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A wide selection of the medieval floor tiles from Cleeve Abbey were examined and samples selected for petrological analysis. The aims of this analysis were: 1) to establish the sources of supply used and 2) to test the classification erected by Ms Harcourt on the basis of design, quarry size and appearance.

Thirty-one samples were prepared at the Lincoln Ceramic Petrology Laboratory and entered into the Laboratory`s reference collection using the reference codes L2031 to L2062. All were stained using Dickson`s Method in order to distinguish between ferroan calcite, non-ferroan calcite and dolomite.

Sub samples were then submitted to Dr N Walsh for IPCS analysis and the resulting data were recorded in an Excel 5 spreadsheet.

Results
The samples could be divided into five groups on the basis of their petrological characteristics. In one case a group could be reliably identified as being the products of a well-known tile factory whose products were widely distributed around the Bristol Channel (Vince 1984 Chapter 2: St Bartholomew`s-Type Tiles). These tiles, Harcourt`s Groups 4 and 5, are usually associated with Cleeve Abbey, following Ward-Perkins` publication of the heraldic tiles from the site, but can now be seen to be a minor element in the Cleeve assemblage. Furthermore, it is also clear that they are at the extreme western edge of a distribution which stretches as far north-east as Gloucester. The remaining groups are likely to be of more local origin and can be divided into those in which the inclusions are of Cretaceous origin and those where the inclusions are probably of Devonian origin. The latter tiles are most likely to be of local origin whereas those in the former group probably originated further east. These petrological groups are here given Fabric Group letters, so as not to be confused with the design groups, which have numbers.

Fabric Group A/B
Six samples of this group were examined. By eye the group was subdivided into two, Fabrics A and B, but in thin-section this distinction cannot be sustained. The tiles in this fabric include examples of design groups 1, 2 and 3, together with a rogue example of design group 9 (visually identified as Fabric E).

The petrological characteristics of this fabric are:

Abundant rounded clay pellets, up to 4mm across.
Sparse rounded sandstone and siltstone fragments, up to 1.0mm across.
Abundant quartz silt
Moderate flakes of muscovite up to 0.1mm long
Fabric A/B is distinguishable from Fabrics E and F, to which its ICPS analyses are otherwise very similar, in the quantities of K2O and P2O5 present.

Fabric Group C
Six samples of Fabric C were examined. They include examples of design groups 4 and 5.

The petrological characteristics of this fabric are:

Abundant silt-sized angular quartz
Moderate flakes of muscovite
Moderate fragments of ?altered glauconite
A plot of Nickel vs Chromium content using the ICPS results clearly separates Fabric C from all others.

Fabric Group D
Five examples of Fabric D were examined, all of design group 6.

The petrological characteristics of this fabric are:

Abundant rounded quartz sand, with grains up to 1.5mm across (but mainly less than 0.5mm)
Moderate to abundant rounded grains of altered glauconite, up to 0.4mm across. Some of these grains retain a zoned structure, with a darker core.
A plot of Nickel vs Chromium using the IPCS analyses clearly separates Fabric D from all others.

Fabric Group E/G
Ten examples of Fabric E/G were examined. They include tiles of design groups 7, 8, 8A, 9 and 12.

The petrological characteristics of the fabric are:

Abundant ill-sorted quartz sand with grains up to 0.5mm across
Sparse rounded fragments of Lower Greensand Chert (angular quartz sand c.0.2mm across, in a matrix of silica) up to 1.0mm across
Sparse angular opaque to dark brown iron-rich inclusions up to 3.0mm across (section L2407 only)
Sparse rounded flint up to 1.0mm across.
Abundant clay pellets of varying colour. Some have dark brown staining (?manganese?)
Moderate muscovite flakes up to 0.1mm long.
Fabric G was distinguished visually as having a higher quantity of rounded quartz sand and this is bourne out by petrological analysis. However, it is also clear from this analysis that the range of inclusions in the two fabrics is identical.

Fabric G, however, is distinguishable from the remaining fabrics using the ICPS results, which show a high Barium content for four of the five samples.

