Dating back to the days of ancient Greeks and Romans, large clay jars have been used to store and age wine. I found a number of these clay vessels throughout Spain, repurposed as ornamental containers for plants in urban gardens.
The scale of these vessels is impressive, as can be seen in the image below, featuring my wife standing by some smaller examples.
Many of these jars were made in the town of Colmenar de Oreja, south of Madrid. At one point, there were 32 separate large-scale kilns in operation in Colmenar de Oreja, each producing enormous vessels for use in the Spanish wine-making industry. In the late 1950s, clay storage vessels were replaced with concrete containers at many wineries. Starting in the 1980s, wineries replaced concrete storage containers with stainless steel.
As interest in organic wine production methods increases, some wineries are returning to clay storage vessels in their production facilities, replacing or supplementing aging wine concrete and stainless steel holding tanks with clay. Winemakers say that clay, being porous, provides micro-oxygenation to wine during the aging period and unlike oak barrels, does not add aroma to the wine. Clay vessels also naturally protect the aging wine from temperature fluctuations. As a consequence, winemakers are able to achieve more fruity wines in clay aging vessels – sometimes a sought-after effect.
I found this website of a clay vessel vendor in Spain: Tinajas Orozco. From their website they sell historic clay vessels as well as smaller modern clay vessels. There is an interesting video chronicling the decline of the era of large vessel production traditionally done at this factory:
JTW Ceramics 
Interesting stuff. What a shame this tradition seems to be dying.
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