Archaeometry 35 (1)

Jet has been widely used in the past for jewellery but was just one of a range of black lithic materials employed. When worked into small objects these materials can be visually indistinguishable, yet discrimination is important because of the restricted occurrence of jet. X-ray fluorescence (XRF) is a rapid non-destructive discriminatory technique, but not all samples are clearly classified, and inhomogeneity may pose problems. A reassessment of the use of XRF is reported. Other techniques considered include electron spin resonance, infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, scanning electron microscopy and x-radiography. These were tested on geological materials to define an effective methodology for archaeological material. The recommended approach is a combination of XRF and x-radiography, although preliminary Fourier transform infrared results showed promise. The x-ray techniques were tested on Romano-British assemblages from Catterick and Stanwick.

There has been debate about whether standard principal components analysis is appropriate for the multivariate analysis of compositional data (eg oxide composition of glass). Loglinear transformation has been recommended by Aitchison as a prerequisite. This paper argues that previous comparisons of methodological merits have tended to circularity of argument by making assumptions about the form of a good multivariate result. To break the circularity of argument the authors have introduced randomised variables into five data sets. A good result must recognise these randomised variables as noise and place them near the centroid of the principal components scattergram of variable loadings. Standard principal components analysis is found to perform better than loglinear transformation in its ability to recognise the randomised variables. It is concluded that loglinear transformation tends to introduce spurious structure into a table of compositional data. There is a comment from M J Baxter (112--15) who is not wholly convinced by their reasoning.