Applying CPR to Baetican oil amphorae

The amphora workshops of the conventus Cordubensis, like their modern-day counterparts, were located in an easily accessible rural landscape. Repeated ploughing has progressively scattered archaeological materials across field surfaces and has standardised the degree of sherd fragmentation; it is rare to find complete rims still attached to their necks. Between 2016 and 2018, as part of the OLEASTRO research programme, I conducted surface surveys with a team of students, investigating 79 sites. Following a detailed review of the various additional criteria, other than amphora wasters, used to identify the sites as workshops (i.e. kiln fragments, firing supports and potters’ stamps), 35 sites were classified as probable workshops and 23 as definite workshops (Figure 2).

Figure 2. Map of the south of the Iberian Peninsula, with the position of the Baetican oil amphora workshops along the Guadalquivir and Genil Rivers. The study area is indicated by the frame to the upper right (figure by I. González Tobar, O. Bourgeon and Q. Desbonnets. ©LabEx Archimède).

The corpus used here comprises a total of 1018 Dressel 20 amphora rim sherds. By way of comparison, 1000 is the initial minimum number advocated by Fentress and Perkins (Reference Fentress, Perkins and Mastino1988: 208) as necessary for their own statistical analysis of tablewares. The sample sizes of other projects include the database of the ICRATES Project, with 3660 entries (Bes & Poblome Reference Bes, Poblome and Humphrey2009: 75), and the excavations at Augst (Switzerland), which have yielded 1015 Dressel 20 rim sherds (Martin-Kilcher Reference Martin-Kilcher1987: 198). Each rim sherd in the current corpus was drawn (González Tobar Reference González Tobar2020), prior to classification. The chrono-typological classification of Baetican oil amphorae is based on finds from consumption contexts, notably Augst (Martin-Kilcher Reference Martin-Kilcher1987; Berni Reference Berni2008, Reference Berni, Carreras-Monfort and Van Den Berg2017). The present study uses the Martin-Kilcher (Reference Martin-Kilcher1987) typology. The latter resembles that subsequently advanced by Berni (Reference Berni, Carreras-Monfort and Van Den Berg2017: 186), with some variation of ±10 years in the dates assigned to specific types. For consistency, the current study retains the well-established categories of A to G from Martin-Kilcher (Reference Martin-Kilcher1987), rather than the revised A to F scheme of Berni (Reference Berni, Carreras-Monfort and Van Den Berg2017) (Figure 3).

Figure 3. Drawing of a Dressel 20 amphora, photograph of the state of fragmentation of a typical rim (left) and typo-chronological data (right) (figure by I. González Tobar).

Several issues merit attention. The first concerns undatable amphora rims. In this case, as MNI is not used and since any inability to a date a rim sherd can be assumed to be the same for all Dressel 20 subtypes, their exclusion here has no impact on overall chronological trends. In research for the current corpus, their small number (n = 7) is insignificant and they are excluded from the final total of 1018.

It is also necessary to comment briefly on the definition of wasters. The vast majority of Dressel 20 wasters are, in fact, indistinguishable from the sherds of Dressel 20 amphorae that were deemed to have been successfully fired and used for the export of oil. The number of confirmed over- or underfired examples is negligible, probably reflecting the success of workshop specialisation and a high level of control over firing temperatures. In the case of Dressel 20s, as for other amphorae, the high discard rate of vessels that were not visibly deformed can be explained by the strict quality requirements for containers whose breakage during transport would have led to the loss of approximately 70l of oil per vessel, and by the fact that production problems may have affected the amphora bodies, leaving the rim sherds studied here without obvious evidence of deformation. Each amphora, fresh from the kiln, must have undergone demanding quality control checks to cull any potentially defective vessels. The definition of a waster used here therefore encompasses both deformed sherds and those without any visible production faults. In order to be certain that the latter were indeed discarded before use, and not later imported, the corpus includes only sherds from confirmed production phases of the 23 definite workshop sites (for an overview of the controversy around workshop identification and how to distinguish non-local amphorae recovered from workshops, see González Tobar Reference González Tobar2020: 855–77).

