Pottery and tile - detailed analysis

5.2.1 The pottery and tile were examined, classified and quantified

Pottery and tile fabrics were analysed, given fabric numbers and assigned to chronological periods associations by Steven Willis. A summary of these fabrics is provided in Tables 5.1 (pottery) and 5.2 (tile).

Table 5.1 Summary of pottery fabrics. (For quantities found, see Table 5.9)
Links go to fabric description

Table 5.2 Summary of Roman tile fabrics. (For quantities found, see Table 5.11)
Links go to fabric description

5.2.2 The quantities of each individual pottery and tile fabric were converted to values per ha.

The quantities of each tile and pottery fabric were measured by sherd numbers and weight, before both sets of values were standardised as quantities-per-hectare. Since individual fields were used as the collection units, and thus as the sample areas on the distribution maps, the number of sherds of each fabric recorded from each individual field had to be standardised in relation to its area. The following formula was used:

The field areas were calculated within the GIS in ArcView, and the formula was executed on the sherd numbers and weights for each field using Excel.

Query pottery quantities by field | Query tile quantities by field

5.2.3 The dated pottery was grouped by fabric into broad chronological phases for analysis

On the basis of the pottery analysis, each fabric for which there was evidence was either assigned to a broad chronological phase, or described as 'possibly' linked to a particular period (Tables 5.3 and 5.4). The ceramic phases used are: Iron Age; Roman; Medieval. Fabrics of less certain date were assigned as follows: Possibly prehistoric; Possibly Roman; Possibly Roman and/or later.

Table 5.3 Summary of fabrics of known date

Table 5.4 Summary of fabrics of uncertain date

5.2.4 The median and percentile values were established for each chronological group, and also for selected individual fabrics

The ranges of data values for the pottery densities in the walked fields do not conform to a statistically Normal Distribution. Consequently it was decided to map the density plots using scales derived from the median value and percentile bands, as opposed to the arithmetic mean and deviations from it. The median is the middle value of a set of data that has been ranked in decreasing order of size. In this instance, the median is the middle value of the ranked data from 855 fields, i.e. the 428th value. For the production of density scales, the ranked values were also sub-divided into 10% blocks, or percentiles. The threshold of the tenth percentile thus represents the ranked value 85th from the highest value in the data, the threshold for the ninth percentile is the 171st from top, and so on.

The densities of pottery and tile contained within the top percentile bracket for each group are thus the highest found, and are thus the most likely to be of archaeological significance. This method of scaling has the advantage of allowing comparison of fabric and period distributions of pottery based on the overall distribution of a particular fabric or group of fabrics, regardless of absolute quantities of pottery in the study area. Although this method allows comparison of high densities of pottery for different phases, any assessment of the significance of such concentrations must be based on an examination of the material recovered and its landscape context.

Median and percentile values were established for phases and fabrics using Microsoft Access and Excel. Both sherd numbers and weights of pottery per-hectare were used to provide a comparison between respective densities for each phase or fabric.

Query pottery fabrics per hectare | Query tile fabrics per hectare

Visual comparison of ranked quantities of sherds of specific phases, together with initial analysis of pottery densities spatially, led to the decision to provide an additional value at the mid-point of the top percentile of each group, thus separating the top 5% of the values from the remainder. These values were calculated because of the low absolute densities of Iron Age and medieval pottery which meant that all fields in which pottery of these periods had been found were included in the top percentile range (Table 5.5). This sub-division was thus designed to help us to assess the significance of fields falling within the top percentile.

Table 5.5 Percentile values of quantities of pottery and tile by period

5.2.5 Maps showing the density of the various pottery and tile distributions were created using the percentile values as scales

The median and percentile values shown in Table 5.5 were used as the scales for maps showing the density of pottery of the three ceramic phases and also Roman tile.

5.2.6 High density concentrations of pottery and tile relative to the background distributions were identified on these maps, allowing the definition of probable sites

The top percentile values thus defined the peaks of density in relation to the background distributions. Fields with values in this range were located on the maps using ArcView 3.1 software at Southampton University. Any clustering of fields with high ceramic densities was noted, together with any pattern of decreasing densities in adjacent fields. Such concentrations of material were encircled on the maps in ArcView and considered as potential sites).

| View newly-identified sites density plots |

5.2.7 The distributions of fabrics of uncertain phase were compared with those of the dated material in order to establish whether there was any correlation between them

The distributions of ceramics of known date were then compared with those of pottery which had been defined as 'Possibly prehistoric', 'Possibly Roman' etc. to assess whether they showed any spatial association. In particular, this involved comparisons between pottery of definite Roman date and that categorised as 'Possibly Roman', and 'Possibly Roman and/or later'. Fabric 308, which accounted for a substantial proportion of 'Possibly Roman and/or later' pottery was also compared separately with the concentrations of dated pottery (Tables 5.6 and 5.7).

Table 5.6 Percentile values of quantities of pottery of uncertain date

Table 5.7 Percentile values of quantities of fabric 308

© Internet Archaeology URL: http://intarch.ac.uk/journal/issue9/millett/analysis.html
Last updated: Mon Dec 18 2000