Example: Examining Antique Porcelain for Lead Glaze
Above: Ming period (1368-1644 CE) porcelain bowl. Credit: AskAsia.org.
My friend Donald loves old things, which is possibly why he studies rocks. He'd be thrilled if nothing in his apartment was less than one-hundred-years old (except his television and DVD player for watching old films). He sews his own clothes using patterns from a century ago and all natural fabrics and materials -- we actually analyzed a button in the electron microprobe because he suspected that it was actually plastic, not bone, as he'd ordered. Donald also loves cooking, and he has a full complement of antique kitchen tools, utensils, and dishware. He insists that everything he has must be useful -- no unused keepsakes are allowed in the kitchen or anywhere else.
About a year or two ago, Donald inherited a set of antique porcelain dishes from a beloved great uncle. He admired those dishes for years and was very happy to own them as a remembrance of his great uncle. But Donald's rule still applied: if he keeps something, it must be utilized. I pointed out to him, though, that these porcelain dishes were old enough that they could have a lead-rich glaze. Fortunately, a saucer had broken, and Donald still had the pieces. I mounted one of the pieces in cross section and examined it using the microprobe.
The backscattered-electron (BSE) image and X-ray maps below show the glaze, temper, and paste of this porcelain. The first thing one sees is the bright glaze layer in the BSE image -- bright areas in BSE images correspond to areas with high mean atomic number; dark areas correspond to areas with low mean atomic number. The glaze layer is so bright because it contains a high concentration of lead, as the lead map shows. Quartz particles are abundant in the body of the porcelain -- these are the orange particles in the silicon element map. One little calcite particle sits at the glaze-paste interface, visible in the calcium map. One alkali feldspar is visible in the potassium map. The rest of the porcelain body is clay -- probably kaolinite, Al2Si2O5(OH)4 -- and a few other silicate minerals.
Below: BSE images and element maps of a porcelain sherd; the field of view is 500 x 500 microns.
Now we knew that the dishes had a lead-rich glaze. In fact, lead was once a very common ingredient of glaze. The earliest known lead glaze occurs in ancient Mesopotamia, and it was adopted by the Romans. Their technique then spread to China and Europe. Leaden glaze is quite common still among decorative pieces not intended for use with food or drink -- never use decorative ceramics for cooking, eating, or storing food!
The worry is that lead will leach out of the glaze and into food or drink. Donald thinks that the leaching is minimal. I contend: why take a chance? I have tried to persuade Donald to stop using these old dishes and get his lead levels tested, but he refuses to do either. Oh, well -- I'll let you know he gets lead poisoning. Anyway, this example shows how the electron microprobe can be used to examine ceramics and how one can individually analyze glaze, temper, inclusions, and clay paste -- one can do petrography and chemical analysis at the same time.
12/1/07
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
