Two good reasons to decant.

The usual mistake is to decant every red for the same length of time, out of habit. How long to decant wine depends on why you are doing it. For sediment, decant just before serving. For air, a young, tannic red may want one to two hours, tasted as you go. A fragile old wine may want no time in a decanter at all.

Get the reason right and everything else follows: whether to decant at all, which decanter to use and when to start. Get it wrong and a wine you waited thirty years to open can fade in the decanter before anyone tastes it.

Decanting for sediment

As red wine ages, tannins and colour pigments bind into ever larger particles until some of them drop out of the wine as sediment, which is one of the ways tannin softens with time. Vintage Port throws a heavy deposit, as do many older reds and wines bottled without filtration. Sediment is harmless but gritty and bitter. It also clouds the wine.

1. Stand the bottle upright for a day or longer, so the sediment slides to the bottom. A bottle that has just travelled needs longer.
2. Open it without shaking it. Old corks need care and a different tool; the method is in opening an old bottle.
3. Pour slowly and steadily over a light. A candle or small torch under the neck lets you watch the wine as it passes the shoulder. Keep the bottle moving in one direction, since tipping it back and forth stirs the sediment up.
4. Stop when the sediment reaches the neck. A little wine stays behind with the deposit, a small price for a clear glass.

Sediment on its own is no reason to wait. Decant, then serve, unless the wine also needs air.

How long to decant wine for air

The second reason is air. A young, tannic red, a powerful young wine or a white that smells of struck match straight from the bottle can all be better after time in a decanter. Air does not age a wine in an hour. It does let closed aromas open and reductive notes blow off. Many tasters also find the tannin of a young wine feels less severe.

Pulling the cork early and leaving the bottle to stand does far less than people expect, because the wine meets the air only across the narrow neck. If a wine needs air, decant it.

As a rough guide, give a young, tannic red one to two hours and treat that as a starting point.

• Taste when you decant. If the wine is already open and expressive, serve it.
• If it is tight and quiet, taste again every half hour or so. Barolo, young Bordeaux and Northern Rhône Syrah are the usual candidates for the longer end.
• Stop when it tastes the way you want. Air keeps working and a wine can go past its best in a decanter as well as in a bottle.

Young vintage Port needs both kinds of decanting at once. It has a deposit and it benefits from air, so it is often decanted a few hours before serving. The styles that throw a deposit are set out in our guide to Port.

A decanter also warms the wine. A wide surface of wine in an air-conditioned Hong Kong room at 22 to 24°C climbs out of its range faster than wine in a bottle, so give a red a few extra minutes in the fridge before decanting it. The ranges for each style are in the guide to serving temperature.

Fragile old wines

Old wines are the opposite case. After decades in bottle, a wine can be at its most beautiful in the first minutes after opening and fade quickly in air. The decanter that helps a young Barolo can finish an old one.

• Decant just before serving, into a narrow decanter, purely to leave the sediment behind.
• Or skip the decanter. With the bottle stood upright for a day or two, pour gently straight from it glass by glass. Stop before the sediment.

Very old vintage Port still needs decanting because its deposit is so heavy, but do it shortly before serving instead of hours ahead.

Taste every old bottle as soon as it is open. Old bottles vary and it is better to know before your guests do.

Decant old wines at the last moment and young ones well ahead.

Double decanting

Double decanting means decanting the wine, rinsing the bottle and pouring the wine back into it to serve. The wine gets its air and loses its sediment while the original label stays on the table. It is useful at dinners where guests like to see what they are drinking and in restaurants when you have brought your own bottle.

Rinse the bottle with water to clear the sediment, let it drain upside down for a few minutes and use a clean funnel to pour the wine back. The second pour adds a little more air, which a young wine will welcome.

The funnel earns its place again at the end of the evening. Pour any unfinished wine back into its bottle, stopper it and keep it in the fridge. It has already had plenty of air, so drink it sooner than a bottle that was never decanted; how long opened wine lasts gives the times by style.

Choosing a decanter

Shape follows purpose.

• A wide, flat-bottomed decanter spreads a young wine into a large surface of air. Use it for young, tannic reds.
• A narrow decanter or carafe limits air. Use it for old wines, where the only aim is to leave the sediment behind.
• No decanter at all suits the most fragile old wines, poured gently from the bottle.

Any clean glass jug does the job. Rinse decanters with hot water only, since detergent residue ends up in the wine. Smell the empty decanter before you pour and if it smells of anything at all, rinse it again.

What not to decant

• Most whites. They rarely need air and warm up quickly in a decanter. The exception is a young white with a struck-match note, which may clear with air.
• Most light reds. Beaujolais and most Pinot Noir have little tannin to soften and plenty of fragrance to lose.
• Most Champagne. The bubbles are the point. A few people decant a powerful or mature Champagne to open it up, which is a matter of taste.

Decanting will not rescue a faulty bottle either. A corked wine stays corked however long it breathes, so taste before you decant and read how to tell a corked wine if something seems off.

Timing a dinner

Work backwards from the moment each wine will be poured.

1. The day before. Stand any old bottle and any vintage Port upright.
2. A few hours ahead. Decant a young vintage Port.
3. One to two hours ahead. Open and taste the young reds. Decant the ones that are tight and leave the open ones in the bottle.
4. When the previous course is cleared. Decant the old reds or pour them gently from the bottle.
5. Before every pour. Taste first.

That schedule covers almost any dinner at home. If you would rather taste the difference than manage it, a vintages dinner puts young and old bottles on one table, each opened on its own schedule.