Many questions arise about the need to decant a wine. It is often affiliated with aeration, which is a different subject (see Wine Knows aeration). The two however are often performed together and with good cause. The practice of decanting is to remove the wine from solids that as part of the natural aging process form from the polymerization of tannins as well as the cross between the pigments and acids in the wine. This causes, especially with older wines, a sludge at the bottom or on the side of the bottle. Decanting is merely the process of removing the liquid from the sludge.
The proper technique is to remove the bottle from your cellar 12 to 24 hours in advance and give it a swirl or a quick shake to loosen the sludge. Then you place the bottle in a cool place upright for the 12 to 24 hours so the sludge will settle to the bottom. Carefully open the cork especially, on an old wine, (see Wine Knows – recorking) and then pour into a decanter with a light behind the flow. Pour the wine on to the side of a decanter watching the clarity of the flow. As soon as the flow starts to darken or looks like it has a little sludge stop. At this point I like to take the remaining wine and pour it into a glass just to see what the sludge is like. Sometimes it is the best smelling glass of the bottle but usually the worse tasting or at least texturally the least desirable.
My father taught me a trick if you really need to skip half these steps and grab a bottle from the cellar for immediate decanting. Use a funnel and a coffee filter (because they are pre acid washed, any other paper leaves a paper character) and filter the wine into a clean vessel. You don’t get the same aeration but you do get the sludge out.