Investigations of Paints on Ancestral Puebloan Black-on-white Pottery Using Magnetic and Microanalytic Methods

A previous study of black paints on Ancestral Puebloan black-on-white potsherds showed that SEM-EDS (scanning electron microscopy—energy dispersive X-ray spectrometry) can unequivocally identify mineral based, iron-bearing paint and differentiate it from non-mineral based paint. From ethnographic accounts and replicative experiments, the latter is generally assumed to be carbon (i.e., plant extract) based. However, actually demonstrating the presence of carbon in any of the paints was problematic. Here, a new study of the same potsherds demonstrates that magnetic susceptibility and magnetic hysteresis are also powerful criteria for differentiating between iron based and non-iron based paints. Both methods are non-destructive, rapid, and require no sample preparation. Susceptibility is inexpensive and can be tested by supervised persons with minimal training, although hysteresis is quite expensive and demands sophisticated expertise. A surprising insight provided by the magnetic work is that coloration by non-iron based paint might be caused by the elemental carbon acting to reduce iron within the neighbouring pottery clay matrix, as opposed to, or in addition to, simply the blackness of residual carbon, as is generally believed. This possibility adds importance to the need for a method of detecting, as opposed to presuming, the presence of carbon. Preliminary attempts at carbon detection using a new SEM-EDS system with capabilities beyond that used for the previous study, as well as using XPS (X-ray photoelectron spectroscopy) are discussed briefly. In any case, magnetic properties can help to crosscheck and refine the categorization of mineral versus non-mineral paints via visual criteria traditionally used by archaeologists.