ATHENS, Ga. Antique lovers have a perennial problem. How can you tell if the lamp shade you have is a Tiffany original or a clever copy? Americans in the Colonial period had something of the same problem, it appears. A new study by a student at the University of Georgia using electron microscopy shows a wide variety in the amount of elements or oxides used in 18th century Staffordshire-style earthenware.
The research, done on sherds found at archaeological sites in the U.S. and Canada, hints at the intriguing possibility that cheap knock-offs were being made and passed off as real Staffordshire pottery. This is the first study using electron microscopy to study the composition of 18th century British transfer-printed earthenware.
"We studied what oxides were added for color in these sherds, and found a great difference in concentrations between some of them," said Michael Douglas, a master's degree student in the department of geology at UGA. "There could be several reasons why these differences occur, but at least one reason may be that someone was making cheap knock-offs of the fine Staffordshire pottery."
Douglas will present a paper on his research at 8 a. m. on Oct. 26 at the annual meeting of the Geological Society of America in Toronto.
Staffordshire is a county in the English Midlands where fine clays abound. Beginning in the 17th century, a number of potteries opened in the area to make lead-glazed earthenware, unglazed or salt-glazed stoneware and porcelain. During the 18th century, the pottery industries of Staffordshire became very well known and were especially associated with the works run by Josiah Wedgwood. Other manufacturers included Spode, Minton and New Hall. Large amounts of Staffordshire pottery made their way to colonial America and thus wound up as sherds found in archaeological sites all over the continent. This is particularly important because researchers use these sherds as dating devices.
The potteries in the 18th century began to use colors to transfer designs on to vessels rather than coloring them by hand as in the past. These vessels were then generally called "transfer-ware."
"The chemical composition of ceramics is an intriguing question for many reasons," said Douglas. "A ceramic manufacturer would keep pigment, glaze and clay compositions a close secret because of the monetary loss he'd face if anyone were able to reproduce his vessels. But knowing chemical composition can also help archaeologists pinpoint exactly where a vessel or plate was made if a marker's mark weren't available on the sherd."
Douglas examined six 18th and 19th century sherds of Staffordshire style using a scanning electron microprobe, an instrument that uses a finely focused electron beam to interact with a sample and determine its composition and locate, identify and quantify compositional differences. He used a fine grinding powder to expose the surface of the sherds.
Douglas found that oxides added for color bear out what the historical record has reported: cobalt for blue, tin and manganese for green and tin alone for green. What he found most interesting, however, is that the amount of the oxides differed greatly from sample to sample.
"There's no reason why a manufacturer would use different amounts of cobalt in mass-produced ceramics to create a consistent shade of blue," said Douglas. While the differences could indicate a quality control problem at the Staffordshire potteries or the unknown effects of cooling or glazes on pigments, Douglas thinks it's just as likely that another company was making inexpensive reproductions for the export market to America.
In addition to the use of the scanning electron microprobe, Douglas also examined the sherds using backscatter electron imaging in his analysis. When an electron beam interacts with a sample, a proportion of the electrons from the beam will be reflected or backscattered. The amount reflected back depends on the atomic density of the region being probed. Thus, compositional changes can be used in later analyses of pottery fragments.
While some scientific work using electron microscopy has been undertaken in Canada on porcelain sherds from archaeological sites, this is the first study to use the technique to examine Stoffordshire transfer-ware.
Douglas's next studies will involve examining cross-sections of transfer-ware sherds to see what is happening with compositional structure and glazes deeper in the vessel's surface.