Thursday, August 30, 2012

How Old is That Hunk of Clay?

While researching my WIP, I wondered how archaeologists determine the age of pottery fragments. 14C dating is useful for organic substances (bones, teeth, parchment, fabric, pollen, or other remnants of carbon-based life forms). The clays used in ceramics are made from inorganic silica compounds that don’t contain much carbon. Pottery fragments can be dated by: 

14C dating of burned food residues (link) 
In the days before dishwashers, steel wool and foaming detergents, food stuck to the crockery. 

14C dating of lipids that were absorbed by unglazed pottery (link)
It’s always been hard to scrub grease off dirty dishes. This method was used to demonstrate that dairy farming has existed in Britain for 6000 years. 

Determining the water of re-hydroxylation (link, link)
After bricks or pottery are removed from a kiln and exposed to the atmosphere, the clay begins to chemically combine with water at a time-dependent rate. This water can be removed with heat. The change in mass (before and after heating) is used to determine when the ceramic was first fired. This method appears to be accurate except for bricks from buildings bombed during WWWII where high temperatures reset the hydroxylation clock. 

The age of 14C-dated objects is expressed in years BP (Before Present) (link) with January 1, 1950 as the origin because radiocarbon dating expanded in use in the 1950s. BP is sometimes interpreted as Before Physics since nuclear testing during the 1950s spiked the atmosphere with radioisotopes of carbon and increased the amount of 14C in samples.


SueBE said...

Did anything you read discuss dating other items within a midden (trash heap). Any burned wood, burned bone etc can be dated. So if the pottery is found in a storage pit later used for trash, they can date other items within the pit.

Ann Finkelstein said...

That's an excellent point. Thanks for bringing it up. Dinosaur bones are sometimes dated by the strata where they were found. In my WIP, there is a ceramic flask that is in the wrong place at the wrong time, and I was curious if its age could be determined independently.

SueBE said...

Ah, that makes sense. It can but it isn't as easy as dating a feature (fire pit, storage pit, midden layer, etc).