MATERIALS AND METHODOLOGY
EXPERIMENTAL DATARESULTSACKNOWLEDGMENTS
REFERENCES

SMOOTHING AND CLAY COATING: REFERENCE COLLECTIONS FOR INTERPRETING SOUTHERN LEVANT CHALCOLITHIC FINISHING TECHNIQUES AND SURFACE TREATMENTS

Pour citer cet articleRoux V., “Smoothing and clay coating: reference collections for interpreting southern Levant Chalcolithic finishing techniques and surface treatments", The Arkeotek Journal, no2, 2017, www.thearkeotekjournal.org.

Mots-clés

ceramic finishing techniqueexperimental collectionsmoothingcoatingsurface treatment

INTRODUCTION

This article presents results obtained from experiments carried out in 1998 and 2002 at the Historical-Archaeological Center of Lejre (Denmark). The scope was to provide comparative reference data to interpret the finishing techniques and the surface treatments of the southern Levant Late Chalcolithic ceramics (2nd half of the 5th millennium BC). The hypothesis was that these vessels were systematically clay coated whatever the morpho-functional category, and thus that clay coating could be a hallmark of the ceramic Late Chalcolithic tradition.

Clay coating, also called stuccoing (Schiffer et al. 1994), is a surface treatment aimed at covering wall surfaces. It is made of a grainy slip and can be applied on wet, leather-hard or bone-dry surfaces before firing, although there are ethnographic examples of clay coating applied after firing (Heidke & Elson 1988). Clay coating is smeared on with soft or hard tools. One effect is to hide any traces left by the roughing out and/or the shaping operations (figure 1). Another effect is to diminish thermal spalling and cracking (Schiffer et al. 1994: 208). Identification of clay coating may be difficult when it is made with the same clay material as the clay paste. Indeed, unlike to colored slip, even at high magnification, no discontinuity is visible in section, i.e. no visible clear-cut grainy slip layer. It is then easily confused with smoothing operations.

Smoothing is a finishing technique aimed at evening the superficial layer of the clay vessels. It can be done on wet or leather-hard clay surfaces, with soft or hard tools or dry or wet tools.

Smoothing and clay coating are distinct from surface treatments by rubbing (softening, burnishing, shining) whose attributes are well described elsewhere (Lepère 2014; Roux 2016; Rye 1981; Rice 1987). Since there is no problem of confusion with the coating technique, the latter have not been included in this comparative study.

Figure 1. Coiled bowl half coated. The clay coating hides the coils left visible intentionally.

MATERIALS AND METHODOLOGY

In order to highlight attributes significant of coating and test the hypothesis according to which southern Levant Late Chalcolithic ceramics were systematically clay coated, we conducted experiments with one parameter varied at a time.

Table 1 indicates the main parameters tested. Three technical operations were carried out: smoothing wet clay, smoothing leather hard clay and clay coating. They were carried out on clay pastes presenting three sizes of coarse temper (small [200-500µ], medium [500µ-1mm] and large [1-2 mm]) with two types of tool (hard, soft) combined with two hydric states (dry, wet). Granularity of the coating material was coarse or fine and its degree of viscosity, thick or semi-liquid. Smoothing and coating hard tools included: flint, wood, calabash, stone and bone. Smoothing and coating soft tools included: fingers, cloth, leather, horsehair, and paint brush.

Experiments were conducted on complete vessels as well as on briquettes for the sake of replicating the results obtained (figures 2-6). In total, 104 pieces were obtained.

Table 1. Main variables tested for highlighting attributes significant of smoothing wet clay, smoothing leather hard clay and clay coating.
Smoothing Wet ClaySmoothing leather hard clayClay Coating
Clay PasteFineXXX
MediumXXX
CoarseXXX
Smoothing toolHardXX
SoftXX
DryXX
WetXX
Coating granularityFineX
CoarseX
Coating viscositySemi-liquidX
ThickX
Coating toolHardX
SoftX

The surfaces of the ceramic vessels and the briquettes are described following the grid of analysis elaborated in Roux (2016). Topography (bumps, overthicknesses, crests) was observed with the naked eye; granularity (protruding, embedded or floating grains), microtopography (irregular, fluid or compact) and type of striation (threaded, ribbed, thickened or scalloped) were observed with a stereomicroscope (magnifications of 10x to 50x) (see illustrations of granularity, microtopography and striations in Roux 2016, figures 2.15, 2.16 and 2.17, PDF in open access, http://presses.u-paris10.fr/wp-content/uploads/2016/09/ceramiques.pdf).

