Bual (or Boal) is a grape used in the production of medium-rich fortified wines that come from the island of Madeira. According to the Geisenheim DNA Database the name is a synonym for each of four, or possibly more, white wine-producing varieties. The name is indiscriminately applied to any one of several varieties including Codega, Dõna Branco, Rabigato, or Portugal and Spain where the fruit is used in the same way for fortified wines.
Of the four styles of Madeira, Bual is on the sweeter end of the spectrum.
The high acidity in the Bualgrape combined with Maderization, a process in which the wine is fortified and then slowly cooked in barrel over a period of many years, allows the wine to acheive incredible longevity.
BualMadeiras from the 1800s are still in good condition and some examples from the 17th century though rare are not unheard of.