People, Pots, and Portable XRF: Towards developing a non-destructive method of analysing the chemical composition of Caribbean ceramics
As archaeologists, we attempt to understand the choices and movements made by people in the past. One method used to do this is the study of the objects people left behind. We can study the pots, bowls, and other objects made of ceramic and use these to reconstruct past behaviour. Ceramic pots are a brilliant resource for achieving this aim because everyone used them. Indigenous communities on the islands made and used ceramic objects before and after the contact period. We can study these materials to determine the changes that occurred over time, and begin to understand the influence of various cultural interactions.
To make a ceramic object, a suitable type of clay needs to be found. This needs to be malleable enough to shape and strong enough to hold that shape.Unfortunately for potters, not all clay is suitable for making pots. It is possible to look at the chemical and petrographic composition of a ceramic object and to determine the type of clay from which it was made. Then, we can begin to match the ceramic object to its clay source. Once these connections have been made, we can begin to assess how different people made production choices in the past. [For more detailed information about ceramic provenancing can be viewed in this video: “CARIB: A journey through cultural ceramics”. See here for the Spanish version “CARIB: El Estudio De La Ceramica En Carib”]
Usually, in order to begin the process of matching the ceramic to its clay source, we need to destroy a small fragment of the object. However, for complete ceramic objects held in museum collections, this destructive analysis is often impossible. Likewise, these precious collections often cannot be exported to laboratories for study. Therefore, we need to use a technique which is both portable and non-destructive. Portable X-ray Fluorescence Spectrometry (pXRF) uses X-rays to excite the elements in the sample. Each element of the Periodic Table, when excited, has a unique fluorescence ‘fingerprint’ and this is what we measure with the spectrometer. However, pXRF only looks at the surface of the object, if this is dirty, painted, or uneven then the measurement will not be a true reflection of the ‘fingerprint’ of the original clay. [The image to the right shows how incident X-rays excite an inner shell electron, the gap left by the ejected electron is filled by one from a higher shell ‘dropping down’; as this happens, the excess energy is emitted as fluorescence. Each element has a unique fluorescence pattern]
Portable XRF can give us some information about the chemistry of the ceramic object, however, it will not be able to give the same level of information as a destructive study. The current research is trying to find the best way of using the pXRF to generate the most accurate and useful information possible. Sometimes, if fragments of pots are available, we are allowed to do destructive studies. So, we also want to develop a method of relating the non-destructive pXRF signature to the information we get from the destructive petrographic and chemical studies. In this way we can be sure that the information gathered from the different techniques is compatible. [On the image you can see; Left – cross section and surface spectra from the same ceramic fragment, notice the difference in signal for certain elements; Right – a petrographic thin section illustrating the depths from which different elements fluoresce]
By building up a pattern of the different ceramic signatures across the Antilles, we can begin to identify the clay fingerprints of specific islands and/or sites. If we relate this information to the shape, style, and design features of the ceramics we can begin to see where objects may have moved i.e. an object found on one island has the fingerprint of a different island. Or, where people and ideas may have moved i.e. a ceramic pot looks similar to those found on the mainland, but the fingerprint says it is from a specific island. Ultimately, we hope to compare ceramics from a variety of collections which may otherwise remain unstudied.