Rock Heating

Experiment #ROCKS

(If you have not read about our previous adventures in attempting to brew Viking Age beer, the write up is one level up from here :) You should also see Brewing Nordic (brewingnordic.com), The Viking Food Guy (Ref http://vikingfoodguy.com/wordpress/), and The Draughts are Deep (Magnus https://thedraughtsaredeep.wordpress.com/), other people learning about and/or recreating similar items to our research)

Viking Age Scandinavian (hereafter often abbreviated VA Scand) cooking technology was in some ways quite limited. Metal was expensive, and most of the cooking utensils found to this point are relatively small in volume (Gronnesby 2016). However, VA Scand people were successfully brewing large volumes of beer on a fairly regular basis (Hauge 1996, Zori et al 2013, Gronnesby 2016, also see mentions in various sagas such as Heimskringla, Orkneyinga, Vatnsdaela, etc). We previously tried adding small amounts of hot water to our mash-tun to sustain our mash temperatures with only mixed results, so we consulted the ethnographic and academic sources for other possible options (yes, you are correct to read that as “that sucked, how else could we do this?”)

An ancient human technology for boiling/cooking/brewing is to heat rocks in the fire, then put them in liquid (Thoms 2008, Thoms 2009, Nakazawa et al 2009). Viking Age, and continuously inhabited sites in Scandinavia can have millions of fire-cracked rocks, the layers can be several meters deep over large areas (Gronnesby 2016). Several authors hypothesize that these rocks are the remains of stones used to heat water for brewing (Gronnesby 2016, Dinely and Dinely 2013). There is a single mention in the sagas of milk being heated with stones (Skre 1988). Eyrbyggja saga mentioned a lack of hot stones being very annoying, but does not specify what their use was.  

We have been unable to find any analysis of the geologic source (what KIND of rocks are they) of the Viking Age boiling stones. In other cultures, specific rock-types are used, and transported to where they are used (Brink and Dawe 2003). The geology of Scandinavia and the Baltic region is varied, but is primarily composed of glaciated metamorphic and crystalline rock types, covered with sedimentary sections (see list of websites below). Igneous and metamorphic stones are well suited to use as brewing stones, while sedimentary are less suited due to their layered and porous structure, which leads to having interior water and easy fracture lines (Brink and Dawe 1989, 2003). The situation is much different in Iceland, as the island is composed of recent igneous extrusions.

https://www.sgu.se/en/geology-of-sweden/

https://www.ngu.no/en/topic/bedrock-geology

http://www.dabasmuzejs.gov.lv/en/Geology-of-Latvia

http://en.ni.is/geology/rocks/

Now, while we SOMETIMES forge ahead in project-land full speed, the many accounts of exploding rocks, our unfamiliarity with how many/much rocks we would need to maintain our desired temperatures, and the fact that we need very SPECIFIC temperatures for the enzymatic carbohydrate transformation, led us to believe that some pre-testing was in order. THAT is this document. 

THERE IS NO ACTUAL BREWING HERE.

NOTE! We do know what we are doing when dealing with hot rocks!

This was not a hare-brained adventure in girls-blow-stuff-up. We have used rocks to heat other things (like saunas), we have a stone griddle for VA cooking (it is granite, btw), we are well aware of what happens when you heat a river rock in the fire (pro tip, it explodes), etc. HOWEVER, all our ethnographic and archaeological research led us to believe that it is fairly common for rocks to fracture when heated in the fire and then plunged into water. The meters-thick layers of boiling rocks are all those that had fractured so small they were no longer useful. "Exploded" is the end fate of ALL cooking rocks.

We also are aware of the different properties of rock types. We have taken several terms of geology, and are aware of the basic thermal properties, sources, and characteristics of rocks. That said, so far we have not been able to find a source for analyses of the Viking Age boiling stones, so if you know of any sources there, we'd LOVE to hear about it!

