Journal of Archaeological Science 1983, 10, 383-384

Application of Tin to Ancient Pottery Krister Holmberg

It has earlier been assumed that the tin cover found on certain antique vases has been attached as a thin foil. In this paper it is shown that the metal may simply have been applied by an immersion technique. Before dipping into the tin melt the pot- tery must be treated with an adhesion promotor. Animal glue which was known as a binder in ancient times has been found to be useful for the surface treatment.

Keywords: TIN FOIL, BINDER, TIN MELT, SURFACE TREATMENT, ANIMAL GLUE.

Introduction In the beginning of the century, Evans, at his excavations at Knossos, found Minoan vases covered with “imperfectly fixed black varnish” (Evans, 1905). Sixty years later, a closer examination of vases from Mycenaean excavations revealed that the black coating was not a varnish but a layer of tin oxide (Immerwahr, 1966). This led to the assumption that the vases had been covered by a tin foil. It was believed that in funerary use this pottery took the place of silver pottery (Wace, 1932). In a recent work, the surface layer was found to consist of a mixture of stannous and stannic oxides (No11 et al., 1980). On some sherds traces of phosgenite and cerussite were also detected, indicating the use of a tin/lead alloy. It was concluded that a foil had been fixed to the pottery by the use of an adhesive. It has been claimed, however, that from a practical point of view application of a thin foil of tin to the irregularly shaped vases is difficult (Holmberg, 1983). In this paper an alternative way of application of the tin is proposed.

In view of the low melting point of tin (232 “C), the easiest way of application would be to immerse the pottery in a tin melt. Especially for vases with handles and other fragile parts, a coating by immersion seems to be rational procedure.

Tests were made to evaluate the “immersion method” on freshly made pottery. A vase made from kaolin clay was dipped in melted tin. The result was discouraging; the metal did not adhere to the surface of the ceramics. The wetting ability of the melt seemed in- sufficient. Evidently, the forces of attraction between liquid and solid are smaller than those between liquid and liquid.

The free energy change for immersion of a solid in a liquid is given by

--G=YSG-YSL,

where YSC and YSL are the surface tension of the solid-gas and solid-liquid interface, respectively.

The obvious way to raise -AC (and improve the wetting) is to reduce ys~. This can

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384 K. HOLMBERG

be done either by adding a surface active agent to the tin melt or by applying a suitable surface coating to the pottery. Surface active agents, although known and employed in ancient times, seem less probable in this case since they, being organic compounds, would decompose in the hot tin melt. A surface coating, however, would be expected to stand the short period of time in contact with the melt required, even if the coating is of organic origin.

Traces of a nitrogen-containing compound have previously been detected underneath the tin foil on some of the vases investigated (No11 et al., 1980). It was assumed that this material derives from the adhesive used to bind the foil to the pottery. It may well be, however, that this is, instead, the remainder of a material used for surface treatment of the vases in order to facilitate the wetting process.

Which nitrogen-containing material was then used for the surface treatment? A pos- sible candidate is the protein-based steam extraction product from animal bones and butcher’s offal which is normally referred to as animal glue. This polymeric material, being readily available everywhere, is known to be one of the oldest types of binders employed. Other protein-containing, sticky materials, such as egg white and casein, are, of course, also conceivable.

To test the idea of a surface treatment of the pottery, a solution of animal glue was applied and, after a short drying period, the vase was immersed in melted tin. The ceramic was now properly wetted by the metal and a thin tin layer was obtained on the surface. The metal coating adhered reasonably well to the substrate but could be peeled off by hand. After removing the foil, the nitrogen-containing material was still present on the surface of the pottery but could not be detected on the inside of the peeled-off metal sheet. The adhesion could probably be improved by optimizing the process.

Thus, it has been shown that the tin coating which has been found on antique vases need not have been applied as a metal foil. Provided a suitab!e surface treatment was made prior to the metal coating, the tin may simply have been applied by an immersion tech- nique.

References Evans, A. J. (1906). The prehistoric tombs at Knossos. Archaeofogiu 59, 26. Holmberg, E. J. (1983). A Mycenaean chamber tomb near Berbati in Argolis. Actu Regiue

Societatis Scientiarum et Litterarium Gothoburgensis, Humaniora 21,49. Immerwahr, S. A. (1966). The use of tin on Mycenaean vases. Hesperia 35, 381-396. Nell, W., Helm, R. & Born, L. (1980). Mineralogie und Technik zinnapplizierter antiker

Keramik. Neues Jahrbuch fiir Mineralogie, Abhandlung 139, 2642. Wace, A. J. B. (1932). Chamber tombs at Mycenae. Archaeofogiu 82, 182.