Canadian Historic Sites: Occasional Papers in Archaeology and History No. 16

The Western European Coarse Earthenwares from the Wreck of the Machault

by K. J. Barton

Appendix A. Microprobe Examination of Blackened Glaze on Earthenware from the Machault.

by R. H. Packwood.

A request was received from J.-P. Cloutier of the National Historic Parks and Sites Branch to identify the nature of the blackening that had been found on the glazed earthen ware recovered from the wreck of the Machault. The vessel analyzed was a large green-glazed earthenware bowl.

The appearance of the glaze on the bowl varies and can be categorized as an essentially clear green glaze, presumably unaffected; silvery black areas, and a rough matte black.

Representative specimens were taken from each of the three areas, mounted in epoxy and a polished cross-section prepared at about 45 degrees to the surface of the glazes.

Examination of the cross-sections under the optical microscope showed the reasons for the differences in the appearance of the glaze. The clear glaze areas were transparent and obviously undamaged; the silvery black areas were generally transparent with only a very thin layer of either discolouration or deposit on the surface of the glaze, and the matte-black areas were basically pitted and black throughout the glaze.

Spot analysis was performed on the matte-black glaze. By weight, it was found to contain large amounts of lead (40 per cent), sulphur (17 per cent) and silicon (15 per cent) together with smaller quantities of copper (circa 1 per cent) and chlorine (circa 2 per cent).

By traversing the microprobe beam across the specimens it was found that sulphur was present throughout the matte-black area, but absent from the clear glaze and only detectable at the surface of the silvery black areas. By analyzing a second sample of the silvery black area with the surface of the specimen perpendicular to the micro-probe beam, it could be estimated that the affected layer is of the order of 500 to 1,000 Angstrom units thick.

How the change in the glaze was effected is another question. The fire prior to the scuttling of the ship may have had sufficient sulphur content from pitch or gunpowder to have reacted with the glaze; 200 years in the estuary with appropriate bacteria may have caused it; or the effluent from the nearby pulp mill may have caused it.