EDAX is, loosely speaking, an acronym for "Energy-Dispersive X-Ray Analysis". (You'll see lots of these in several of Robert Tichane's excellent books, but that was a few years ago, so he calls it an "EDXR" if I remember correctly.)
This is a breakdown of the wonderful clay that I mention in my journal and in my description of how I became a student potter. It is high-quality brick clay.
Unfortunately, I don't have a good photo of a fired sample of the clay by itself; I'll have to do one at some point. There is a photo of the unfired clay in the archived journal entry in which I describe what I've been doing with the stuff. (There's a link at the bottom of this page.)
There are several parts to this, because in fact the analysis covers three areas or regions of the sample. The three areas are indicated by arrows in the following photo, which is an overview. (Click any little photo here to get a bigger version. Use your "back" button to return.)
Here are detailed views of the three regions:
You'll notice that region 3, even though it looks fairly smooth in the overview picture (and is entirely smooth to the naked eye or low-power optical magnification), is actually filled with small crystals. I think that they are mixed into (and probably precipitated from) a glassy matrix.
Modern EDAX techniques are quite wonderful. In addition to giving you weight and atom percentages, they even give you X-ray pictures of the regions being analyzed, and those pictures are broken out into the wavelengths corresponding to the various elements. (This seems to me to be little short of magical.)
Here are the element maps for the three regions:
Is that amazing, or what?
In the early days, an EDXR result looked about like this:
These days, they also give you a fairly accurate breakdown. Please forgive me for not presenting these as HTML tables -- I haven't had time, and they're too small to bother to make miniatures of, so you get 'em full scale.
First of all, you can ignore the gold. They put a thin coating of it on the sample so they can do the pictures.
That said, I need to point out that the target audience here is mainly potters, and that potters take a peculiar view of their materials. As a potter, I'm going to pretend that clay is made up of various oxides. What we need to do, then, is get a sense of the amounts of those oxides that "make up" this clay. In fact, of course, they are mostly not present as such. (Sure, if I don't strain the clay through a fine enough sieve, I end up with a few undissolved boulders of quartz in it, and that's reasonably pure silicon dioxide, but I usually do strain it.)
If you're operating in atom percentages, it is very easy to get from the percentage of an element to the percentage of its oxide: for Silicon, for example, you add double the Si amount of O, and now you have silica.
Of course, if we ignore the gold, and figure that the carbon is not combined into oxides (which are gases and would have escaped), the sum of the oxides (plus the carbon) for region 3 is 110.86, as I calculate it. This is uncomfortably high. I would have been happier if it had been, say, 102. Such, however, is life.
The next problem is that potters tend to deal in weight percentages, because when you have a bucket of red iron oxide in front of you, there is no way you are going to worry about how many moles of the stuff you're putting into a glaze. One can, of course, generate weight percentages directly from atom percentages and vice versa, but fortunately we are given weight percentages directly in the analyses. It does, however, take a little time to generate molecule weight percentages, because you can't just double the Si to find out how much O to add. When I get around to it, I'll generate these numbers and post them here.
Beyond that, however, there is one more little problem: we don't really know how much of the region 1 and region 2 materials there are, so we don't know how much of them to add to the region 3 material to come up with the actual analysis of the fired clay as it needs to appear in glaze calculations. About the best we can do is guess. (If you go back to that first photo and look carefully at it, especially close to arrow #2, you'll see that there's a slightly darker region around each set of crystals, indicating that as the crystals grow, they are depleting the melt around them of something. That something is mostly, but not entirely, iron oxide, as you can tell by comparing the amounts of iron that were found in the three regions.)
...And that's all I've got for now. When I've gotten all
the numbers worked out and have made what I think are
appropriate guesses, I'll post the revised analysis.
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