Figure 2. Smoothing wet clay with fingers.

Figure 3. Smoothing leather hard clay with a wooden tool.

Figure 4. Semi-liquid coating applied with horse hairs.

Figure 5. Thick coating applied with a leather square.

Figure 6. Clay coated briquettes.

EXPERIMENTAL DATA

Smoothing wet clay surfaces

P0/3 Smoothing wet clay paste with dry hard tools

– Topography: depending on the degree of hygrometry of the clay paste and the angle of action of the smoothing tool, the smoothing motion can cause thickened edge overthicknesses, and the surface topography irregular.

– Microtopography: irregular.

– Striations: depending on the tools and the humidity of the clay paste, types of striations vary: from fine to wide and deep, from unevenly to evenly spaced, from straight to undulating. When the clay paste is not too wet, there is a trend towards threaded striations. When the clay paste is very wet, ribbed striations can form. In both cases, they can be combined with deep striations caused by the dragging of the coarse grains by the hard tool.

– Granularity: exposed protruding and embedded grains.

Diagnostic features do not vary depending on the clay granulometry (fine, medium or coarse).

Figure 19. General aspect of a bowl smoothed with a dry wooden tool. Overthicknesses and deep striations are due to the application of the wooden tool on a not very wet clay paste. (Experiment no 98.5)

Figure 20. General aspect of a bowl smoothed with a dry pebble. Note the difference of surface features between the rim and the wall. (Experiment no 98.8)

Figure 21. Fine wet clay paste smoothed with a dry flint blade. Variety of type of striations due to the application of a hard tool on very wet clay. (30x)

Figure 22. Fine wet clay paste smoothed with a dry flint blade. Deep striations. (44x)

Figure 23. Coarse grain wet clay paste smoothed with a dry wooden tool. Threaded striations, irregular microtopography and exposed embedded grains. (10x)

Figure 24. Fine grain wet clay paste smoothed with a dry calabash tool. Threaded striations, irregular microtopography and protruding grains. (5x)

Figure 25. Medium grain wet clay paste smoothed with dry bone tool. Threaded striations, irregular microtopography, protruding and embedded grains. (5x)

Figure 26. Medium grain wet clay paste smoothed with a ceramic tool. Threaded striations, irregular microtopography and exposed protruding grains. (10x)

Figure 27. Fine grain wet clay smoothed with a dry pebble. Threaded striations, irregular microtopography and protruding grains. (10x)

P0/4 Smoothing wet clay paste with wet hard tools

– Topography: idem P0/3.

– Microtopography: fluid.

– Striations: ribbed striations in the majority; variability in the spacing of the striations; bands of striations of different dimensions.

– Granularity: protruding grains partially covered with clay and embedded grains.

Diagnostic features do not vary depending on the clay granulometry (fine, medium or coarse).

Figure 28. General aspect of a clay wall smoothed with a wet calabash tool. Overthicknesses visible near the rim indicate the start of the tool course. Ribbed striations and fluid microtopography are visible with the naked eye. (Experiment no 98.3)

Figure 29. Close-up view of the vessel of figure 28. Note the ribbed striations, the fluid microtopography and the protruding grains partially covered with clay. (10x) (Experiment no 98.3)

Figure 30. Fine grain clay paste smoothed with a wet calabash tool. Ribbed striations, fluid microtopography and embedded grains. (10x)

Figure 31. Fine grain wet clay paste smoothed with a wet flint blade. Ribbed striations, fluid microtopography and protruding grains partially covered with clay. (30x)

