D r. William Vernon is one of the Ban Chiang Project’s rare gems. Dr. Vernon, retired Chairman of the Department of Geology at Dickinson College in Carlisle, PA, initiated, designed, and carried through a pioneering study of the Ban Chiang crucibles. He is also the team geologist for the Thailand Archaeometallurgy Project (TAP) and a Research Associate in the Museum Applied Science Center for Archaeology (MASCA). His pioneering studies of crucibles from Ban Chiang and Phu Lon have revealed a strong technological link between these two sites.

     Crucibles are ugly ducklings. At first glance, a crucible fragment appears to be merely a piece of fired clay, perhaps a very thick sherd. Even a whole crucible is not beautiful. Appearances can be deceptive, however, and crucibles, properly studied, can make very important contributions to the study of a culture and its technology.

    A crucible is a vessel used to hold ore during smelting or metal during melting. Study of crucibles therefore opens a window on two separate skill sets within a society. The vessels themselves reveal the knowledge of their makers in selecting, modifying, and shaping materials to create containers able to withstand the very high temperatures used in metalworking. Studies of the metal or slag (non-metal detritus from smelting or melting) residues left in them illuminate the techniques of metal workers and the metals and alloys which were being worked at the site. I studied eighty-five crucible fragments and two whole crucibles (a rarity) from the major temporal phases of Ban Chiang. Most came from the Middle Period (900-300 B.C.). The Ban Chiang crucibles are relatively small, approximately 8 to 12 centimeters in diameter and generally no more than 5 centimeters deep. The capacity of the larger crucibles was probably no more than 200 cc. This is a little larger than the Phu Lon crucibles (see Ban Chiang Update Issue #5), which had a maximum capacity of approximately 65 cc. Whereas the Phu Lon crucibles were used primarily for the production of ingots, the Ban Chiang crucibles were probably used for the production of relatively small artifacts such as bracelets.

    I n order to study the fabric from which the crucibles were made, I prepared thin sections. A thin section is a very thin slice through a piece of pottery which is mounted on a slide and polished until it is thin enough to be translucent. A petrographic microscope, which shines a light upward from below the slide, is then used to examine the sections and identify their constituents by color, shape, and appearance in polarized light. The majority of the pieces consisted of a fine gray clay paste with many very small (generally 0.1 to 0.3 mm in diameter) quartz inclusions. Spicules from fresh water sponges, visible in the clay under magnification, show that the clay source was under water, and from a pond rather than along a stream. All of the fragments show additions (temper) made to the clay to improve its physical properties. Ninety-five percent of the fragments studied have rice chaff as the principal temper, either alone or with other organic matter, grog (fired clay), slag, or minor amounts of other minerals. The remaining 5 percent of the fragments contain grog as the principal temper.

    S eventy percent of the crucibles were also lined on the inside with a layer of fine quartz-rich sand or silt held together by clay. This technique is called lagging. The quartz used is very similar to the quartz in the paste, and may have been prepared by washing the quartz out of some clay from the same source used for the crucibles. The purpose of this lagging would be to prevent the molten metal from sticking to the crucible, and to reflect heat, thus helping to protect the crucible from disintegrating during use due to heat damage. This indicates sophisticated knowledge of materials on the part of the crucible makers. Some crucibles have multiple laggings separated by layers of slag, showing that the crucibles were reused, sometimes several times.

    S eventy percent of the crucible fragments contain slag or dross, ranging from very small patches to a layer almost covering the interior. The slag, prills (tiny blobs of metal), and other melting/slag products are often incorporated in the lagged layers. The slag is black, often frothy, and is sometimes accompanied by multicolored glass, usually black, brown, or red copper colored. The prills are very small (rarely greater than 0.1 mm in diameter) copper, bronze, and silvery colored spherules. Three prills were extracted and analyzed by X-ray emission spectroscopy. Two of the prills were copper, and the third was a bronze containing twenty-two percent tin. The silvery gray prills may be high in tin, since they were identical in appearance to prills found in Phu Lon crucibles which proved to be tin rich. This tallies nicely with the presence of copper alloy casting detritus at Ban Chiang.

    I n sum, study of the crucibles from Ban Chiang is contributing key data to the definition of a prehistoric "Southeast Asian Metallurgical Province" with a distinctive and similar technology not only in northeast Thailand, but also possibly extending from Burma to Vietnam.

Originally published in Ban Chiang Update, Issue #7, Spring/Summer 1998

For more detailed readings concerning Dr. Vernon's research see the following:

Vernon, William
1997    Chronological variation in crucible technology at Ban Chiang: a preliminary assessment. Bulletin of the Indo-Pacific Prehistory Association 16:107-110.

1996-97    The crucible in copper-bronze production at prehistoric Phu Lon, Northeast Thailand:  analyses and interpretation.  In Ancient Chinese and Southeast Asian Bronze Age Cultures, Vol. II, edited F.D. Bulbeck and N. Barnard, pp. 809-820. SMC Publishing, Taipei.

White, Joyce C., William Vernon, Stuart Fleming, William Glanzman, Ron Hancock, and Andrew Pelcin
1991    Preliminary cultural implications from initial studies of the ceramic technology at Ban Chiang. Bulletin of the Indo- Pacific Prehistory Association 11:188-203.