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
"Brewing stones," says the farmer.
"Yes, boiling stones."
"Yes. They were used for boiling in the old days, when people didn't have metal kettles."
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. 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.
Brewing stones. Photo: Åge Hojem, NTNU University Museum
In Norway, this method seems to have been in use from roughly the 7th century until the middle 16th century, 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.
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.
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, and Räsänen 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. In Estonia and Latvia stones were used in the mash, and the wort appears not have been boiled . 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. 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.
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. 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.
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.
 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.)
 Bryggestein og kulturlag - spor etter gårdens opprinnelse, Geir Grønnesby, Spor, no 1, 2014, Museumsforlaget.
 Kokstenshögar, Karl Alfred Gustawsson, Fornvännen, 1949.
 Anmärkningar öfver finska allmogens bryggningssätt, Carl Niclas Hellenius, Åbo Academy, 1780.
 Vom Halm zum Fass: die volkstümlichen alkoholarmen Getreidegetränke in Finnland, Matti Räsänen, Kansatieteellinen arkisto, Helsinki, 1975. ISBN 9519056181.
 Oxford Companion to Beer, editor Garrett Oliver, Oxford University Press, 2011, ISBN 0195367138.
It's only the last few centuries that metal kettles have become something that most people could afford to own
Read | 2016-12-18 12:47
Dave Pawson - 2014-12-10 03:27:54
You forget clay cooking pots? Until you can make them sufficiently strong, they cannot stand being in flame? Hence this method.
Also boy scouts - today, both the fire pit and hot stones to make a cup of tea!
Lars Marius - 2014-12-11 06:03:47
@Dave: I did forget those, but I think the conclusion is the same as with soapstone vessels. They're probably too small for brewing purposes.
Are Gulbrandsen - 2014-12-12 08:33:27
I'm currently working on a project at Humanities and Social Sciences library at the University of Oslo Library, and would be surprised if we didn't have the article on brewing stones by Eilert Sundt (reference 1). I'll se if I can get a copy for you.
Martyn Cornell - 2014-12-13 07:01:58
Fascinating, Lars. Yes, Archaeologists like the Moor Group in Ireland and Merryn Dinely in the Orkneys have found similar sorts of evidence, and both have recreated "pit" brewing with hot stones (Merryn is one of those lucky archaeologists whose husband is a home brewer, and thus wouldn't make mistakes like thinking a long carved-out wooden log was automatically a canoe …) Merryn has also found evidence of large clay pots, up to six gallons (27 litres) used for Neolithic ale brewing, though I assume these were fermentation vessels rather than used for heating. Boiling, of course, isn't something you NEED to do until you start using hops, and what evidence there is suggests that English ale brewers never boiled: indeed, there was one 17th century English author suggesting even beer brewers shouldn't boil their worts.
Jocelyn - 2014-12-13 08:29:57
Hello, I know it's not the focus of this post, but I thought it was worth mentioning that the hot rock method of boiling was the only way to boil for thousands of years in pre-ceramic North America. A hole would be dug in the ground and lined with a skin, and hot rocks from the fire placed in. The "fire cracked rock" that resulted is so ubiquitous that (at least where I work at a national park in Ohio) we don't curate it, only count and weigh it. The use of the rocks as fertilizer is quite interesting, I've never heard of that before!
ringo - 2014-12-13 10:16:04
Also, if the flame does not go above the level of the liquid, you can heat water over a fire in a skin bag (bladder, stomach, etc.). Probably also a full hide.
I wonder if these were actually imu stones.
Pete Mason - 2014-12-13 11:25:49
You do not use the term "Burnt Mound", neither as a comparison nor as a distinction. Should one be considering the possibility that some burnt mounds may be "brewing mounds" or not? Is there a clear distinction, ie no bones, or not. Or could it be that some mounds may be both?
Lars Marius - 2014-12-13 14:03:31
@Are: It would be fantastic if you could find it. I asked a professional archaeologist, but he'd never managed to find a copy.
@Martyn: Note that the misinterpretation I wrote about took place more than a century ago. It seems to have been corrected by the time T. C. Cantrill wrote "Stone Boiling in the British Isles", in Man XIII (1913). I haven't read that paper, but it's quoted in  above.