Figure 32. Coarse grain wet clay paste smoothed with a wet flint blade. Ribbed striations, fluid microtopography and embedded grains. (30x)

Figure 33. General aspect of a clay wall smoothed with a wet ceramic tool. Note the bands of ribbed striations of different dimensions and the fluid microtopography. (Experiment no 98.4.1)

Figure 34. Coarse grain wet clay smoothed with a wet wooden tool. Ribbed striations, fluid microtopography and embedded grains. (10x)

Figure 35. Coarse grain wet clay smoothed with a wet bone tool. Ribbed striations, fluid microtopography and embedded grains. (10x)

Figure 36. Coarse grain wet clay smoothed with a wet pebble. Ribbed striations, fluid microtopography and embedded grains. (10x)

Smoothing leather-hard clay surfaces

P0/5 Smoothing leather-hard clay paste with dry soft tools

– Topography: smoothing leather-hard clay paste does not affect the general topography obtained after shaping. When smoothing causes clay movement, edges of overthicknesses are scalloped because of the leather hard consistency of the clay paste.

– Microtopography: compact, caused by the friction against leather hard clay paste.

– Striations: variety of type and dimensions of striations depending on the degree of hygrometry of the clay paste; trend towards deep striations due to the friction against a compact surface;

– Granularity: embedded grains. In the course of compaction, there is a tendency for grains to be sunk into the clay paste and become embedded. They can be partially covered with clay.

Diagnostic features do not vary depending on the clay granulometry (fine, medium or coarse).

Figure 37. Fine grain leather hard clay paste smoothed with dry fingers. Compact microtopography and embedded grains. (30x)

Figure 38. Coarse grain leather hard clay paste smoothed with a dry piece of leather. Compact microtopography, deep striations and embedded grains. (10x)

Figure 39. Coarse grain leather hard clay paste smoothed with a dry piece of cloth. Scalloped overthicknesses. (5x)

P0/6 Smoothing leather-hard clay paste with wet soft tools

– Topography: idem P0/5.

– Microtopography: combination of compact and fluid microtopography. Fluidity is caused by the wet tools whose load of water makes the superficial clay layer wet again.

– Striations: ribbed striations (caused by the fluidity of the superficial clay layer), deep striations due to the friction against a compact surface and dragging of the coarse grains on leather hard clay.

– Granularity: embedded grains combined with exposed protruding grains. The wet soft tools can expose protruding grains as for wet clay.

Figure 40. Fine grain leather hard clay paste smoothed with a wet leather square. Exposed protruding grains and ribbed striations. (25x) (Experiment no 98.9)

Figure 41. Smoothing leather hard clay with wet fingers. Fine ribbed striations combined with compact microtopography. (10x)

Figure 42. Fine grain leather hard clay paste smoothed with a wet leather square. Compact microtopography, ribbed striations and embedded grains. (10x)

Figure 43. Coarse grain leather hard clay paste smoothed with a wet leather square. Ribbed striations, exposed protruding coarse grains and embedded small grains. (10x)

P0/7 Smoothing leather-hard clay paste with dry hard tools

– Topography: idem P0/5; depending on the degree of hygrometry of the clay paste, the friction of hard tool against leather hard clay paste can cause crevices (shearing of the clay paste due to its low degree of hygrometry).

– Microtopography: compact.

– Striations: variability in the types of striation; deep striations due to the dragging of the coarse grains on leather hard clay; spaced ribbed striation formed by the width of the smoothing tool.

– Granularity: embedded grains partially covered with clay.

