The Freer Gallery of Art has an archaeological collection of approximately 150 Medieval Islamic, glass-frit ceramics from Raqqa, Syria. This collection consists primarily of functional tableware, most of which has a transparent bluish-green glaze, some with black under-glaze painting. A great number of these ceramics have deteriorating glazes. This deterioration is the cause of an iridescence which appears on most of the ceramics. Combined with the deterioration of the glaze is the additional complication of soluble salts. These two factors were the impetus for the following research.
The combination of deteriorating glazes and soluble salts makes it imperative that the most efficient method of desalination be performed. The accepted standard method of desalination, static water baths, may take weeks or months to remove the salts from the ceramic. The longer the ceramics are in the baths the more likely that damage to the glaze will occur.
The research performed was directly related to The Freer collection and has implications for the desalination of all ceramic types. The author compared several methods for desalinating glazed ceramics using: standard stationary baths, stationary baths with a varying time frame, circulating baths, and electrophoresis. The project proceeded in the following steps:
1) Samples of Raqqa ceramics from the Freer Gallery of Art’s study collection and from shards collected on an archaeological excavation administered from the University of Pennsylvania were characterized using a scanning electron microscope and microprobe to determine their chemical composition. Following standard methods, the ceramic’s permeability and porosity was determined.
2) Ceramic tiles, used as standards were fabricated of similar chemical composition and physical characteristics to that of the Raqqa ceramics.
3) Each tile was weighed under consistent humidity and temperature. They were soaked in distilled water to remove any soluble material and to create a more uniform standard. After the standards air dried they were weighed again to determine the base weight of each tile.
4) A common salt found in buried archaeological ceramics was introduced into the tiles by capillary action. After the tiles dried they were weighed to determine the amount of salts absorbed.
5) The tiles were divided into equal numbers and the methods of desalination mentioned above.