If YOU are not familiar with how rocks work, here are some initial guides to not instantly blowing up your rocks:

https://outdoors.stackexchange.com/questions/801/how-to-avoid-exploding-rocks

https://www.hunker.com/12397976/rocks-that-explode-around-fire-pits

Okay, back to OUR experiment!

The GOAL of this experiment was to determine the number of degrees of heating achieved per mass-unit of rock. YES, there are a lot of other potential experiments here (difference in rock type, for example), but we needed to start somewhere

Equipment list:

    Bathroom scale

    Sewing tape

    Rocks (more details on that later)

    A large stockpot

    Tap water

    Instant read thermometer (meat thermometer)

    Notebook and pen

    Timer/watch/cell phone

    Firewood

SAFETY EQUIPMENT

    Impact rated safety goggles

    Face shield

    Heavy leather gloves

    A shovel if your rocks are huge

    Fire resistant clothing

    A fire extinguisher

    If it is NOT pouring rain, any other items you need to keep your fire from getting out of control (sand, pails of water, etc) 

Fire tools

    Poker

    Tongs/log lifter

    A grill (we used a basket for bbqing vegetables)

    “Advanced equipment”

    Infrared thermometer (for the rocks)

    pH meter (or pH strips)

    Camera

Procedure, primarily in the form of a narrative adventure!

We tried to buy rocks, but none of the rock-purveyors were open on a cold and blustery Sunday. River rock is a BAD IDEA (shatter-tastic!), we were determined to get something more…igneous… and dryer. I mean…as dry as rocks in ditches in the rain are going to be. We spent about forty minutes driving around the valley looking for rocks. Local rocks are mostly very recent sedimentary, and those we rejected (also shatter-tastic!). However, many local ditches feature large amounts of riprap (if you are unfamiliar, riprap is the rocks that are piled up to form natural-ish water and soil barriers. It is often in the form of medium size boulders, but local riprap varies from cobble to large gravel, just what we needed!). We made several stops, taking a few rocks from each. The riprap rocks are some kind of close-grained igneous stuff, quite jagged and a light grey when dry. I would very tentatively identify this rock as some kind of basalt. It is not local to the coast range, but could come from the east side of the state? We acquired a total of ten rocks, and then came home.

Figure 1. Igneous riprap rocks. Note jagged cleavage, fine texture, and pale color when dry.

Figure 2. Local sandstone. Seriously, our local rocks are no good for this, we broke this one by tossing it a few feet. The riprap rocks are WAY better!

 We washed each rock in the sink to remove the mud. Each rock was then dried with a towel, and set in front of or on top of the pellet stove to dry.

Figure 3. Drying rocks on the pellet stove.  

While damp, we weighed each rock on a bathroom scale (they were too heavy for the cooking scale), then took two right-angle circumference measures at approximately the center of the rock. We photographed each rock, and assigned it a letter. This procedure was supervised by extremely disapproving cats. They think we are stupid, rocks in their bed is extra stupid, and rocks smell funny (I agree on that last point).

Figure 4. Taking mass of rock A.

Figure 5 (two images). Stupid humans are STUPID 

The next step was to test our POTENTIAL fire/rock lifting situation. We assembled a temporary fire ring from cement blocks, placed the vegetable fry-basket in the end, and put the rocks in the basket. The log-grabbers/tongs were a perfect tool for this task! Gersvinda was able to easily pick up the 2 and 6 lbs rocks, holding them well away from her body, and accurately place them. The 9 lbs rocks we initially picked up with BOTH the grabbers and a long handled shovel. Later Gersvinda was able to manipulate a 9 lb rock without the shovel, but it is a handy tool for this project. 

Figure 6. Test lifting a 6 lb rock out of a so-far-theoretical fire with the log-grabbers. 

