Tawny Ports and full bodied, fruity Ports such as Fonseca BIN 27 do not require decanting, since they have been aged in cask or large wooden vats and filtered before bottling to remove any of the natural sediment there may have been in the wine. Simply open and these Port wines are ready for pouring.
Vintage Ports, on the other hand, only spend 2 years in wood before being bottled, unfiltered. Over time as the Vintage Port ages, the natural sediment in the wine will settle in the bottle, requiring that any Vintage Port be decanted prior to serving to remove this deposit.
Two Fonseca Port wines that have been aged for longer periods in wood, are Fonseca Late Bottled Vintage Unfiltered and Fonseca Crusted. Both Port wines are bottled without being filtered and will 'throw' a very light deposit. So light is the deposit in the Late Bottled Vintage, that decanting is not necessary, but the Fonseca Crusted Port would certainly benefit from being being decanted, as the deposit will be more evident.
HOW TO DECANT PORT WINE
If your chosen bottle of Port has been moved recently, ensure that you stand the bottle upright for several hours before decanting. This will help the sediment in the wine to settle a little and will make the decanting of the Port easier.
Before you start, have a clean decanter at the ready. Alternatively, you could also use a clean empty bottle or a jug. Without shaking the bottle too much, remove the cork seal around the neck of the bottle, wipe off any dust that may have accrued and carefully insert the corkscrew. Depending on the age of the Vintage, the cork may have been in the bottle for many years, so it is important to treat it with care and pull gently from the bottle, so as not to snap the cork.
Slowly pour the Port into the decanter, gently raising your arm so as not to agitate the sediment in the bottle. A funnel can be useful at this point, although not entirely necessary, and particularly with older bottles of Vintage Port which will naturally have more sediment, a small piece of muslin cloth can help to catch any deposit from entering your decanter.