I was beginning to accept that I might never get answers to these questions. The archivist had suggested I try looking in the two histories of the brewery ("Guinness 1886-1939" by Dennison & MacDonagh and "Guiness's Brewery in the Irish Economy 1799-1876" by Lynch and Vaizey). As it happens, I own both. And neither has details of the compostion of their beers.
Along with a no smoking section, it was the bookstall at last weekend's Bokbier Festival that interested me most. I've told you my weakness for beer books. When we buy a new bookcase there won't be piles of them on the living room floor any more. At least for a while. The stall had a couple of Dutch brewery histories that I didn't own and a big glossy volume called "A Bottle of Guinness Please" by David Hughes. It looked like a compilation of old Guinness advertising from the cover. But you never know, so I flipped through its pages. I found a few of the thing I most love - tables of numbers - so reckoned it was worth 40 euros of my money.
It was only when I had chance to peruse it more dilligently at home that I noticed what was on pazge 71. The malt bills and hoping rates for all Guinness's beers in 1883. Words cannot convey my joy. Obviously someone had looked at the brewing records.
I'm a generous type. I don't keep this sort of stuff to myself.
It was sort of what I expected. Mostly pale malt with 5% roast malt for colour. Though I hadn't foreseen the 10% amber malt. Sure enough, no brown malt at all was used. The similarity of the grists for Porter, Stout and Export Stout didn't surprise me either. Breweries liked to keep things simple in the 19th century.
Some claim that the difference between Irish Porter and London Porter was the use of roast malt. As you'll see from the tables below, this doesn't appear to be the case.
To answer another of my questions, there's no significant difference between Guinness Porter and Stout, except that the latter was more heavily-hopped.