As Jeff Rankert wrote above, I wrote an article in Zymurgy on this technique, and I believe I was the one to introduce the term "solera" for it. So far as I know, the term traditionally refers only to the blending of sherry in Jerez, Spain. That technique is somewhat different in that it uses many vessels.
My solera is simply a dedicated corney keg. I started it about 15 years ago simply to improve a flabby, all-pale malt strong ale with some dark old ale to give the blend some needed "bite" from higher bitterness and dark malts. I had no thought of continuing it. It worked, and over the next year or two (as I recall, I suppose I should dig out the article), I blended in other strong ales to keep it going.
After perhaps two years, it developed a nice sour tang, so I brewed new beers to capitalize on that - more or less in the old ale style - 1.065 or so, dark but not a porter, moderately hopped, maybe 45 IBU. At this point, I was thinking of Greene King's Olde Suffolk, which apparently has Walloon origins http://www.beerhunter.com/documents/19133-000646.html
When newly filled on top of perhaps a gallon or so of the old blend, this beer had a nice, mellow tang, and this increased over time. At its best, it had a wonderful winey character.
This article, and previous posts in HomeBrew Digest, inspired homebrewers and homebrew clubs to try it. At some point, clubs and commercial brewers started doing this in oak barrels. As Jeff wrote, our club began one four years ago with an English-style barleywine.
Regarding what style to use - I would suggest a minimum of OG 1.060, but beyond that, experiment.
My own solera has been rather neglected of late with about two quarts left untouched for maybe two years. I wonder if the microbes in it are still viable. I will brew an old ale this spring and top it up.
One of the homebrewers who read my article lives in Sweden, and sent me the following article, showing that this technique has migrated beyond its presumed Walloon roots.
JeffOLD WALLOON ALE
A ’CENTENARY ALE’, NOW TWO-HUNDRED YEARS OLD
THE OLD ALE IS:
-sour, with a rich aroma all its own
-of a reddish brown hue
-unfiltered and live
THE ALE IS BREWED WITH:
-a generous measure of Vienna malt
-wort that has simmered a good long while
--relatively little bitterness and hops aroma
OLD WALLOON ALE IS AGED IN OAK
-about two years
-in a temperate cellar(>14oC/66°F)
-and then tapped and replenished in a special way
Old Ale was brought to Sweden by Walloon immigrants recruited under royal privilege to develop the budding Swedish iron and steel industry. The first ”French”brewers to receive royal permission to brew ale in Sweden were Willem and Gillis DeBesche. That was during the reign of King Carl IX in the early 1600s. The tradition of brewing was carefully nurtured by the families who owned (virtually reigned) Sweden’s rural iron foundry estates until the time of the Great War, 1914-1918. Among the estates known for their quality Centenary Ales were Söderfors, Gysinge and Österbybruk. On display in the entry to the Gammel Tammen restaurant at Österbybruk is a hand-made bottle found in a cellar on the premises that bears the legend, ”Old Ale”.
Perhaps the most famous of these Old Ales was that brewed at Söderfors. The brew was started in 1794, but unfortunately has died out. An article in an Uppsala newspaper from the mid-1960s notes that a local brewery, BayerskaBryggeriet, maintained a handful of family casks at that time. The Geddacaskwould seem to be the last of these that is still going strong. The unique assortment of micro-organisms inside the cask has been the object of the Geddas’ tender care since 1860, when the cask
came into their possession. The brew dates back to 1806 and may, via the af Klercker and LeFeburefamilies, be traced to foundry estates at Gimoand Rånäs(both east of Uppsala). Jeanette LeFebure, grand-daughter of Jean LeFebure, owner of the two estates, married Fredrik af Klercker, who in 1860 gave the cask to GudeAdolf Gedda, his fellow officer in the regiment at Västerås.
Notes on the CARE AND FEEDING OF OLD ALE
The traditional Belgian brewing technique, introduced into Sweden by Walloon mining engineers and metallurgists in the 17th century has been preserved by the Gedda family in the family cask (photo above) through the years. The closest ’cousins’ to the Old Ale today are Belgian Oud Bruin (brown) and the West Flemish Red Ales. Mashing is performed in the usual way using Vienna malt, which lends the wort a reddish tone, accentuated by the long boil. OG varies between 1.055 and 1.060, with an FG of about 1.012, which yields an ABV of 6%. Newly brewed, the ale has a normal pH, but after two years in the oak the pHwill be 3.1.
Together, the long months in the oak and the special procedure for tapping and replenishing the cask with fresh brew preserve a unique flora of micro-organisms that give the ale its very special wine-like character. After two years’ ageing the brew is drawn off and bottled – preferably burgundy bottles (photo) –for distribution, enjoyment or, perhaps, further ageing. The cask, now half-empty, is replenished with new, primary ferment ale. Whereupon it is left to rest another two years until, once again, the procedure is repeated. This routine has been faithfully observed since 1806, a full 200 years!