The system is designed to smooth out the differences between vintage years and is effectively a more subtle, and very much more labour-intensive, version of the blending of inexpensive table wines between one vintage and another, although the solera system concerns barrel-aged liquids and is made up of several different scales. Depending on market demand, a fraction of wine is removed from the oldest scale of the solera, the so-called solera barrels themselves, and replaced (although the barrels are never filled completely) with wine from the next scale of barrels containing wine of the same type but one year younger, the so-called first criadera. They in turn are replenished from the scale two years younger (the second criadera) and so on, the youngest scale being replenished with new wine. This system is particularly useful for flor wines because it refreshes each barrel with younger wine and provides micronutrients to sustain the flor yeast for several years. It takes several years’ operation for a solera to reach an equilibrium average age. Many soleras in Jerez were started decades ago and, since no barrel is ever emptied, there is always some of the oldest wine in the final blend. Fewer scales are needed to produce a consistent amontillado or oloroso sherry than a fino or manzanilla sherry because these fuller, richer wines vary less from year to year.