Regarding boiling, the common practice with hops seems to have been to boil hop tea separately, using a small soapstone or clay vessel, and then pour that into the wort. Or to take off say a bucket of wort, boil that with hops, and pour it in. Raw ale has been combined with hops in Scandinavia and the Baltics for centuries.  above describes exactly that procedure, but no doubt it began long before.
If you have some references for English raw ale I'd be really curious to see those.
And, yes, many raw ale brewers seem to have been firmly of the belief that boiling was wrong. Nordland 1969 quotes several examples.
@Jocelyn: That's interesting, and further evidence that people in many different places have hit upon the same solution to the same problem. Very likely this is quite simply the most practical approach at that technological level.
@ringo: Like I wrote there are two kinds of burned rock found by archaeologists. One type is very similar to Hawaiian imu stones. That is, they're just stones used to heat food. The other type has obviously been dropped into hot liquid, as can be seen from how they have splintered into fragments. The edges of the fragments are sharp, showing clearly that the rock has fractured.
@Pete Mason: You bring up essentially the same point. I don't recall offhand if bones have been found in piles of brewing stones or not.
Bill - 2014-12-13 15:52:11
We have such piles of stones in the New World also and these are commonly assumed to have been used to heat water to cook food.
Jesse Merrill - 2015-01-01 22:46:48
I don't have firsthand experience with this, but it is commonly taught that Native Americans boiled maple sap into syrup using skin or bark vessels and hot stones. I have made syrup many times and it is a very intense and fuel-consuming process even with a cast iron kettle or boiling pan. The rule of thumb is 40 parts sap becomes one part of syrup! It takes a good rollicking boil for many hours to get any worthwhile quantity of syrup. There must be huge middens of boiling stones in the eastern woodlands. But it also leads me to suspect that getting huge quantities of liquid up to boiling this way must be relatively effective and I wouldn't discount the possibility that many old-time brewers did end up boiling their wort, either intentionally or inadvertently.
Lars Marius - 2015-01-04 05:26:21
@Jesse: Yes, it's true that you don't need very many stones to make the liquid start boiling. Rock has very high heat capacity. So probably quite a few brewers did wind up boiling either their mash or their wort. However, there's a big difference between boiling the beer for, say, 5 minutes, and the one-hour boil that's common in modern brewing. The accidental boil won't give you a hot break, and so the protein is still in the beer and you effectively still have a raw ale.
It's also possible that some brewers did boil their beer with stones until they got the hot break. So far I haven't seen any evidence either way. My point was that it makes no sense to automatically assume they must have used the stones for boiling when many farmhouse brewers still don't boil their beer today, a millennium later. That most, or maybe all, did not boil is more consistent with the fragments of information I have so far.
Alex Weaver Crocker - 2015-06-30 13:20:27
Really interesting that the process was so ubiquitous in Northern Europe. I suspect that the use of hot stones results in caramelization and browning reactions. A slow addition of stones may also have help efficiency with the probably under-modified malts of the time. If you've tasted many beers made this way, have you found any striking commonalities between them?
Lars Marius - 2015-06-30 13:32:00
@Alex: I've only really tried one, a German industrial beer. It was pretty caramelized. Finding farmhouse ale made with stones today is really difficult. There's at least one made in Finland, as the blog post says. There may be some still made in Latvia. And who knows about Estonia. Maybe. Unfortunately, this tradition appears to be mostly dead.
Jon - 2015-08-14 21:56:13
There are still modern breweries employing this method to this day. Libertine pub in San Francisco is one such brewery that does so exclusively. Except they use lava rocks.
Fal Allen - 2021-12-14 00:07:19
Lars, thank you for all these great Blog posts (as well as your fantastic book) and the help you gave me on the Gose book in the past. Next week Anderson Valley Brewing will be brewing a variation on a Stein Bier. We will be using igneous rocks gathered from the creek bed at the back of the brewery property and heating the stones with the wood from fallen oak trees from the brewery property. We will add the rock to the wort (not the mash) as we are going for flavor, not heat to convert the mash. We are toying with the idea of taking the caramelized rocks and putting them in the conditioning tank for further flavor development (I saw that in some video somewhere a time long go). With some luck the beer will be drinkable. Cheers !