There are a couple of points of note in the following text. Firstly, it confirms mine and Zythophile's distinction between London and Dublin Porter, namely the use of brown malt. Maybe one day I'll get to see some Irish brewing records to get this 100% straight. (Dolores seems quite keen on visiting Cork, where the Murphy's records have recently been made accessible to the public.)
Then there's the stuff about Mild. It's a pity he isn't more precise about the colour. What he says seems to back up my ideas about Mild becoming first an amber colour and only going properly dark in the early 20th century.
And finally there's the bit about using some wheat malt in Porter and Stout for head retention. I've not come across it brewing records of the period, but I know that post-WW II many Bitters contained a small amount of wheat for the same reason.
For pale ales, of course, only the palest malts can be used.
Mild ales are in most localities brewed of a higher colour than pale ales. When the requisite degree of colour can be obtained from malt dried at the ordinary temperature no coloured malt should be used, but when additional colour is required it may be obtained either by using amber malt, or a very much smaller quantity of black malt. For colouring ales really first-class black malt answers, I think, better than amber, and gives less empyreumatic flavour, but inferior qualities of black malt should never be used for this purpose.
Porter and stout are brewed in Dublin from high-dried pale malt and black malt only, but London brewers generally prefer a grist containing all the three qualities of coloured malt, viz.: amber, brown, and black, in addition to the pale malt. In the case of black beers, as in that of high-coloured ales, I think that if the black malt is only good enough, the amber and brown may be dispensed with, and an additional amount of black substituted for them.
When black malt only is used in brewing porter and stout, one of black by measure, to seven of pale, is sufficient for the blackest beers, and one of black to twelve of pale is about the smallest proportion used, even in Ireland, where the black beers are generally far less highly coloured than in London.
I may here mention that there is a great advantage in using a proportion of wheat malt, prepared by my process, to replace some of the ordinary pale malt, in black beers. A proportion of about ten to twenty per cent, of wheat answers well, and promotes that fullness of palate, and permanent creamy head so much admired by consumers of stout and porter."
Source: "A Systematic Handbook of Practical Brewing", by E.R. Southby, 1885, page 217 - 218.
There's a great section of brewing water in Southby. I'll post that when I've put all the details in a nice, neat table. The excitement never stops.