JAIC 2002, Volume 41, Number 1, Article 2 (pp. 13 to 19)
JAIC online
Journal of the American Institute for Conservation
JAIC 2002, Volume 41, Number 1, Article 2 (pp. 13 to 19)




Initially archaeologists from the Center for Desert Archaeology, a contract archaeology firm in Tucson, Arizona, contacted the Arizona State Museum (ASM) for advice regarding the stabilization of a unique clay-covered basket excavated from a Salado site in the Tonto Basin of central Arizona (Salado culture is identified as the cultural period from A.D. 1150 to 1450). Archaeologists had excavated a complete vessel consisting of a clay coating and painted decoration. Fearing that its fragile state would not withstand the lifting process, they consolidated the clay surfaces. Unfortunately, the surface was not as weak as the structure, and the vessel broke into many pieces during the excavation process. A second clay-coated basketry object had been partially excavated when the archaeologists decided to protect the structure and remove it as a partial block (fig. 1).1 A third object was left intact as an entire “block lift” mass. These objects were brought to the Arizona State Museum conservation laboratory for study. The primary constraints of this project included a limited amount of time before reburial and the stipulation that no materials be added or destroyed during the study.

Clay-covered basketry objects with painted decorations are extremely rare in the archaeological record of the American Southwest. Archaeologists have traditionally regarded the random fragments as exotic elements and generally devote little more than a sentence or two to their occurrence. Several notable Southwest archaeologists have reported the presence of clay-covered basketry fragments during archaeological excavations. A complete bibliographic list of these references appears in Odegaard and Hays-Gilpin (forthcoming).

Fig. 1. Block lift of clay-covered basketry vessel 2 before excavation in the conservation laboratory

Determining the complete nature of all the technologies present in these objects was difficult due to their fragmentary and friable state of preservation. It was obvious that a thorough collection of data would be necessary for comparing these examples with known museum objects and samples reported in archaeological literature. Conservators Nancy Odegaard and Matthew Crawford utilized microscopy, chemical spot-testing techniques, and analytical techniques to gather and record information on a chart.

The conservation study began with a close examination of the layering or sandwichlike nature of the clay covering, the woven basketry impressions left in the clay covering, and the decorative use of pigments and painted designs (fig. 2). Initially, the general lack of basketry impression in one example suggested the possibility that another support material, such as leather, had been used to form the structure. A small fragment of the layered structure was opened to reveal the interior surface of the clay covering, where a chemical test for the presence of protein was used under a microscope. The Biuret test determines the presence of proteins if the copper (II) sulfate reagent solution forms a violet color (Odegaard et al. 2000, 144–45). The results with this test were negative, so a test for the presence of cellulose from within the sealed and layered sandwichlike structure was made. One test for the presence of cellulose includes the use of pyrolysis in the presence of orthophosphoric acid to form a gas that reacts with an aniline acetate reagent to give a pink spot (Odegaard et al. 2000, 160–66). This test resulted in a positive reaction, so the search for evidence of any remaining basketry structure using a stereo-zoom microscope continued until a faint impression was found.

Fig. 2. Detail from Punkin Center clay-covered basketry object 2 illustrates the layering or sandwich-like nature of the clay covering. The woven basketry impressions are left on the inside of the clay layers.

A laboratory excavation of the soil block provided more complete preservation of the decorative areas and facilitated the process of taking comparable measurements of the woven impressions. Because the archaeologists had not attempted a complete field excavation of the third object, its original shape could be maintained through the use of more carefully controlled excavation techniques and the placement of strategic supports. The ability to examine the objects during the excavation also enabled more accurate documentation of the vessels' form (bowls), orientation in situ (both upright and inverted), overall measurements, wall-thickness measurements (ranging from 0.5 to 1.5 mm), weave structure (single-rod close coil), weave direction (to the left), workface, and Munsell notations for the clay and pigment colors (azurite, malachite, limonite).2

Some of the common characteristics previously observed on painted objects in the Southwest include color areas that tend to be separated by definite lines, colors that are not blended or mixed, and paints that are usually applied in uniform thickness (Odegaard 1998, 260). The basic approach to painting technology involves the use of ground pigments and an adhesive binder suspended in a liquid vehicle to form a dispersion. As the vehicle evaporates, the binder holds the pigment particles in place. Fine and uniform grinding ensures that the pigments remain evenly suspended throughout the vehicle so that sufficient color may be seen. In general, the ratio of pigment to binder concentration, as well as the amount of vehicle used to apply the paint, determines how matte or glossy the paint appears and how well it covers the surface when dry. Excess vehicle produced weak paint with poor covering power on one of the basketry objects. Generally, the paint technology on these clay-covered basketry objects includes the use of fine-grained pigments of even density, good application control, and lasting adhesion (fig. 3). On clay-covered basketry object 1, the quality of painting suggested that the artist was low on materials or did not understand the technology well, while the application of the paint suggests that the work was done in haste or that the artist lacked the skill demonstrated in the other examples.

Fig. 3. Detail of a loose fragment from Punkin Center claycovered basketry vessel 3 illustrates the decorative use of pigments and painted designs.

A loose surface sample from another basketry object was analyzed by Fourier transform infrared reflectometry (FTIR) to determine the presence of organic binders in the paint, and none were identified.3 While it is possible that pigments suspended in liquid alone could adhere to the porous clay surface without the use of a binder, it is also likely that any organic binder (it would have been a small amount to produce the matte appearance) would have degraded over time. Another sample was analyzed by scanning electron microscope–energy dispersive spectrometry (SEM-EDS) to semiquantitatively confirm the identification of pigments previously identified by polarized light microscopy and chemical spot tests.3

Fig. 4. (l-r) ASM object #GP46711 from the Rye Creek Ruin, ASM object #GP12601 from the Gila Pueblo Ruin, and Punkin Center clay-covered basketry object 3 are claycovered basketry bowls that had not been analyzed or published prior to these studies.

Fig. 5. Detail from ASM object #GP46711 from the Rye Creek Ruin shows preservation of basketry stitch structure below clay covering and painted design.

Once the technical data were recorded for each of the clay-covered basketry objects, they were compared with cataloged examples in the Arizona State Museum collection (fig. 4). A nearly complete example from the Rye Creek Ruin (located to the north of the Punkin Center site) included the unusual use of a blue copper carbonate–based clay to cover the basketry (fig. 5). The biocidal properties of this clay preserved much of the actual basketry fiber, revealing an extensive amount of stitch detail. Another example from the ASM collection was from the Gila Pueblo Ruin (located to the south of the Punkin Center site). This example revealed areas of exposed foundation material and also offered the possibility of seeing the effects of a previous consolidation process on the original color and texture of the painted surface (fig. 6). The details on these more intact specimens facilitated the search for impression details on other sample fragments where no actual fibers remained. After the examination of similar specimens, archaeological field notes, published reports, and additional groups of cataloged fragments were reviewed. Most examples illustrated the use of remarkably similar basketry technology but different coating materials or slightly different pigments and design motifs.

Fig. 6. Detail from ASM object #GP12601 from the Gila Pueblo Ruin shows preservation of basketry foundation material and consolidation of the painted surface.

Copyright 2002 American Institution for Conservation of Historic & Artistic Works