Internet Archaeol. 17. Willis. Case Study 1: ratios between cup forms Drag. 27 and 33

5.3.2.3 Case Study 1: ratios between cup forms Drag. 27 and 33

A useful indicator of date is the changing relative frequency of two common samian cup forms during the 2nd century AD: forms Drag. 27 and 33. During the middle and later 1st century the Drag. 27 form is far more frequent than the 33 (compare SGLG 27 with SGLG 33; see also, for example, Groves 1993, 126). However, during the 2nd century the situation alters. The Drag. 27 form declines in frequency relative to the 33 and by c. AD 160 the 27 becomes rare as it passes out of production (cf. Willis forthcoming f). The database can be used as an index of this changing balance over time (Chart 1 below) and represent a new perspective (new information), as previously trends in samian forms have not been demonstrable in this manner, since a large dataset of information about samian from stratified contexts has not been brought together until now.

Chart 1
Chart 1: The Changing Relative Frequency of Two Samian Cup Forms: Drag. 27 and Drag. 33 during the Second Century AD (Source: database).

The chart shows a clear sequential decline in the frequency of Drag. 27 relative to the 33, which becomes predominant. This trend should be reliable, as the data upon which it is based represent a large sample. Chart 1 shows that at the start of the 2nd century, during the Trajanic period (c. AD 100-118) the data suggest a ratio of 9 Drag. 27s to every 1 Drag. 33. This ratio drops to around 4:1 in the early Hadrianic period (c. AD 124), and then to 3:2 by the later Hadrianic period. By the early Antonine era, in the early 140s, the ratio is c. 1:1. Thereafter the ratio drops to 7:3 in favour of Drag. 33 during the early to mid Antonine period, and then to 4:1 in favour of the 33 during the latter decades of the 2nd century. There is a marked watershed around the late Hadrianic-early Antonine period. These data are consistent with the conventional suggestion that the Drag. 27 was passing out of manufacture c. AD 160 (cf. Webster 1996). Chart 1 shows that the Drag. 27 does not disappear from deposits after this date but registers throughout the late 2nd century. Some examples of Drag. 27 are only to be anticipated among groups of this date, resulting from the gradual breakage and loss of old vessels in circulation and the presence of some residual items. To some degree this 'presence' is likely to be a function of the fact that actual site groups are often of a lengthy date-range. In consequence, vessels in use in the mid 2nd century are effectively spread across a longer time-span when the groups they form part of are of broad attributed date range, for instance, c. AD 130-190, thus making examples of the Drag. 27 seem to have a longer currency than was actually the case.

Again, this case study is an example of how the data assembled in the database might be employed as a yardstick by which the composition of particular site groups might be compared in order to ascertain an approximate idea of the date of that group. Clearly the data can be expressed as ratios or in percentages. The key point of the example is that if, say, an excavator establishes that they have a ratio in the order of 4 Drag. 33 cups to every 1 Drag. 27 among a phase group this would suggest that the group dates to the second half of the 2nd century, probably after c. AD 170. By looking at the ratios between other form types the excavator could confirm or perhaps refine the dating of their group. Moreover, the impact of residual material in any particular site group is offset since the database itself, based upon actual stratified groups, will include residual elements which are effectively averaged ('normalised') via the aggregation of groups comprising the database. This seems therefore to be a good example of the way in which plain forms can be used as a valid and reliable indicator of date.

The data can be sorted by site type. Hence both the overall ratios of forms might be ascertained over time (as with Chart 1), as well as the pattern for particular site types, as this might be helpful. The pattern of ratios over time among groups from Roman military sites can vary somewhat from those at civil centres and this may need to be borne in mind (see below, Sections 5.3.2.4 and 5.8.2).