APPLICATION OF XRF TO THE CORRELATION OF COLONIAL BRICKS TO A LOCAL CLAY DEPOSIT IN SOUTHWESTERN NEW HAMPSHIRE

Handmade, colonial bricks are eroding from a stream bank along the Cold River, South Acworth, NH. Bricks were regularly made close to local sources. There are three potential sources: the preadvance varved clays, post-glacial varved clays, and clay from the local lodgement till. The predicted source is a varved clay deposit 100m upstream. A sample of this clay deposit was collected along with two bricks for trace element analysis using XRF spectroscopy. The mineralogy was evaluated using a petrographic microscope.

Observed mineral components in the brick include feldspar, quartz and iron oxide. These mineral phases are in keeping with typical firing temperatures. XRF spectroscopy analysis indicates the majority of trace element concentrations are 30-50% higher in the brick than in the clay. However, elemental ratios between immobile elements, such as Zr and Ti are the same in the clay and bricks.

Firing conditions should result in total dehydration and decarbonation of the clay, increasing elemental concentrations in the bricks. Therefore, dehydration and decarbonation reactions can be used to estimate the mass concentration increases expected during firing. Typical dehydration and decarbonation reactions result in an 11-25% increase in elemental concentrations in bricks. However, trace element concentration differences between the clay and brick are not consistent with the predicted increase expected from dehydration and decarbonation reactions. This result could be explained if the source material was heterogeneous, which would be expected if from a varved clay or till, or was from one of the other deposits along the Cold River. To help answer this question, other areas of the Pleistocene varved clay deposits must be sampled and tested using XRF spectroscopy. Different glacial clays from the Connecticut River Valley must also be tested.