Application Of XRF To The Correlation Of Colonial Bricks To A Local Clay Deposit In Southwestern New Hampshire

Abstract

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 two potential sources: glacial lake clays from the Cold River Valley and glacial lake clays further downstream in the Connecticut River Valley. 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. Determination of the mineralogy was attempted using a petrographic microscope. However, minerals in the clay-fired matrix were too fine-grained to be identified. Observed components in the brick include lithic clasts that are likely pebbles or sand grains. Literature suggests that quartz, feldspar, and mica should be contained in the matrix. XRF spectroscopy analysis indicates the majority of heavy metals increased from the clay to the brick. Elemental ratios between these metals, such as zirconium and titanium, 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 should result in an 11%-28% increase in elemental concentrations in bricks. However, not all elemental concentrations measured in the Cold River bricks fell into this predicted range. This can be explained through heterogeneity in the source clay, which would be expected if from a varved clay or till, or if the source is one of the other deposits along the Cold River. To help answer this question, the Cold River clay was sampled and tested using XRF spectroscopy again. Clay from another popular brick source, Glacial Lake Hitchcock, was also tested and analyzed for elemental ratios. Results indicated that the Pleistocene varved clay along the Cold River is the source material, as most elemental ratios between the brick and Glacial Lake Hitchcock did not match.