From: Gary Tefft / locustent@aol.com
Location: Menomonee Falls, WI
Date: 04 Jan 1999
Time: 00:12:14
You pose in intriguing question, Byron. Couldn't we identify the origin of a piece of pottery through the use of forensic science? Well, in order to do that there needs to be something that can be analyzed quantitatively, like the chemical make-up of the clay or glaze. At one time this would have had to be done by traditional "wet chemistry" methods, but today there are some marvelously elegant methods employed that are fast, accurate and reliable.
Gas chromatography and X-ray spectroscopy are a couple of impressive sounding methods that rely on the property of each chemical element to "vibrate" at a unique frequency. The equipment collects the light or other radiant energy emitted from the sample that is either heated or bombarded with x-rays, breaks it into a spectrum and analyses the colors to detect the presence of the individual elements. By further analysis, the proportion by mass of each element in the sample can be calculated.
In the foundry were I used to work, a spectrograph was used to check the chemistry of each "heat" of metal that was melted in order to be certain that the alloy met specification. The equipment is so automated that an analysis could be done in about three minutes. It doesn't even consume much of a sample. It makes a plasma "spark" which vaporizes material from the surface and leaves a "scar" only about 1/4 inch in diameter.
The trouble with clay is that its chemistry is not as unique as the DNA in a stain on a blue cocktail dress. Any example of clay is going to be a mixture of various oxides and carbonates and snips and snails and puppy dog tails. Well, isn't this just what we want? We just have to identify the proportion of snips to snails in Red Wing clay and compare it to the proportion in the piece that we are trying to identify. If it matches, it's Red Wing!
Clay from different sources WILL be different. Unfortunately, clay from different parts of the same deposit will be different, too. Red Wing used more than one pit down through its history and vastly different grades of clay from them at different times. In the early days, they used nothing but the best and just stripped off the rest. Shortly after the turn of the century they built a "washing plant" to refine the sandy or silty material and render it useful. On page 22 of Red Wing Potters & Their Wares are some analyses done by Frank F. Grout in 1919. Two of them show the chemical proportions of clay as-dug and after washing. There are some significant differences, but each is genuine "Red Wing" clay.
It might be possible to determine that, for example, Red Wing clay always has a proportion of lime (calcium carbonate). I assume that this might have something to do with its supposed origin in the Cretaceous Era, notable for its shell-bearing marine life. The results of those millions of years of sea shells accumulating as sediment on the sea bed is limestone (I wasn't just being cute about the snips and snails). Now, if it turns out that Red Wing clay is unique in that respect (Not likely), you would have something. Or possibly, Red Wing clay is unique in that it has magnesia in addition to lime. Western Bentonite (a fine clay used in steel smelting) is distinguished from Southern Bentonite by the presence of Potash (a Potassium compound) in one versus Soda (a Sodium compound) in the other.
Sometime in the 1930's, when art pottery became a more important part of the product line, Red Wing started importing clay. A lot of the old "ink stamped" Red Wing art pottery shows that the local clay was just not up to the task. Iron impurities, which gave the salt-glazed stoneware its warm tan color, spoiled many pieces of translucent-glazed artware. So, what is the unique, characteristic chemistry of this blended clay?
It might be possible, but not very practical.
-Gary-