Eilert Sundt (Oslo Museum: image no. OB.03176 (Byhistorisk samling), via oslobilder.no, CC BY SA)

The year is 1851. Sociologist Eilert Sundt is walking across a field in Hedmark, central Norway, when he notices a pile of stones. They catch his eye because they look peculiar. They're small, about the size of a fist, with obvious signs of burning, and they have been chipped and cracked somehow. He asks a farmer working nearby what the stones are.

"Brewing stones," says the farmer.

"Brewing stones?"

"Yes, boiling stones."

"Boiling stones?"

"Yes. They were used for boiling in the old days, when people didn't have metal kettles."[1]

As Sundt had noticed, these piles of burned and cracked stones were found on many farms in the area. Other farmers told him the stones were used for brewing in wooden vessels in the old days, by dumping hot stones into them. This is in essence the same technique reported from Finland, Estonia, Latvia, Lithuania and Russia. It's also the technique traditionally used for German steinbier.

As Sundt described, many of these piles of brewing stones had been broken up and used as fertilizer in the fields already in the 19th century. Even more have been flattened today, or even built over. In fact, many Norwegian farm buildings are built on a foundation of brewing stones. Very few remain untouched. Archaeologists have surveyed them, however, and found that there are two types of cooking remains[2]. The first is what's known as "cooking holes", where people have made a fire in a small hole, and put stones in the fire. Once the fire has burned out, food was placed among the stones, and the whole covered with turf. The remaining heat cooked the food without burning it.

The brewing stones are different, since these have been dropped into cool liquid. After being used a few times the shock of cooling has caused the stones to shatter, and the pieces have been discarded. It's these pieces that make up the heaps of brewing stones. This is a cumbersome way of heating liquid, but for wooden vessels placing the vessel directly over the fire is obviously not an alternative. It must be added that probably other liquids were also heated this way, although it's thought that in many cases smaller vessels of soapstone were used. These were too small for beer brewing, however.

In Norway, this method seems to have been in use from roughly the 7th century until the middle 16th century[2], when the introduction of copper and iron kettles displaced it. In exceptional cases it may have been used for another century or two. The same piles of stones can be found in the British Isles, too, where they were initially misinterpreted as burial remains or melting ovens, and the hollowed-out wooden logs used as vessels were thought to be canoes. It was only in the early 20th century that English archaeologists started reinterpreting the finds. Swedish archaeologists seem to have become aware of this in the 1950s, and gone through a similar reinterpretation[3].

So clearly this method of brewing has been in use over a huge area, and for at least a millennium. And if people have been using stones every time they brewed for such a long period of time, wouldn't the resulting piles be pretty big? Well, they are. The farm of Vik in Flatanger near Trondheim burned in the late 19th century, and was rebuilt elsewhere on the farm. In 1978, the farmer tried ploughing the area where the original buildings had been, and struck huge amounts of black earth and fire-shattered stones. This was found to be on average about 70cm deep, and circular in area, with a diameter of about 50 meters. That's more than 1300 cubic meters, which is a lot of rock. In another instance, Ranheim near Trondheim, only parts of the pile were removed, but that alone came to 700 cubic meters[2].

Curiously, most people seem to have assumed that the stones were used to boil the beer. That may seem reasonable, given that virtually all commercial beer is boiled, but that does not apply to the farmhouse ales, which are the direct descendants of the beers that were brewed with stones. Surprisingly many farmhouse ales are not boiled at all, merely heated for the mashing. This makes sense if they were originally brewed in wooden vessels with hot stones, where boiling for an hour or more becomes cumbersome and awkward. Obviously not boiling also saves time and fuel, even if you do have a metal kettle.

Copper brewing kettle, Voss

And indeed, in the Finnish tradition, hot stones were used in the mash. Hellenius described this in 1780[4], and Räsänen[5] later gives the same account in the 1970s. I cannot find any mention of Finns boiling with stones. In the 19th century the Russians appear to have boiled with stones[5]. In Estonia and Latvia stones were used in the mash, and the wort appears not have been boiled [5]. For Lithuania I have less information, but it's notable that in the single Lithuanian stone beer I am aware of (Moko Maukas from Dundulis) the stones were used in the mash.

In other words, it is far from obvious that the brewing stones were used for boiling. They may have been. It seems more likely, however, they were used for the mash, and that boiling the wort is an innovation that was introduced together with copper kettles. When this happened is not clear, but there is mention of copper brewing kettles in Voss in the 14th century. In medieval times, copper kettles were expensive and highly treasured items of equipment, so it's likely that it took a long time for all farms to acquire one. Which fits well with the continued use of brewing stones as late as the 16th century.

Steinbier is described as having been made by mashing in wooden tubs with hot stones[6]. Some sources describe it as subsequently having been boiled in a kettle, but some sources omit that part. It seems very likely that at some point in the past, there was no kettle, and south German and Austrian steinbier was a raw ale. I've also seen accounts farmhouse brewing of raw ale (apparently sometimes using hot stones) in Mecklenburg in northern Germany[5].

If you put all of these pieces together, an interesting picture emerges. It's beginning to look like raw ale may have been brewed with hot stones over much of northern Europe many centuries ago. There are at least indications of it for the British Isles, Scandinavia, Germany, the Baltics, and Russia. Probably there is much more evidence I simply haven't found yet. Being cumbersome and awkward, this method of brewing gradually died out. Today it survives only in a few places.

Apparently southern Germany and Austria are blessed with a type of stone called greywack, which does not split when used for boiling[6]. Very likely this is one reason why the use of brewing stones survived for so long in that relatively rich and modern part of the world. The only other place where I know it has survived until the present day is Finland, where Hollolan Hirvi makes a sahti with hot stones.

Norwegian farm, Sandøya, Møre og Romsdal.

Sources

I didn't want to interrupt the text with bibliographic references, but on the other hand I think readers will want to know where I got this information from. So here follows a list of the sources which were referenced in the text above.

[1] Lidt fra Oldtiden. I. Brygge-Sten. Eilert Sundt. Folkevennen volume 14, Kristiania, 1865, ISSN 0808-5161. (Scan from Oslo University Library, courtesy of Are Dag Gulbrandsen.)

[2] Bryggestein og kulturlag - spor etter gårdens opprinnelse, Geir Grønnesby, Spor, no 1, 2014, Museumsforlaget.

[3] Kokstenshögar, Karl Alfred Gustawsson, Fornvännen, 1949.

[4] Anmärkningar öfver finska allmogens bryggningssätt, Carl Niclas Hellenius, Åbo Academy, 1780.

[5] Vom Halm zum Fass: die volkstümlichen alkoholarmen Getreidegetränke in Finnland, Matti Räsänen, Kansatieteellinen arkisto, Helsinki, 1975. ISBN 9519056181.

[6] Oxford Companion to Beer, editor Garrett Oliver, Oxford University Press, 2011, ISBN 0195367138.