits are filtered it must be diluted to under 50%. At such lower alcohol concentrations the activated carbon can work much more effectively (with maximum efficiency at approximately 38-42% alcohol) and at the same time we have diluted the impurities to half the original concentration. The spirit should be diluted with boiled, softened, or preferably distilled water (not distilled water sold for topping up batteries as this is only purified for its intended purpose) to prevent the deposition of calcium in the finished spirit.
Then we filter the spirit through the tube. The procedure takes about one hour per litre of spirit. If a large funnel is used it need only be charged a couple of times and the filtering can look after itself, for example during the night. The pipe will normally manage 4-5 litres of spirit. When the carbon stops functioning as a filter it is soon noticed by the deterioration in the taste and odour of the spirit. If one is not satisfied with the results then re-fill the pipe half full and start again. Activated carbon is relatively cheap so it is better not to be mean with it and use its full capacity to the last drop. Even if there is still some capacity left in the carbon one can discard it without trepidation. Most suitably down the drain. When one filters spirit through a high layer of activated carbon a peculiar thing happens at the start. Spirit is lighter than water so the first to come out of the pipe is water. You have not been cheated, and the spirit has not been converted into water. It is just a natural physical phenomenon. Very soon afterwards spirit will issue from the pipe.
When all the spirit has been filtered one can pass a litre of water through the funnel. The water will drive out the spirit that remains in the pipe and spirit is collected until water emerges. The gain is quite a few centilitres.
If one want to purificate more then 4-5 liters one can use a tube with a larger diameter, but the same length.
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