Figure 44. Leather hard clay wall smoothed with a dry wooden tool. Note the scalloped overthicknesses, the deep striations and the compact microtopography. (Experiment no 98.7.2)

Figure 45. Leather hard clay paste smoothed with a dry wooden tool. The compact microtopography contrasts with a central zone which has not be smoothed and whose microtopography is irregular. (10x) (Experiment no 98.7.2)

Figure 46. Coarse grain leather hard clay paste smoothed with a dry wooden tool. Crevices (visible at the top of the photography) combined with varied types of striations and embedded grains partially covered with clay. (10x)

Figure 47. Coarse grain leather hard clay paste smoothed with a dry calabash tool. Compact microtopography, embedded grains partially covered with clay and crevices. (10x)

Figure 48. Medium grain leather hard clay paste smoothed with a dry pebble. Compact microtopography, embedded grains and crevices. (10x)

Figure 49. Fine grain leather hard clay paste smoothed with a dry bone tool. Crevices. (5x)

P0/8 Smoothing leather-hard clay paste with wet hard tools

– Topography: idem P0/5. Topography tends to be more regular when smoothed with fine working profile tools (calabash, bone) than when smoothed with thick working profile tools (ceramic). Scalloped overthicknesses.

– Microtopography: combination of compact and fluid zones.

– Striations: ribbed striations; deep striations due to the dragging of coarse grains on leather hard clay; depending on the tool, variability in the spacing of the striations and the dimensions of the bands of striations.

– Granularity: embedded grains partially covered with clay.

Figure 50. General aspect of leather hard clay wall smoothed with a wet wooden tool. The ribbed striations combined with compact microtopography contrast with the irregular microtopography of the rim. (Experiment no 98.11)

Figure 51. Leather-hard clay paste smoothened with a wet ceramic tool (ceramic convex paddle used in Ecuador). Ribbed striations of different sizes; combination of compact and fluid zones depending on the contact zone of the tool with the clay surface. (Experiment Ecuador, C. Lara).

Figure 52. Coarse grain leather hard clay paste smoothed with a wet ceramic tool. Ribbed striations, embedded gains and combination of compact and fluid zones. (10x)

Figure 53. Coarse grain leather hard clay paste smoothed with a wet calabash tool. Compact microtopography, embedded grains partially covered with clay and ribbed striations. (10x)

Figure 54. Coarse grain leather hard clay paste smoothed with a wet flint tool. Scalloped overthicknesses. (10x)

Figure 55. Coarse grain leather hard clay paste smoothed with a wet bone tool. Compact microtopography and embedded grains partially covered with clay. (10x)

Clay coating

P0/9 Fine grain coating with soft tools

– Topography: irregular, marked by numerous small bumps (lump type) and overthicknesses formed when coating.

– Microtopography: fluid with a high concentration of coarse grains covered with a thin clay layer.

– Striations: spaced ribbed and/or threaded striations (threaded striations when coated with fingers; thick ribbed striations when coated with a piece of cloth; pitted with a honeycomb structure when coated with a piece of leather).

– Granularity: floating grains, alignment of the floating grains along the striations, clusters of floating grains and protruding grains covered completely or partially with a layer of clay.

Figure 56. Coating with fingers. Irregular topography with lumps of clay formed by the concentration of coarse grains while coating. (15x)

Figure 57. Semi-liquid coating applied with a leather square. (15x) (Experiment no 02.15.3)

Figure 58. Semi-liquid coating applied with fingers on half of the briquette. The coating is visible on the left. Note the contrast between the floating grains on the left and the protruding grains of the smoothed surface on the right. (15x)

Figure 59. Coating with fingers. Floating grains and threaded striations. (15x) (Experiment no 02.48.4)

Figure 60. Semi-liquid coating applied with fingers. Floating grains, ribbed striations and fluid microtopography. (x12)

Figure 61. Thick coating applied with fingers. Fluid microtopography with a high concentration of coarse grains covered with a clay layer and floating grains; spaced threaded striations. (x22)

Figure 62. Semi-liquid coating applied with a piece of cloth. Ribbed striations. (25x) (Experiment no 98.20).

Figure 63. Semi-liquid coating applied with a leather square. Honeycomb structure. (20x) (Experiment no 02.15.3)

P0/12 Coarse grain coating with hard tools

– Topography: irregular characterized by bumps (lump type), overthicknesses and/or crests (accumulation of slip underneath the tool).

– Microtopography: fluid, with a high concentration of coarse grains covered by a layer of clay.

– Striations: straight threaded and/or ribbed striations; variability in the striations and their arrangements.

– Granularity: idem P0/9.