Informed by foundation and abandonment dates derived from a wider study (González Tobar Reference González Tobar2020), CPR graphs were generated for each amphora workshop (for individual values and percentages, see Table S1 in the online supplementary material (OSM)). These CPR graphs visualise the temporal development of production. The graph corresponding to the Mohíno workshop CPR (Figure 4a), for example, indicates that 40 per cent of its total production dates to just two decades, between AD 30 and 50. In contrast, production at Cortijo de Romero (Figure 4b) was more constant, maintaining relatively stable percentages for more than a century, between AD 30 and 50 and the mid second century AD, corresponding to approximately 80 per cent of its total production.

Figure 4. Examples of CPR charts based on samples from two workshops in the area of Córdoba: a) El Mohíno (Palma del Río) and b) Cortijo de Romero (Palma del Río) (figure by I. González Tobar).

The combination of all the CPRs of all the workshops across the study area (Figure 5a: grey) yields a Mean CPR (Figure 5a: black); the data simplified in Figure 5b provide a clearer view. As noted above, the raw figures of the Mean CPR can then be scaled to account for uneven time intervals used by the two different chrono-typological schemes. The resulting plots reveal the strong influence of the choice of specific typology on the proportion of production over time (Martin-Kilcher versus Berni (Figure 6)).

Figure 5. a) Cumulative CPR chart based on the data of all the individual workshops of the study area (grey) and Mean CPR (black) combining the cumulative data; b) a simpler view of the Mean CPR (black), showing general economic trends (figure by I. González Tobar).

Figure 6. Scaled CPR charts indicating the intervals of the chrono-typological classifications of (a) Martin-Kilcher (dark dashed line) superimposed on the cumulative data of the Mean CPR (thin grey lines); and (b) Berni (dark dashed line) superimposed on the cumulative data of the Mean CPR (thin grey lines) (figure by I. González Tobar).

Assessment of method

The Mean CPR (scaled or unscaled) offers an overview of the percentage rates of amphorae discarded during each of the Dressel 20 chronological phases relative to the total number produced. It represents a first quantification of the relative volume of Dressel 20 amphorae produced in an area in a specific chronological period. The results corroborate the impression gained by a simple count of the changing number of active workshops—that is, a peak of activity towards the second century AD; at the same time, the results qualify the pace of that growth and decline in production. The reliability of these findings rests on a representative sample of datable amphorae and an understanding of effects of the unequal lengths of the periods used (Figure 6). Two major issues thus require discussion. The first pertains to data collection, notably the validity of surface sampling; the second is statistical, notably the problem of unequal chronological intervals.

With regard to the validity of the materials recovered by surface sampling, two phenomena relevant to the districts producing Dressel 20 amphorae bolster confidence. The first is post-depositional and relates to the acute anthropogenic disturbance. Throughout the Guadalquivir Valley, irrigation works, deep ploughing and levelling of agricultural fields have inverted deep archaeological stratigraphy raising representative samples of sherds to the surface (see Table S2). The second process which encourages confidence in the representativeness of surface materials derives from the way in which ancient potters disposed of misfired amphorae. Large surface concentrations of sherds generally correspond to ploughed out waster dumps (Figure 7). These dumps may stem from two different disposal strategies: the filling of deep clay extraction pits with wasters, as suggested by earlier research (e.g. Ponsich Reference Ponsich1979; Chic García Reference Chic García1985), or the horizontal spreading of wasters around the kiln site. More recent stratigraphical analyses of Dressel 20 workshops within the framework of the OLEASTRO project demonstrate that although deep pits were used for the discard of some wasters, potters also discarded them in low heaps at floor level (or near floor level, in large, shallow clay extraction hollows). The heaps rarely grew to any great height before the locus of production and waste disposal shifted, leading to a general spreading out of wasters and incremental distancing of these heaps from the original workshop centre (Figure 7). Indeed, typically, rural environments have fewer restrictions on space than urban contexts and hence favour such horizontal shifts in production and waste disposal. Crucially, this spatial displacement, combined with post-depositional disturbance, permits us to track with some confidence the chronological development of production through surface materials.

