Solera is a process for aging liquids such as wine, beer, vinegar, and brandy, by fractional blending in such a way that the finished product is a mixture of ages, with the average age gradually increasing as the process continues over many years. Solera means literally "on the ground" in Spanish, and it refers to the lower level of the set of barrels or other containers used in the process; the liquid is transferred from barrel to barrel, top to bottom, the oldest mixtures being in the barrel right "on the ground". Products which are often solera aged include Sherry, Madeira, Lillet, Port wine, Marsala, Mavrodafni, Muscat, and Muscadelle wines; Balsamic, Commandaria, and Sherry vinegars; Spanish brandy; beer; rums; and whiskies.
In the solera process, a succession of containers are filled with the product over a series of equal aging intervals (usually a year). One container is filled for each interval. At the end of the interval after the last container is filled, the oldest container in the solera is tapped for part of its content, which is bottled. Then that container is refilled from the next oldest container, and that one in succession from the second-oldest, down to the youngest container, which is refilled with new product. This procedure is repeated at the end of each aging interval. The transferred product mixes with the older product in the next barrel.