Fabric Group F
Four examples of Fabric F were examined, all of design group 11.

The petrological characteristics are:

Abundant angular quartz silt/very fine sand (average c.0.1mm across)
Sparse angular sandstone fragments up to 1.5mm across
Sparse ?Lower Greensand Chert up to 1.0mm across
Sparse rounded flint
Sparse to moderate muscovite up to 0.1mm long
Fabric F is distinguishable from Fabrics A and E, to which it is otherwise very similar, through its lower P2O5 content and higher average K2O content.

Discussion
The main petrological division present in this material is into fabrics which contain wholely or mainly rocks and minerals of likely Cretaceous origin and fabrics containing inclusions of probable Devonian origin. Only one fabric, F, includes material from both sources and even here it is clear that the fabric is mainly tempered with Cretaceous material. The characteristics of these Cretaceous fabrics are:

Rounded quartz sand, often including well-rounded grains, some with brown-stained veins (Fabrics D and G)
Lower Greensand Chert (Fabrics E, G and F)
Flint (Fabrics E, G and F)
Glauconite (Fabrics C and D)
Abundant quartz and muscovite silt (Fabric C)
It is known that this material of ultimate Cretaceous origin is found widely in the sands and gravels of Somerset but that as one moves south and west so the material is mixed with rocks and minerals of south-western origin. Glauconite, especially in quantity and with relatively large grains, is unlikely to occur in these secondary contexts to anything like the same extent as chert and flint and its present in fabric D is an indicator of an origin somewhere close to the outcrop of Cretaceous rocks. Whether its presence as much smaller fragments in Fabric C is also significant for the source of the clay is less certain, especially so since the known distribution of tiles of this type does not include areas on or close to the Cretaceous outcrop (for example, Bristol, Gloucester and Tintern, in addition to Cleeve).

Fine-grained sandstones, often with a high proportion of non-quartzose inclusions (ie. classifiable as greywackes), are the other distictive inclusion type in the Cleeve assemblage. Although only present in two fabric groups, A/B and F, and only common in the former, it should be remembered that fabric group A/B is actually the most common at Cleeve and there is no reason to doubt that fabric group A/B is relatively local, although it is not possible without clay and potential temper samples from the area to show whether or not the raw materials could have been collected on site. The large quantity of clay pellets in Fabric A/B suggests a casual attitude towards clay preparation, although it may simply be a characteristic of the sources used.

Fabric F, which includes a mixture of southwestern and Cretaceous inclusions, is similar in petrology and texture to the fabric of the products of the Donyatt potteries and this may likewise be the source of these tiles.

The ICPS analyses confirm that each of the fabric groups recognised here is chemically distinct. Fabrics D and C are clearly distinguished from the remainder using their Nickel and Chromium contents. Fabric G can be distinguished in four out of five cases by its Barium content whilst the remaining three Fabrics - A/B, E and F - are similar in most respects but can be differentiated by their K2O and P2O5 frequencies.

Statistical analysis of ICPS (added 1999)



In 1999 the Cleeve Abbey data were examined using Principal Components Analysis (the Bonn Archaeological Statistics Package for Windows - WinBASP). Fourteen components were computed by the program, the first of which accounted for the majority of the statistical variation in the dataset, the second of which accounted for next and so on. Some of the petrological groups formed clearly separated clusters whilst others overlapped with neighbouring groups. Using a combination of different components it was possible to separate Fabrics C, D, E and G from each other and from the remaining samples.

The plot of PC2 versus PC4, shown here, is the clearest.



The contribution of the different elements to this plot is shown here. It indicates that Fabric D, tempered with a glauconitic sand, is distinguished by a group of elements: Ni, Co, V, Zr, Yb, Y, Dy and Cu. Fabric G, a quartz and chert sand-tempered fabric, is distinguished by CaO, Sr and Ba.


Appendices
List of samples
ICPS data (HTML table format)
ICPS data (Excel 4 spreadsheet)


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© Alan Vince 1999