You're probably thinking "I know how Bass fermented. They used unions." As always, the truth is slightly more complicated. But take my word for it. Listen to the words of someone who went around the brewery in the 1880's: Alfred Barnard.
"Thither we went, and found ourselves in a lofty and light chamber of similar dimensions to the cooling department, called the fermenting square room. It contains 117 square wooden vessels, each holding forty-five barrels, and all as clean as a new pin. In the middle of each is placed a tall mixing tunner or spreader, the shape of a doctor's stethescope, through which the added liquor is allowed to flow, so as to mix it well with the liquor which has first been run in. Here it is that the yeast is added, and the wonderful change of wort into beer takes place. This is a delicate process, as there are certain leading features to be distinguished by the watchful brewer, for, although he may have been successful in all the preceding stages, yet. should he fail in this, the advantage obtained will be of little avail. At the corner of this room, as usual, there is a capital office for the use of the Excise Officers, for it is here that their duties, as described in our first article, commence.
On returning to the staircase, we noticed on either side of the fermenting room a gas engine, which our guide informed us was used for hoisting yeast the union room below, and, on descending a few stairs we came to an intermediate low-pitched chamber, the floor of which is covered with asphalte and possesses side drains to run off the water that falls through the upper floor when the fermenting vessels placed therein are cleaned. In the centre, is the large receiving vessel, which acts as an intermediary between the fermenting squares and the union vessels, and the remainder of the floor is used for cleansing pipes and sundry other purposes. As there was nothing else that claimed our attention here, we proceeded to the floor below, which is another spacious and lofty apartment containing 640 union casks, each union holding 160 gallons, in which the fermentation of the liquor, now "beer," is completed. The ale from the fermenting squares is conducted to the union room through a series of pipes which discharge it into a long trough over the union casks, passing it from thence into the casks, by what are called union pipes ; and, fixed in the top of each cask is another pipe, called the swan neck, through which the barm passes or works itself from the cask to the top of the barm trough.
"Noted breweries of Great Britain and Ireland, Vol. I" by Alfred Barnard, 1889, pages 66-68.
If you could be arsed to read that, you'll note that the fermentation began in open squares, not unions. The unions weren't used so much for fermentation as for cleansing. The final phase, when the yeast was removed. Brewers were obsessed with cleansing and had many different ways of achieving it. Like the dropping system used at Fullers. Or the pontoes used in many London breweries.
I've always had a rather romantic view of unions. These wonderful, if expensive, devices for producing authentic Burton Pale Ale. Until John Bexon, head brewer of Greene King, who formerly worked at Bass in Burton, put me right. "They were impossible to clean properly," he said "and were a constant risk of infection." Makes it easier to understand why breweries ditched them for more conventional methods. The surprise is that Bass kept them for so long, until the 1980's.