If you know me at all, you know that I am a curious and avid reader. So, a couple of weeks ago, while down with bronchitis, I started researching the most current pottery dating methods, which of course led me to the oldest pieces of pottery known so far.
Estimated to be 20,000 years old, fragments of what are thought to be cooking vessels were found in layers of sediment in the Xianrendong Cave in China. The shards came from pottery made by migratory hunter-gatherers during the Mesolithic. As a point of reference, that’s about 10,000 years before farming trumped nomadic hunting and gathering. (That’s right, farmers weren’t the ones who first started making pots.) But how do scientists know this?
Fired clay, being comprised of clay or earth, is impossible to carbon date. When clay is first fired, the water that it originally chemisorbed is removed (adsorption) and the clay itself becomes vitrified into a hard ceramic form. But over time, that same piece begins to absorb moisture from its environment at a very slow but predictable rate. This process is called ‘rehydroxylation.”
“A ceramic sample may be dated by first heating it to determine its lifetime water mass gain, and then exposing it to water vapour to measure its mass gain rate and hence its individual rehydroxylation kinetic constant. The kinetic constant depends on the temperature the measurements are taken at,” says Dr. Clelland at the University of Manchester, who has issued a truly fascinating white paper on the subject.
Originally developed to date brickwork, rehydroxylation has also been used to explain the crazing that sometimes occurs in old glazed pieces of pottery. And it’s how scientist dated the Xianrendong Cave pottery.
Discovering all of this made having bronchitis almost bearable.