This was a surprise. Finding a method of spontaneous fermentation in ione of the old English manuals. I'm not sure how common it was. It sounds more like a technique for private brewers than for commercial operations.
Levesque details a method of spontaneously fermenting Ale. After the boiling wort had broken "discharge the whole together, hops and all, into the cask in which the liquor is intended to be kept, and bung down, for the present, the cask then being quite full: at your leisure fix the safetly-valve, and there let the liquor remain untouched, to ferment and depurate, without any addition of yeast, which will require 12 months for ordinary ale. The vacuum caused in cooling, will furnish room for the expansion occasioned by this mode of spontaneous fermentation. The time required for fermentation and depuration will be from eighteen months to two years, for the strongets ales; or of a gravity of 45º, in a temperate cellar." (Source: "The Art of Brewing and Fermenting" by John Levesque, 1836, pages 46-47.)
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