SECTION 3.10:CLAYDON PIKE MORTAR AND PLASTER by Graham Morgan

INTRODUCTION

DESCRIPTIONS
Group 1
Group 2
Group 3
Group 4
Group 5

DISCUSSION

Table 1: Soluble or lime percentages


Introduction

Samples from the excavations at Claydon Pike were examined microscopically and then subjected to chemical analysis, to determine the lime to aggregate ratios and, particle size distribution analysis. The local, mainly calcareous, geology meant that the traditional forms of chemical analysis, using dilute acid to remove the lime matrix from the lime mortars and plasters, was made difficult or even impossible due to the use of limestone sand and gravel as the aggregate. This meant that lime: aggregate ratios were un-measurable in most samples. It was however possible to get some idea of composition of the crushed tile based mortars, and, in the case of the soft lime samples, to simply gently crush them and then use physical separation of the residues by sieving. It is assumed that most of the very fine residues then consist of silt, clay and lime.

Descriptions

Microscopic examination was used to initially separate the mortar/plaster types. They were all lime based. The term intonaco refers to the final layer of fine, usually white, lime plaster to which any paint was applied. For the true or buon fresco technique, the pigment had to be applied whilst the lime was still wet. This enabled the pigment to be actually bonded into the lime plaster.
Opus signinum is the mortar or plaster with water resisting properties. In Britain it usually consists of crushed brick or tile with lime. Other ceramic materials were also used, as is attested here by the fragment of “Samian” ceramic in sample /3163\ [No 11] The ceramic additives gave the lime additional hydraulic properties, making it capable of setting in wet areas and giving soft lime greater strength.
Five groups were distinguished;

  1. Fine plaster containing little aggregate
  2. Coarse wall plaster with gravel up to 10mm
  3. Very coarse floor or wall mortar with gravel up to 40mm
  4. Tile mortar or plaster (opus signinum type)
  5. Lime residue, or waste lime
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Group 1

Only one sample seems to be represented in this group; No 4692, of red (0.05mm) on white intonaco (0.25-0.5mm) on fine off-white mortar (9mm). This sample also contained a fragment of green on pale yellow painted plaster (as seen in sample 4691). Analysis indicates about 65% lime and 35% coarse to fine limestone "sand".

Group 2

This is a coarser group of painted or finishing plaster.
4690: Red on white intonaco (0.5mm) on off white mortar (12mm), also fragments of green spots on orange on white intonaco. Analysis indicates 15% lime and 85% fine gravel.
4691: White on green on pale yellow on dark yellow on white intonaco (0.5mm) on whitish mortar (7mm), also a fragment of red on white intonaco on 11mm of mortar.
4696: Yellow (0.1mm) on white intonaco (0.75mm) on buff gravely mortar (12mm) on 12mm, on 20mm of mortar. These mortar layers are very similar and may represent the application of the mortar in three stages. Analysis indicates 20% lime and 80% fine gravel.
4698: Two layers of fine gravel separated by a white lime layer. Both the mortars are very similar, the thicknesses are as follows: 5mm, 1mm, 24mm. This may represent a re-plastering or a break in the application of the mortar.
4699: White intonaco (0.5mm) on fine gravel mortar on coarse gravel mortar, total thickness 20mm.
4718: Fine gravel mortar, analysis indicates 15% lime and 85% fine gravel.
4731: Tile mortar with gravel mortar, up to 10mm limestone gravel.

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Group 3

A coarser gravel mortar, probably more structural than finishing. This group was too hard to be crushed for physical particle analysis or lime/aggregate measurements. It contains gravel up to 40mm. Samples include: 4706, 4708, 4710, 4716, 4723, 4725. They all contain up to 50% large gravel.

Group 4

Crushed brick or tile based mortars - opus signinum. All are very similar in composition, being mainly tile with small amounts of silica sand (from the tile) and varying amounts of limestone gravel, but probably not more than 10%. The precise composition varies with purpose, for either finishing or lower layers. Samples include; 3163 (36% lime), 4731 (30% lime), 4732 (54% lime), 4733 (52% lime), 4738 (75% lime), 4749 (50% lime), 4754 (57% lime with some sand), 4755 (white intonaco (0.25mm) on tile mortar (11mm), about 50% lime), 4757 (35% lime), 4767 (white intonaco on 25mm tile mortar, 50% lime), 4695 (white intonaco (0.5mm) on tile mortar (17mm), 56% lime).

Group 5

Unused or waste lime. 4770 is almost pure lime (99.5% soluble) and could possibly be intonaco lime. 4773 is 97.4% soluble.

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Discussion

The mortars all contain limestone or hard chalk gravel and therefore the analyses must be considered to be approximate. The % lime figures should perhaps be called " % soluble" except for samples 4690, 4692, 4696, and 4718, which were crushed and sieved without the use of acid.
The pigments present were all natural: red is red ochre, orange is red and yellow ochres, yellow is yellow ochre, green is glauconite, and white is pure lime.
The following are wall plasters with intonaco and pigments: 4690, 4691, 4692, 4695, 4696, 4767.
The following samples appear to be lower layers of wall plaster, being fine and fairly thin: 4698, 4699, 4718, 4731, 4738.
The particle distribution analysis graphs show the range of sizes of tile or gravel. Two distinct curves are shown; the gravel/sand curve with a peak in the middle of the graph, and the tile curves which decrease from coarse to fine. The ends of the curves (>0.15mm mesh) show the totals of the very fine particles, being mainly silt and clay. In the case of the crushed samples, the >0.15mm mesh gives the silt and lime content. The silt is less than 10% on average.
The results, as shown in Table 1, indicate that the lime content is quite different from the soluble component (normally lime and soluble are very similar) due to the use of calcareous (limestone) aggregate. Lime aggregate ratios are often 30:70, 50:50, or 10:90.
A National Analytical survey of Romano-British mortars and plasters (Morgan 1992) showed that the instructions of Vitruvius and observation of Pliny were more, or less, followed by builders in Roman Britain. The results of the analyses of these samples make it very difficult to make safe comparisons. The presence of considerable amounts of added calcareous aggregates made the quantification of the original lime content to be somewhat tentative, but, the opus signinum type mortars and plasters, being mainly made with crushed ceramic material, are much more likely to show an accurate analysis. Lime : aggregate ratios for opus signinum type mortars and plasters frequently show high lime : aggregate ratios, typically 40 : 60 or even 50 : 50. Vitruvius also gave precise instructions on how to apply the plaster to walls. For the finest results six layers of plaster were to be applied, each getting finer, with the last layer, to which the paint was to be applied, containing crushed marble and being burnished. The samples from Fairford Claydon Pike conform the lowly majority of most Romano-British wall plaster, with two or three layers, including the intonaco. Only two sites in Britain have produced six layer plaster, of which Fishbourne Roman Palace was one.

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Table 1: Soluble or lime percentages


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