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Experimental Workshops

Heating water

Although it is unsure what exactly burnt mounds were for, most excavated examples have a tank in addition to the fire-cracked stones. Literary sources suggest that the stones were placed within the tank to heat water - but how practical is this? Can this produce boiling water? And how do you get the stones from the hearth to the tank? In 2008, we conducted several firings at the replica hearth cell to see how hot we could get the water in the tank, and to check on how long the process of heating water took.

Checking temperatures within the hearth cell with a thermocouple

Temperatures within the hearth cell reached over 900 degrees centigrade, and the stones were soon hot enough to move to the tank. This was done by raking them out of the fire and pushing them down the paved corridor. At the excavation site, the corridor sloped down from the hearth to the tank.

The hot stones in the water cause steam and bubbles

Stones added to the water cause localised boiling, with lots of bubbles and steam giving the impression that the whole tank is boiling. However, we found this not to be the case, and although the tank looked as though it was boiling, there were a range of temperatures within the water, depending upon proximity to a hot stone.

Checking temperatures within the tank with a thermocouple

We checked the temperature of the water with a thermocouple, and found that a batch of stones could raise the temperature by about 30 degrees centigrade in under ten minutes. However, after the first and second batches of stones went in, there was significant heat loss, with the temperature reaching a maximum of 70 degrees centigrade by the end of the second firing, yet dropping to 45 degrees before the third batch went in. The third batch of stones brought the temperature up to 80 degrees in just seven minutes, and it had only dropped ten degrees within half an hour. In order to retain high temperatures, it seems necessary to have plenty of hot stones initially to raise the temperature and prevent heat loss. A cover of some sort for the tank is also a good idea.

Burnt stones after just one firing

The local stone is Old Red Sandstone, and this did not survive heating well. After just one firing, many of the lumps of stone had shattered into small, unusable lumps. Many of the beach pebbles had cracked also.

After four firings, the wall of the hearth cell was damaged, and many of the stones were cracked.

More worryingly, the hearth cell showed signs of damage after just four firings. At the excavation site, the hearth cell was seen to have been lined several times, and numerous floors were recorded.

 

More detailed experiments will be held next year - check back to this site to get more details.

 

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