Figure 7. Cross section of the succession of amphora waster dumps at the workshop of El Sotillo (Almodóvar del Río, Córdoba) (figure by I. González Tobar).

Regarding the variation in the length of specific production periods, we should note that, although the scaling of the graphs leads to results that appear only to differ slightly from the unscaled Mean CPR results (dark dashed lines versus thin grey lines in Figure 6), the key point to note is that the two scalings each correct the Mean CPR in the same manner. In other words, the scaled production of the first century AD consistently surpasses that suggested by the Mean CPR, while the situation reverses for later centuries.

Another means of addressing this particular issue is to apply the method developed by Fentress and Perkins (Reference Fentress, Perkins and Mastino1988) cited above. Their technique, which standardises time into units of equal length—designated the ‘linear method’ by Willet (Reference Willet2014: 44–46)—is applied here to the Mean CPR using intervals of 50 and 25 years (Figure 8). The chart based on 50-year intervals (Figure 8a) is of limited value as resolution is lost within a single general expansion and contraction of production. The chart with the 25-year intervals (Figure 8b), in contrast, offers a better view as it reveals a sharp decline in production from the mid third century AD, which coincides with the cessation of three important indicators: the use of Monte Testaccio for the dumping of imported amphorae, the use of Dressel 20 for the export of oil, and the use of the complex tituli picti system, all of which permit the tracking of production. The use of 25-year intervals, however, may be somewhat artificial, as the typology of the pottery is not always as precise as this. The real value of the Fentress and Perkins method is that it can combine several different ceramic forms with varying chronologies into a single, standard time-interval series. Yet, as the current study pertains to only a single type of amphora which evolves over time, the calculated CPR does not obviously benefit from this type of standardisation. A more precise future typological amphora classification, taking into account overlapping periods of production, will likely lead to a more accurate diachronic mapping of the data. For the time being, however, I believe that the simple CPR scale offers the most reliable results.

Figure 8. Mean CPR scaled by the Fentress and Perkins (Reference Fentress, Perkins and Mastino1988) linear graph method, following the amphora typologies of Martin-Kilcher (Reference Martin-Kilcher1987) and Berni (Reference Berni, Carreras-Monfort and Van Den Berg2017): a) 50-year intervals; b) 25-year intervals (figure by I. González Tobar).

Finally, it is necessary to note that the potential application of the CPR method presented here to other types of pottery production requires consideration of several problems. The main drawback concerns the issues that arise when considering two or more different types of pottery, where it cannot be assumed that wasters will be generated, and discarded vessels fragmented, in the same ways. Although ethnographic research suggests regular discard rates (Nicholson & Patterson Reference Nicholson and Patterson1985: 231), these can vary considerably in practice. Indeed, piles, and sometimes entire kiln loads, of vitrified, melted sigillata pottery (Genin et al. Reference Genin, Hoffmann, Vernhet, Genin and Vernhet2002) can hardly be compared with the discarded amphorae discussed above; the vitrified cases of the latter, as noted above, are relatively uncommon. Hence, presently, CPR seems best suited for the analysis of amphora types but may also serve, when adopting certain precautions, to determine production rates of other vessel types, even if direct comparisons between types are not appropriate.

Results and discussion

While recognising that the sites considered here comprise approximately only one third (n = 23) of the total number of workshops in the Guadalquivir Valley, it is still possible to advance several observations on the basis of the CPR method. To date, assessments of the Baetican oil trade and the volume of its exports have relied on pottery stamps (Remesal Rodríguez Reference Remesal Rodríguez1986, Reference Remesal Rodríguez and Remesal Rodríguez2018: 387). Yet, as the practice of stamping fluctuated in frequency over time, these stamps provide an unreliable basis for gauging economic activity (Rodríguez Almeida Reference Rodríguez Almeida1984; Berni Reference Berni2008: 33–34; González Tobar et al. Reference González Tobar, Mauné, Tiago-Seoane, García Vargas and Leveque2018: 335). Mean CPR, which is independent of stamping, demonstrates the inappropriateness of resorting to stamps for statistical propositions. The CPR method modifies, for example, the dramatic decline in the number of Dressel 20 stamps observed along the Germanic frontiers from the middle of the second century AD (Remesal Rodríguez Reference Remesal Rodríguez and Remesal Rodríguez2018: 387), suggesting that the drop in the number of stamped amphorae reflects an organisational change in the workshops producing amphorae rather than a decline in the export of oil to the frontiers.

