"The Kingdom of Hannover and the province of Saxony have been famous since the days of old for an unhopped Weissbier, whose inventor called it after his own name of Broyhan and this name has stayed with the drink to our days.Interesting that he describes it as unhopped. That wasn't always true. There were some versions with a small amount of hops.
Broyhan (sometimes also written Broihan) belongs to the pale, cooling, thirst-quenching, sweet-sourish tasting, non-intoxicating malt drinks. It is seldom clear, cannot be kept long, turns quickly sour in the summer . . . . ."
"Der Bier-Brauer als Meister in seinem Fache"by A.F. Zimmermann, 1842, page 91.
Here he discusses the grist:
"For brewing Broyhan are variously used one part wheat malt to two parts barley malt, the opposite proportions, equally in other breweries a third raw wheat and two thirds barley malt. In times when wheat is expensive, it is often left out and barley malt alone used, and in the present day, where starch syrup is available and cheap, this is also used along with barley malt and it produces a stable sweetness, which doesn't change so quickly, as does malt sugar, through fermentation into a wine flavour, so mostly replaces the wheat flavour. I've even succeeded in producing, using my own brewing process which I have demonstrated on pages 123 to 136, a wort from half raw potatoes and half barley malt from which Broyhan as well as other famous beers of extraordinary beauty and stability could be made and I will try in this treatise to give the necessary instructions to this purpose and thereby highlight a greater profit for the brewing industry."
"Der Bier-Brauer als Meister in seinem Fache" by A.F. Zimmermann, 1842, page 92.
Note that this is one of those Weissbiers that wasn't necessarily made from wheat. This is the older meaning of Weissbier, where it means a beer made from air-dried, pale-coloured malt.
Now for the recipe itself. The text doesn't specify which temperature scale is being used, but, based on the pitching temperature, it looks like celsius to me.
To brew 30 to 32 barrels (of 100 Berlin quarts) [3425 to 3664 litres] of Broyhan with a specific gravity of 1045 to 1050º from barley malt and starch syrup.
24 Berlin bushels of barley malt [+-690 kg]
220 Berlin pounds of yellow starch syrup [102 kg]
0.5 pound of Irish moss
Bring 720 Berlin quarts [824 litres] of water to the boil.
Let the water cool in the mash tun to 30º, then add the malt and stir well with brewing paddles.
While mashing, bring more water to boil in the kettle.
Once the mashing is completed slowly add 1680 Berlin quarts [1924 litres] of boiling water to the mash, mixing in well.
When this is complete, the mash should be 50 to 55º.
Transfer the entire mash to the kettle and bring to the boil.
Simmer for a few minutes then transfer back to the mash tun.
Leave to stand for half an hour then run off the wort into the mash kettle along with the potato syrup.
When the grain bed appears, add boiling water to cover it to a depth of 6 inches.
Continue to run off the wort and to cover the grain bed with 6 inches of water every time it appears.
When half the sparge water has been added this way, the remaining half can be added at once, taking care not to disturb the grain bed.
Boil until the wort breaks. Add the Irish moss to the boiling wort.
Quickly cool the wort in a shallow cooler.
Pitch the yeast at 15º in the summer, 17-18º in the winter.
Paraphrased from "Der Bier-Brauer als Meister in seinem Fache" by A.F. Zimmermann, 1842, pages 98 - 104.
That's quite a high gravity for Broyhan. It's another of the watery Northern German styles, that rarely poked its head above 2% ABV. It must be a Doppel-Broyhan.
I suppose that's a sort of simple decoction mash. It sounds fiddly to me.