Introducing ceramics 8 – the dimpled-base experiments

By Roy Towers

Ceramics research at the UHI Archaeological Institute is not always about the Ness of Brodgar assemblage.

Through ORCA (Orkney Research Centre for Archaeology) we also do commercial archaeology work across Scotland and in all time periods, including the Iron Age, which is today’s subject.

We do, however, use equipment and techniques learned at the Ness and similar procedures, so the link is strong.

The last piece had an example of high-tech research. Now it is time for the low-tech, not to mention explosive.

Dimpled bases have always been a problem.

They are basal sherds with finger-impressed dimples on the interior surface and they are found on Early and Middle Iron Age sites right around the Atlantic coast.

In Orkney, they are found at Skaill, Howe of Howe, Riggan of Kami and a number of others and in the Western Isles at Dun Mor Vaul, Loch na Beirgh and A’Cheardach Mhor.

Sometimes they have one central dimple, but more often multiple randomly-scattered dimples. In the literature they are traditionally described as decoration, but I have always doubted this. In the dark recesses of a cooking pot they would have been invisible, and also difficult to clean.

Could the dimples have thinned the pot base and made heating more efficient?

Together with Edinburgh archaeologist and potter Orlene Mcilfatrick, I decided to find out.

Orlene fired sets of four pots of identical size and composition. They had, respectively, one central dimple, three dimples and five dimples with a control pot having none.

We heated water in them with the water at an identical temperature and with a controlled heat input. Industrial thermometers measured the changes.

The result was spectacular, but not in a good way.

We watched, in horror, as the heating water migrated into the porous pot walls and tried to escape…by exploding.

This happened every time. Luckily Professor Jane Downes, the UHI archaeology boss, never knew what we did to her lab (she does now!) but the clean-up was lengthy.

The problem was the porous pot walls so, for the next experiments, Orlene glazed the interior pot walls. We were not trying to replicate Iron Age firing techniques but to test heating efficiency. In any case, it is possible that Iron Age people sealed their pot interiors by boiling milk in them, thus allowing fats to enter the walls.

Orlene tried this and cooked her morning porridge which, she said, was disgusting.

We had learned some things. Our laboratory heating pad had a thermostat which led to inconsistent heating. Bunsen burners were no better due to varying gas pressure and we found that by far the best heater was a domestic gas cooker.

We began again and the results were spectacular, this time in a good way.

The water did not heat more quickly because the dimples had thinned the pot. They had merely created ridges round the dimples and not thinned the base at all.

To our surprise the water in the pots did not heat more quickly, indeed the more dimples in a pot the slower it heated. In fact, with a consistent calorific input (heat) the pots with more dimples refused to boil.

Why? A detailed explanation involves some mathematics, Newton’s law of cooling and an understanding of Steady State Conduction (we had help with these) but I won’t inflict it on you.

There is a simpler description of what happened: As heating progressed a column of hotter water rose above each dimple. As the heated water reached the surface if cooled and tumbled down again. With one dimple this didn’t matter much. But with more dimples the columns rose together and the descending cooler water interfered with the heating process. More dimples meant less efficient heating.

We believe this effect was designed by Iron Age people as a response to an observed effect. Again, why?

A possible answer is that the dimpled base pots were designed not to boil contents because they were being used for simmering food or, perhaps more likely, making a form of cheese.

You will search in vain for this discovery in the literature because, despite kind words and encouragement, the major archaeological journals refused to print it. They were correct. We had run out of money and time and did not repeat the experiments a sufficient number of times.

So, the idea is still out there for aspiring experimental archaeologists.

Just remember to duck!