Several years ago I watched a documentary (I don't recall the title) about ancient pagan sun-worshipers who built these mysterious stone houses and shrines in Britain. I don't recall who they were exactly. They may have been Celts or Druids--or a different group. I do remember the problem the archeologists were having. They were trying to figure out how these ancient people managed to build these stone structures.
They weren't just ordinary stone structures. The stones were heat-fused together. An incredible feat by these ancients, considering their stone-age/bronze-age technology. The popular theory, of course, is that ancient people were lunkheads who couldn't do anything without the help of visiting extraterrestrials.
Being scientists, the makers of the documentary set out to prove the popular theory wrong. They tried to construct one of these stone structures, using only the materials available in ancient times. They first created a mound of dirt, then piled stones on top to get a kind of igloo shape:
Next, they surrounded the structure with timber:
They then set it ablaze:
What happened next? Well not much. They hoped the stones would fuse together after being exposed to a pyre that would make any Viking funeral a hit. Unfortunately, the stones failed to fuse together. But these archeologists were stubborn and did not give up. They tried the same procedure again and again, hoping for a different result: success. But the stones "stubbornly" refused to fuse together.
At the time I watched this documentary, I wasn't even a physics student yet, so I, like the archeologists, had no clue what went wrong. But that was then. Now it's patently obvious what went wrong. There wasn't enough heat because the second law of thermodynamics reared its ugly head!
When the fire burned, most of the heat was lost to the open air. The convective-heat-transfer equation below spells this out:
The fire's temperature is approximately 1571 degrees Fahrenheit or 855 degrees Celsius. The air temperature was far colder, so nearly 100% of the heat was lost. What these archeologists needed to do was somehow raise the air temperature to 855C, then little or no heat would have been lost.
What I suspect the ancients did was something the archeologists failed to do: The ancients used some form of insulation. If insulation is used, the following equation shows the benefits:
Thick insulation with low thermal conductivity will not only save energy, but can also cause the temperature to increase. This is good news, considering the minimum temperature needed to melt stones is higher than the fire's temperature. Here are the numbers:
So now the question is what materials were available that could be used to insulate? Here is a short list that includes each material's thermal conductivity (kt):
Using stones for insulation seems like an obvious choice, but if there is no mortar to work with, then we would need to heat-fuse these stones together so we can insulate the fire so the stones we started with will be heat-fused.
Wood has a slightly higher thermal conductivity: .5. It is easy to build a wooden insulating frame around the stone structure. The main drawback is it will burn up. That leaves soot. Soot has a very low thermal conductivity: .07. Insulation made from soot will raise the temperature approximately seven times higher than stone or wood--up to 10,500 degrees Fahrenheit or 6,000 degrees Celsius.
I don't know what those ancient people did exactly, but here's how I would engineer a heat-fused stone structure. I would pile stones on a mound of dirt, leaving openings where I want doors and windows. I'd put the timber on top, and over all that I'd build a wooden insulating structure. (I use cutaway views in the diagrams below.)
Of course the wooden planks I'd use would be coated with a paste containing soot, or be charred wood. I would also use a bellows to feed the fire more oxygen. I'd also bury the whole shebang under a pile of mud that would be allowed to harden. Finally, I'd light the fire and insert the bellows and pump away.
To save some time, it might not be necessary to coat the wood with soot or use burnt planks. Fresh wood could be used. It would no doubt burn and may become soot that sticks to the mud structure that remains. This would be ideal.
When enough time has lapsed, the fire would be doused, the mud structure removed. The final step involves removing the dirt from inside the stone structure. If all goes well, it should stand firm because the stones would be heat-fused together.
