I'll go ahead and confirm Jeff's presumption that you cannot make a historic porter using 100% modern brown malt. Modern brown malt has no enzymatic activity, no diastatic power.Quite true. Since everything I know about historical British brewing I learned from Cornell and Pattinson, I went to them. Ron to the rescue:
Before the hydrometer revealed that pale malt was far more economical, Porter had been brewed from 100% brown malt. That in itself tells us that the brown malt of that time was very different stuff to that of the 19th century onwards. Later brown malt had no diastatic power. It didn't need to, because there were sufficient enzymes in the pale malt base. The purpose of brown malt in a brew had changed. It was just being used for flavour and colour, as Carr wrote.That earlier brown malts were highly dried, but not flashed with searing fire like the brown malts a century later. Moreover, their quality depended on the source of fuel. Again, via Pattinson, a contemporary observer:
"The brown Malt is the soonest and highest dryed of any, even till it is so hard, that it's difficult to bite some of its Corns asunder, and is often so crusted or burnt, that the farinous part loses a great deal of its essential Salts and vital Property"Hope that clarifies things.
"Brown Malts are dryed with Straw, Wood and Fern, etc. The straw-dryed is the best, but the wood sort has a most unnatural Taste, that few can bear with, but the necessitous, and those that are accustomed to its strong smoaky tang; yet it is much used in some of the Western Parts of England, and many thousand Quarters of this malt has been formerly used in London for brewing the Butt-keeping-beers with, and that because it sold for two shillings per Quarter cheaper than Straw-dryed Malt."
(London and Country Brewer, 1736)