Internet Archaeol. 17. Willis. Ratios of samian form types as an index of the date of a group

5.3.2 Considering ratios of samian form types as an index of the date of a group

5.3.2.1 Introduction to the approach

Some workers have noted the fact that changing proportions of different samian vessel types over time can provide a general guide to the date of a group or site phase. Put simply, there are clear rises and falls in the incidence of different forms over time, relating to the life of the form type. As certain forms become more common and others become less frequent, establishing the changing ratios or proportions between form types should provide an index of the date of a group. This is a version of seriation (cf. Millett 1987a, fig. 1).

Felicity Wild was one of the first specialists to pursue this line, which she adopts in her recent review of the Roman military occupation of north-west England (2002). Groves (1993, 122-3 and 126) in the case of Leadenhall Court, London, Tyers (1993) in respect of Usk, Millett in reviewing the Cala Culip IV wreck (Millett 1993b) and latterly Dickinson in some of her samian reports, such as that relating to Shepton Mallet (2001b, 145), have also looked at form ratios in this manner in order to clarify the chronology of particular assemblages.

By assembling within the database a structured sample of stratified/chronologically discrete samian groups, it is possible to establish the frequency of common form types in proportion (or ratio) to each other, for any period (c. AD 30-280). Given that the database draws on the evidence of a large sample from a wide range of sites, it should be a fairly reliable index of such trends (see Table 80 and, especially Table 81). Both plain and decorated samian forms changed over time, as did their relative importance within assemblages. By examining these trends it has been possible to establish some normal 'finger prints' for samian groups over time. That is to say, what proportion of one form type might be expected to occur as opposed to another. Hence the non-specialist, by establishing which forms they have present in their phase groups, etc., and in what proportions (a comparatively straightforward task), can examine the database to find a 'fit' between the proportions of samian forms they have in their group. This should suggest a likely date for the particular group, which might be verified by looking at the proportions of another pair of forms. An exact fit is of course not to be anticipated; individual groups have their own particular make up. Bearing this in mind, and anticipating that there will be a degree of difference between dates suggested by different ratios, the outcome of the exercise is likely to be a 'best-fit' date range which is likely to be a satisfactory outcome.

This method may liberate the non-specialist from the task of needing to identify the fabric or source of the items (a task which, for the inexperienced, can prove too taxing), or the need to refer to a specialist. The method may be particularly helpful for those producing archaeological assessments, interim reports, and so forth.