A second key insight of the CPR method is a greater representation of Dressel 20 amphorae in the Claudian and pre-Flavian phases (AD 30–70). To date, their presence in these phases has been underestimated due to the low frequency of stamping during these decades. Yet, the Mean Scaled CPR clearly reveals significant production during these early phases when, despite fewer active workshops, production output resembled that of the subsequent Flavian period (AD 70–110). Based on the CPR, this period of approximately 40 years (AD 30–70) saw the manufacture of 35 per cent of all output across four centuries (Figure 6a). The graph therefore reveals a period of great expansion of oleoculture, a phenomenon that is concomitant with a surge in the power of Hispanic families, and more precisely families from Baetica, within the Roman world, culminating in the elevation of the emperors Trajan and Hadrian (Des Boscs-Plateaux Reference Des Boscs-Plateaux2005). Although the origins of the oil trade lie in the Augustan era at the end of the first century BC and start of the first century AD, its development surged over the following century. The results of the CPR method used here reveal that the capacity of oil production in the conventus Cordubensis quintupled between the time of Augustus and Hadrian's accession to the throne in AD 117.

Lastly, it is important to recognise the method as a means for conducting comparisons with consumption sites. Despite the great frequency of Dressel 20 amphora throughout the Western Roman Empire, diachronic studies of the changing proportions of sub-types (Dressel 20A to 20G, plus the late form, Dressel 23) are scarce. Large quantities of precisely dated Dressel 20 rims recovered from consumption contexts are only available for two sites: Augst and Ostia. A visual review of these published data (Ostia, 169 rims; Augst, 988 rims) forms the basis for a comparison between these consumption contexts and the Baetican production sites. Converting the Ostia and Augst data into percentages, as if they were the CPR of a workshop, and applying the simple scale with the time intervals of the Martin-Kilcher typology, leads to Figure 9.

Figure 9. Comparison of the diachronic distribution of olive oil amphorae at the sites of Augst (Martin-Kilcher Reference Martin-Kilcher1987: 183) and Ostia (Panella & Rizzo Reference Panella and Rizzo2014: 210–13), with Mean CPR data of the Baetican Dressel 20 amphora workshops in the conventus Cordubensis (figure by I. González Tobar).

This comparison reveals notable differences between the two consumption sites. Baetican oil exports (at least those from Córdoba) were guided more by the demand of Ostia than Augst. Although the inclusion of oil in the annona explains Córdoba's reliance on the Ostia/Rome markets, we should not expect one consumption site to reflect the production trends of every specific import. A good example of this is the fact that Dressel 23 amphorae (AD 280–500) (Rizzo Reference Rizzo and Járrega2018: fig. 2) demonstrate that oil exports from Baetica continued into late antiquity—evidenced by the CPR of the production sites, as well as by fieldwork in the Hispalis (Remesal Rodríguez Reference Remesal Rodríguez, Blázquez Martínez and Rodríguez1983) and Genil areas (Bourgeon Reference Bourgeon2017)—but these amphorae are not found at Ostia. The Augst data identify a different flow of oil towards the German frontier, with a progressive reduction of imports from the middle of the first century AD (Figure 9). Comparisons of CPR of production and consumption sites are therefore useful as they reveal that a decrease of imports to specific sites is not tantamount with a decline in global production, but rather linked to other reasons, such as changes in local demand. The key point here is that mean CPR can serve as a reliable tool for determining economic tendencies and can offer insight into local demand and global production.