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Barclay Perkins were not alone in considering blending the defining feature of Irish Stout. Frank Faulkner, writing in the 1880's, concurred. His description of the blending process is well worth reporducing here.

"I suppose that Dublin porter, especially that produced by one firm, stands out preeminently as a special and distinct type of beer, and just as Mr. Steele, in his work on brewing, asserted that the whole success of the original London porter brewer turned upon the adoption of deep and closed-in coppers, so much of the distinctive character of the Dublin porter spoken of depends undoubtedly upon the immense capacity of the store vats.

According to Mr. Steele's idea, restricted convection and increased temperature of ebullition decide more perfect caramelisation of wort extract; and, on the other hand, depth during storage determines definite fermentative changes and the development of special varieties of alcoholic ferments. Now let me, in full view of these two distinct influences, which depend altogether upon plant arrangement, and which have been undoubtedly accidentally adopted, describe the main features of the Irish process.

First of all, there is the undoubted softness of water, that used in Dublin being a mixture of mere canal Dublin water, and river water, the organic impurity of which is counterbalanced, comparatively, by the antiseptic nature of wort constituents; then follows the selection of material, there being but little diversity of opinion as to what proportion of the different descriptions are most suited to the special flavour requisite, while as a general rule the Dublin brewers strictly adhere to exceptionally good pale and black malts, the proportion varying from 90 to 94 of so-called pale, according to its exact colour yield, and the balance of black. This is a very simple matter a mere question of colour ; and all I want to point out is that brown, amber, or crystalline malts are not used in this centre.

There is little to say about the actual mashing process, boiling, or collection of wort.

Invariably using good material there is no necessity for stewing or any of the careful manipulation that is usual when dealing with inferior malt, when employed for the production of pale beers. The wort is frequently, indeed, procured by making up lengths, the liquor of which has been sparged over goods at the boiling temperature, while enormous boiling quantities leading to extreme caramelisation are submitted to ebullition in closed coppers.

It is pretty clear, then, that by employing material of the nature referred to, mashing it at comparatively low temperature, and rapidly boiling off the collected worts, a dry extract is obtained, abnormally rich in dextrin, albuminous, and inert bodies, a combination which invariably ensures palate fulness; but it is not exactly the character of wort that has gained for Dublin the renown undoubtedly attaching to its black beers, this being due more to a peculiarity of flavour, partly acquired through the fermentation of immense bulks, the storage of similar bulks for prolonged periods in vats, and the very careful system of "blending" carried out.

For instance, a Dublin brewer, we will say, brews in the season large quantities of a high-gravity stout at 32 to 35 lbs. [1089 to 1097] saccharometer weight, this being stored in the immense vats mentioned for some twelve months, although it is possible that this period of storage varies according to the time when the matured flavour commences to develop, this matured heavy stout constituting, as I may describe it, the flavouring portion of the mixed or blended beer that is afterwards disposed of locally or exported.

Next we have the mild porter, brewed daily according to requirements upon the usual lines, but not finally vatted; and, thirdly, we have the "heading," which, in several of the breweries, consists of a portion of very strong first wort partially fermented, say to half original gravity, and clarified to a definite extent by skimming; this prepared day by day, and employed, as I may express it, perfectly fresh, and in that exact condition of quietude that each brewer finds necessary.

There is nothing difficult in seeing that if these three distinct beers the one matured, the other mild and clean, and the third half-fermented, be mixed together in different proportions, we can secure a great many varying flavours, degrees of fulness, and tendency to early cask condition.

In other words, the required flavour and condition for the several trade outlets are arrived at, not by uniformity of intermixture, but by a perfectly distinct variation in the several percentages of vatted, mild, and heading descriptions. I have no hesitation in saying that the success of Messrs. Guinness depends on good material, great bulk during fermentation and vatting, and ingenious intermixture of different qualities of produce, whereby a perfectly uniform palate flavour is secured.

For instance, what would be the proceeding for a mere local demand? It might be summed up thus : A large and definite proportion of mild, a dash of matured, a heavy quantity of what is termed gyle or heading, and a warm racking room, to give the immediate cask condition; while, on the other hand, and to simply give the extreme case for export, the proportionate intermixture would be entirely different and the heading would sink to a minimum.
This general principle underlies the whole of the several processes common to Irish breweries, but I think it would be manifestly unfair to enter into further detail. Many people have attempted to imitate the treble intermixture process by substituting returned porter or old beer, which has been bought, for what I have described as the matured or vatted stout, and by using, for heading purposes, actual fermenting wort or simple malt flour, but in every case complete failure has resulted. The old, or returned beer is different in every way to stout matured under pressure, while the fermenting wort or the malt flour, however vigorous they may be in the direction of inducing condition, are in no sense comparable with a strong wort partially fermented; since, in one case, we are introducing developing yeast wholesale, in the other merely wort with high fermentative capacity, freed from excess of yeast-forming matter by the semi-fermentation that it has passed through.

I do not think that the Irish brewers touch sugar at all; it certainly would not answer for the special kind of beer produced by the one firm that exports alone 350,000 barrels a year, and I hold that, as this export does not represent tied trade in any sense, it practically means perfect system of production. In main, I have pointed out the principle on which such production depends, while there is no doubt that, theoretically speaking, it is both interesting and instructive.

I have often heard brewers extol the peculiar softness of their own stouts, but they have generally ended with the remark that, do what they will, they cannot procure the exact Dublin flavour. They will not, perhaps, wonder at this after reading the above description of the Irish process; and I again lay special stress upon pressure, grist proportion, and uniform blending of distinct quantities of three definite beers."
"The theory and practice of modern brewing" by Frank Faulkner, 1888, pages 260-264.
Though he only mentions them once by name, he's clearly discussing Guinness and their methods.

The Guinness grist, as I've said many times, was different to that used in London, consisting of just pale and black malt. But, if we're to believe Faulkner, it wasn't this that set it aprt from English beers. The large volume in the fermenters, vatting in large quantities and careful blending that lent it a unique character.

If you think about it, there's a certain irony there. Large volumes, ageing and blending: they were are all important in London Porter brewing until around 1850 or so. The success of Guinness demonstrates that there was still a considerable market for partially-aged beer, even though the process had been largely abandonned by English brewers.

What surprised me about the Guinness process was the third beer, or "heading". Never come across something like that before. It's not clear exactly when this was blended in. Perhaps at racking time to serve as a sort of priming.

Given the failure of other English brewers to emulate Guinness, I wonder if Barclay Perkins had any luck with their Irish Stout type?