Something to bear in mind: this is a description of private brewing. It doesn't necessarily reflect what commercial brewers were up to.
"To brew Porter in a Private Family.
Take eight bushels of porter-malt, or any other very high-dried brown malt. Let it be ground carefully, so as only to crack the grains, not to let out the flour. Lay it in a cool place two days and one night. Then set on a hogshead of soft water, and so much more as will allow for waste, according to the directions before given. This must be covered with a head of malt, to keep in the spirit; and, when it has once boiled up, the fire must be immediately damped, and about one third part of it must be let into the mash-tub. Then it is to stand till cool. It must be cooler than what is required in the common method of brewing ; and then the malt to be poured gradually in. While it is pouring in, it must be stirred very well about; and, when all is in, a person should work it still round and round, first one way and then another, for half an hour together ; but this must be done gently, not to bruise or break the malt. The water in the copper should be kept at a little more than the heat of that which is used for the mash in common brewing : and when the malt has been thus mashed a full half hour, there must be as much more let in as will make it in the whole something more than half the quantity of the water. This must all be very well stirred once together, and then covered with the malt that was left out for that purpose : it is then to be covered close up in the vessel to keep in the heat, and thus to stand two hours and a half.
Then bruise four pound of hops between the hands; and tie them up in a bag ; put them into the receiver or under-back, and let the wort run out upon them in a fine, small stream. When this is running, stir up the fire under the copper, and make the remaining water considerably hot ; then run it on upon the grains, when the other is nearly run off ; and, after stirring them well about, cover the mash-tub, and let them stand two hours more. Then run this second wort upon the first, with the hops still in it ; and let them standtill quite cold.
Then lay a cask a little above the bottom of the receiver; draw off the whole directfly, so as to leave the coarser that has settled behind; pour the wort into the copper, put in the hops With it ; and boil them about twenty minutes. Then let off the wort into the upper back or cooler ; in which let it stand till so cool that you can bear to put your hand in it : then draw it off (leaving again the sediment behind) into the other, or under cooler.
In this let it stand till only milkwarm, and then prepare for workings put into a bowl three pints of good and moderately thick yeast ; work this gently about with a little of the wort, and then put it into the tun, Let the wort out of the cooler run gradually into the tun, so as to blend with this, and to leave its own sediment behind.
A small quantity must be saved to fill up as it wastes in the working, and the full time allowed for this last fermentation in the barrel: then a little isinglass, dissolved as before directed, must be put into the cask, and a quart and half a pint of elder-berry. When these last ingredients are put in, the vessel is to be left with a little opening at the vent-hole two days, and then stopped up entirely. The rule for tapping is when it is fine : and that generally happens in about fifteen days. If it be then drank from the cask, it will be very bright, clear and pleasant, well-coloured, and of a good body. It will have all the flavour of porter; tho' not the sound and peculiar taste of what has been kept a considerable time in a large body ; which is the case with most of the porter that is drank at the famous houses in London.
The flavour which a mixture of elder-juice gives even in this small quantity, is truly that which we expect in fine old porter; and, what is very singular, it is of the same kind with that which porter gets by being, long kept in a large quantity. This must not appear wonderful ; for in chemistry, and even in the common affairs of life, we find the taste of peculiar things may be given to a mixture, by those which seems of a very different nature: in particular, the root of masterwort, with common fennel seed, gives its tincture the flavour of sassafras. Other instances might be given, which indeed are frequent, tho' they are not known. This may be sufficient.
The other great article of time, and Keeping in a body, is what a private family cannot have opportunity of doing ; and 'tis for that reason, and that only, the public brewed porter will always be superior. The brewers Of this liquor have, large caskS; in which it is kept two years and more : and in those it undergoes a last fermentation; which, as it is slight and slow, produces no other change than mellowing of the drink ; that, is a perfect mixture of the malt and hops: it lasts a long time, and consequently the effect is greater: in time, this last fermentation, perfect rest; and a cool air from good cellarage, produce a fineness and clean sound taste in this liquor ; which is what we admire and what is not to be found in any other; because the same degree of keeping in any other kind than a brown malt beer would soften it, but take off the spirit.
In the same manner, if a butt of porter be too mild, they will throw into it a small quantity of some that is very strong and too stale ; first dissolving in it a little isinglass. This produces a new tho' slight fermentation ; and the liquor, in eighteen or twenty days, fines down, and has the expected flavour. These, and many such advantages, none but the public brewers can have: and therefore none but they can brew this beer in that degree of perfection. We do not propose the brewing it in private families in London. But the extent of this enquiry into its nature is, that those who prefer this to other malt-liquors, and live in places where it cannot conveniently be bought, may brew it for themselves ; and that such as may intend to erect public breweries for it, may proceed with regularity. The construction of those large brew-houses, where it is usually made, favours also greatly the excellence of the drink: and this is the third article of which it was proposed to treat in this enquiry."
"The complete English Brewer", by George Watkin, 1773, pages 123-133.
I'm so busy. Still working on Barclay Perkins water chemistry, amongst other things. As well as a couple of grander projects. I would tell you more, but I don't want to spoil the surprise.