Feeling very pleased with ourselves, we then went out to start the fire! Starting a fire, outside, in a heavy downpour, is not easy. We laid a fire and promptly failed at making flammable items burn, several times. This part of the scientific process was discouraging, frustrating, and took a very long time. And a lot of bickering. And several new plans. And a “fire umbrella” (a deconstructed cardboard box held several feet over the fire, that occasionally smothered it). The majority of the wood we burned was oak, with some douglas fir to increase the potential for initial flame. This was a very efficient way to quite thoroughly smoke ourselves, our general environment, and annoy one another.

However, we prevailed, and eventually got a brisk fire started, after the weather cleared a bit. I should mention, however, that it was also very breezy, and about 6 C (42 F). So now sopping wet, hours later than we planned, and quite cold, we proceeded to the ACTUAL experiment (yes, this IS an excuse for a small sample size!). 

Figure 7. SIGH.

We filled two large stock pots with water. Specifically, an old canner with busted enamel, and a slightly mangled stainless steel kettle. 5 liters in the small pot, and 12 in the canner. My tap water is from an older well, and contains high levels of iron, sulfur, and occasionally mud. Luckily, for this experiment, we don’t care what the results TASTE like (we tried the "after" water, the answer is “not worse than when we started”). My tap water is also quite COLD. Specifically, about 11 C.

Ok, back to the rocks. People who learned this type of technology as generational knowledge, would, we are VERY sure, mock us roundly, but we were unclear on things like “how hot do rocks GET in the fire?”. The first time we heated them, when the rocks got above 100 C, we were DELIGHTED, and promptly tossed them into the pots of water. That didn’t do much. So, the next time, we tried turning the rocks OVER while they heated, and building the fire up much higher, so the flames were coming up around the rocks. This time the rock was about 200 C on all sides when we put it into the water, and this was a better result. We cycled the rocks through, working with 3 rocks and two experimental pots. The third “round” of rocks the surface was closer to 300 C, and the response in the water temperature much larger (see Table 2). The rocks turned quite black (they continue to be black a day later). We did not replace the water each time, as the eventual plan is to try to get the water VERY HOT, and we needed to know how many rocks that would take. The results below are in "increase in water temperature". 

I had been concentrating on the MAXIMUM heat transfer, and had not thought about the time component to our experiment, and therefore there is no time-series data for the first few rocks. Subsequent trials we recorded temperature until it began to fall, at which point we determined that the rock was cooled sufficiently and put it back in the fire.

We had heard many horror stories of shattered rocks, but in this case, the only cracking that happened was to the largest rock, the first time we heated it. Small flakes came off the sides, but NONE of the rocks cleaved explosively in the water, nor did any shatter on the fire.

Figure 9. Relationship between mass and volume of the three rocks used in this study.

As for heating, that’s a bit more complex. I analyzed this two ways. First is the heating over TIME. I divided the amount of HEAT by the number of liters of water in the pot AND the mass of the rock, so this is heat per 1 lb of rock imparted to 1 liter of water.

We found two different patterns. Over about 20 minutes, a small rock imparts MORE heat per liter of water, while a large rock imparts LESS heat. The 6 and 9 lb rocks were similar to each other, and the 2 lb rock different. The maximum heat imparted by the small rock was 1 degree C per 1 lb of rock in 1 liter of water (2 lb rock in 5 liters of water for 7 minutes). The maximum heat imparted by a medium size rock was 0.12 degree C/1 lb/1 L (6 lb rock in 12 L of water for 17 minutes). The maximum heat imparted by a large rock was 0.09 degrees C/1 lb/1 L (9 lb rock in 12 L of water for 14 minutes.

Figure 10. Timed test of heat transfer to water. The units on the y axis are how many degrees C a single pound of rock would impart to a single liter of water. All three rocks are represented here.

The pattern is more clear if we look only at the 6 and 9 lb rocks. Here, the heat increases for about 12 minutes and then begins to fall slightly. The 6 lb rock does impart more heat total to the water (per lb per liter), but the differences are smaller than between a 2 and a 6 lb rock.