Q. What wines should be decanted?
A. Young tannic reds and old reds both benefit from decanting.
Young tannic red wines should be decanted to soften the wine and release the aromas. Splashing the wine through the air and mixing it with that oxygen softens the tannins in the wine. The term for this is opening up the wine.
Old red wines should be decanted ― after sitting upright for a number of hours ― by pouring very slowly and carefully to get the wine off its sediment. There's a process that involves a little bit of useful ceremony and a little bit of voodoo. Sommeliers set a decanter in front of a light source, it's often a candle. The candle may seem like voodoo but it allows the sommelier to see through the wine as it's being poured, to see where the sediment begins.
Most of us could stand to decant a lot more wines than we do. You don't need a fancy container for decanting; a clean mayonnaise jar will work just fine.
Wines that should be decanted:
Bordeaux, Cabernet Sauvignon, Merlot, Cabernet Franc, Petit Verdot, Bordeaux blends (sometime called Meritage blends), and sometimes even big, tight Syrahs.
Wines that should not be decanted:
Delicate red wines and most white wines.
You don't want to decant delicate reds because they will oxidize too fast. The oxygen will destroy the wine. It's a love hate relationship wine has with oxygen. Sometime you don't want it in there and sometimes you do. It can be a friend or foe.
Q. How long should a decanted wine stand before it's served?
A. The bigger structured the wine is, the longer it should sit. You may even notice a wine's taste improves if you pour it in a glass and swirl it occasionally while having a conversation. The Alban Vineyards Syrah Reva 2004 that we are drinking has changed drastically from being in a glass for just 20 minutes. It's a blend of Rhône grapes, Syrah, and Grenache. It's not incredibly tannic but it was fairly dense and needed to open up.
Some wines can be double decanted or decanted the day before serving.