Figure 76. Thick coarse grain coating applied with a wooden tool. Lumpy topography, clusters of floating grains and coarse grains covered with a layer of clay. (Experiment no 98.16.1)

Figure 77. Semi-liquid coarse grain coating applied with a bone tool. Crest formed by the accumulation of clay beneath the tool. (Experiment no 98.29.2)

Figure 78. Semi-liquid coarse grain coating applied with a flint blade. Overthickness and floating grains. (20x)

Figure 79. Thick coarse grain coating applied with a flint blade. Floating grains on the edges of the threaded striations. (13x)

RESULTS

Diagnostic attributes of smoothing techniques

P1/1. Threaded striations combined with irregular microtopography and exposed protruding grains are diagnostic of smoothing wet clay paste with dry tools (hard or soft)

Dry tools on wet clay remove clay material and expose coarse grains creating an irregular microtopography and protruding grains. Striations are threaded because of the low degree of humidity of the clay paste. They can be created by hard and soft tools.

Hard tools may create a greater variability of striations depending on the working profiles. There is also a tendency for hard tools to move the clay paste and cause overthicknesses. These remarks are valid also for P1/2.

P1/2. Ribbed striations combined with fluid microtopography are diagnostic of smoothing wet clay paste with wet tools (hard or soft)

Wet tools moisten the superficial layer of the wet clay paste, causing fluid microtopography and ribbed striations through the accumulation of fluid clay. Exposed grains can be protruding and embedded.

P1/3. Combination of compact microtopography with deep striations, embedded grains and/or scalloped overthicknesses and crevices is diagnostic of smoothing leather hard clay paste with dry tools (hard or soft)

The passage of dry tools on leather hard clay paste compacts the microtopography of the clay surface and leave deep striations. In the course of compaction, there is a tendency for grains to be sunk into the clay paste and become embedded. Overthicknesses with scalloped edges are caused by the leather hard consistency of the clay paste. Crevices get formed by tearing of the clay paste.

P1/4 Combination of compact and fluid zones with ribbed striations, embedded grains and/or scalloped overthicknesses is diagnostic of smoothing leather hard clay paste with wet tools (hard or soft)

Wet tools, whether soft or hard, partially moisten the superficial layer of the leather hard clay paste. The effect is a combination of compact and fluid zones with ribbed striations. There can be exposed protruding grains caused by the passage of the wet tool.

P2/1. Combination of irregular/fluid versus compact microtopography, threaded versus ribbed striations, thickened versus scalloped overthicknesses distinguishes between smoothing wet and leather hard clay paste, and between smoothing with dry and wet tools

The diagnostic attributes do not vary depending on clay granulometry (fine, medium or coarse grain clay paste).

Exposed protruding and embedded grains partially covered with clay can be obtained on both wet and leather hard clay paste, with dry and wet tools. Their significance depends on the other descriptive attributes.

Diagnostic Attributes of clay coating

P1/5 Lumpy and irregular topography combined with fluid microtopography and clusters of floating grains is diagnostic of a clay coating

These diagnostic attributes are met whether the coating is fine, coarse, semi-liquid or thick.

The lumpy topography is formed while coating the surface. In this process, the coarse fraction is unevenly distributed on the surface and covered with a layer of clay. Differences in topography, namely size and frequency of the lumps, depend on the coarseness of the coating.

No univocal descriptive attribute are significant of the type of tool (hard versus soft).

ACKNOWLEDGMENTS

The experiments were conducted in 1998 and 2002 thanks to grants from the Historical-Archaeological Center of Lejre (Denmark). I thank Inger Hildebrandt and Lizbeth Tvede-Jansen for their patient assistance with the preparation of the experimental ceramics as well as for profitable discussions and extremely thoughtful suggestions. I also wish to thank Catherine Lara for her careful review and her constructive suggestions and Blanche Barthelemy de Saizieu for her help with infography.

The experimental collections are located at the lab Prehistory & Technology (Nanterre, UMR 7055) and are accessible to all.

The photographs have been taken by V. Roux and C. Lara.