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More Porter I'm afraid. I'd forgotten to finish off Faulkner's description of Black Beer. So here it is. In a nice indigestible form.

Despite supposedly being about the country method of brewing Black Beer, most of the text below is actually about "heading" or primings as they would be called today. Surprisingly, sugar wasn't the only material used. Some brewers preferring flour. Throwing some flour onto the wort had been practised since at least the 18th century, though the chemistry behind its efficacy wasn't understood. Something to do with the enzymes, if I recall correctly.

That country brewers had "no heavy demand for their black beers" is a good indication of Porter's decline outside London. This period is when regional brewers began to drop Porter from their range. In London Porter would struggle on for another 50 or 60 years.

"Country black beers.
I now come to the country as a whole, for outside London and Dublin the production of black beer is carried on in no very distinct manner; some brewers softening water, some using sugar, others employing malt-flour, and sugar solutions for heading purposes, and most falling back upon some definite preservative agent to prevent early deterioration. As a rule, country brewers have no very heavy demand for their black beers, and they have to brew them accordingly— i.e., if for immediate sale, and if prompt draught can be relied upon, country brewers imitate, to a certain extent, the example set them by Londoners, using sugar as a portion of the extract, raw sugar solution as the heading.

On the other hand, the majority, bound to produce an article of some stability, and one that will only come into condition after considerable storage, strictly adhere to entire malt brewings with low initial temperatures of mash, comparatively brief standing periods, fermentations progressing with free range of heat, racking their beer sometimes as high as third of original gravity. Finally, they employ some definite kind of heading, either introducing it at the racking stage, or at period of shipment. Many different varieties of heading have found favour, some of them being substances easily fermentable, others practically wort in a state of fermentation, or when in the state of dry flour forming, as we may suppose, the food of ferments.

Quite recently it has been suggested that flour only acts in the sense of being the store-house of so much air; but this view seems hardly correct in face of the act that the addition of flour to black beer undoubtedly leads to secondary fermentation, more or less prolonged in character, and I think there is no doubt that the crude albuminous matters of raw or malted grain become slowly modified into yeast-forming material when placed in a fluid undergoing fermentation.

The use of flour does not, however, commend itself to many, since it is apt to lead to the slow generation of gas in place of that high condition that is considered so essential, so the popular plan at the present time is to introduce a prepared solution of sugar, either perfectly quiescent or brought into a state of incipient fermentation a few hours previous to the shipment of the beer. This operation would be costly if the sugar so used was not taken into account when calculating original gravity; it is customary therefore to fix upon a certain quantity of sugar solution to be added per barrel, and then reducing the brewing gravity of the beer so that the final addition of sugar brings it up to standard.

The best variety of sugar to use seems to be either dextrin-maltose or some pure saccharine. A boiling-hot solution is made, cooled, and added to each cask, the ordinary quantity being some three gallons per barrel of a gravity corresponding to 1,150, those desiring very rapid condition inducing a quiet fermentation in the strong sugar solution by adding a small weight of yeast. It will be evident that such a solution requires constantly making afresh, and it is well even then to treat it with salicylic acid to prevent any deterioration. To admit of its use it is necessary to keep the black beer in stock more or less quiet, since it is not customary to add this form of dressing before the beer is required for use, very rapid fermentation immediately following its addition.

I need hardly say that if this heading has been treated with a little yeast, or if a little malt flour be added with it, it puts an end at once to all possibility of flatness, while the degree of condition that results may be increased or diminished at will by varying the quantity of sugar heading employed, or the proportion of flour or yeast that is added with it.
"The theory and practice of modern brewing" by Frank Faulkner, 1888, pages 268-270.
That figure of 1150 for the gravity of primings is just about spot on. At least for Fullers, whose logs list a gravity of 1145 for their primings.