(25 December, 2007, ff)
This page describes my ongiong attempt to make porcelain that is more translucent than bone china. This, no surprise, is a nontrivial endeavor.
Before I came to Joss Research, I had already developed a porcelain that is about as translucent as decent English bone china. It looks about like this:
That isn’t bad; in fact, it is slightly more translucent than Southern Ice at the same thickness, and as you can see, it is plastic enough to throw on a wheel. (I have to do things to it to achieve the required degree of plasticity, but we can get into that later.)
I have continued to think about the issues involved in translucency, and have reached certain conclusions, about which I am now beginning to be dubious. Some things appear inescapable, however. For example, it seems quite clear that bubbles and voids are inimical to translucency. The refractive index of aluminosilicate glass is in the vicinity of 1.515, whereas the index of air is 1.000, near as makes no difference to us as potters. Any bubble or void will cause reflections, which bounce light around inside the clay body instead of letting it pass through. The further it travels inside, the more chance that it will be absorbed; moreover, if it doesn’t go straight through, it makes the body appear milky even if it does eventually escape on the far side. (Clearly, light that escapes on the same side it entered on is not contributing to translucency!)
Beyond that, however, what interferes with the passage of light through a fired porcelain?
It is clear that impurities, most especially iron and titanium, are among the culprits. In oxidation, both of them contribute yellow to brown coloration. In reduction, they are even worse — they combine to form a deep blue color by charge transfer. (This is, in fact, the same color you see in sapphires and in Rutile Blue glazes.) While we tend to see blue-white colors as being whiter than yellow-white colors, they are not necessarily brighter, as you can tell from the photo below.
The description of ordinary porcelain involves only three materials: kaolin, feldspar, and “filler”. The most common filler is silica, which in the US is available as powdered quartz. (I understand that flint, a microcrystalline form of silica, is available in England; but even though many US potters use the term, I have never actually seen flint for sale here.)
Early in my career as a potter, I abandoned the idea of adding silica to my porcelain body, for reasons that are not of note here. This resulted in a body that is perhaps best described by the old term “Parian” — porcelains that sit low on the Clay-Silica-Feldspar triaxial and are known for translucency. I also add a small amount of MgO (originally in the form of Magnesium Carbonate), because I was able to verify a claim that doing so enhances the translucency slightly. (I originally discovered the claim in the English translation of a book from about 1931, by Hermann Salmang.)
I do not have a copy of the porcelain triaxial diagram here, but you can find it in various references.
It turns out that in typical porcelain compositions, some of the silica fails to go into solution, and Professor William Carty, of Alfred University, believes that this is the main hindrance to translucency. I am somewhat doubtful about this, though it is clear that quartz remaining in the body after firing cannot be helping. On the other hand, my porcelain does not contain any free quartz, and if all other things were equal it should be nearly as transparent as glass.
Well over half of a fired porcelain is aluminosilicate glass, similar in composition to a clear glaze. Granted, bubbles can be expected to make it milky; but if bubbles were the only problem, I would still expect the fired body to be better than it is.
In thinking about this, I decided that the refractive index difference between mullite and aluminosilicate glass was a likely culprit. Although the individual mullite crystals are much smaller than the wavelength of light along their two short axes, they are acicular, and can easily be more than 1 μ long. In addition, the mullite crystals clump together, and the clumps are often considerably larger than the wavelength of light. (Visible light ranges from 0.4 to 0.8 μ. Many people, myself included, can see well beyond 0.8 μ at the long end; but the sensitivity of the eye to far red wavelengths is quite low, and it seems reasonable to give the spectrum as a single octave.)
Mullite is not isotropic, but both of its refractive indices are close to 1.645, and it seemed to me that if I could increase the index of the glassy phase in my porcelain to that level, the translucency should improve quite a bit.
Lanthanum oxide is used in some optical glasses to increase the refractive index, so I elected to try that as an additive to my porcelain. The initial results are not particularly encouraging as far as translucency is concerned, but I am seeing an odd effect in oxidizing firings: perhaps in a way that is similar to the way cerium oxide decolorizes glass, the lanthanum oxide seems to make my porcelain less yellow. The effect is subtle, and difficult to see even by eye except in very good light; I have hugely increased the color saturation and contrast in the next photo, in an effort to make it visible here.
On the left are porcelains that do not contain lanthanum; on the right are porcelains with roughly 1% La2O3. The two columns in the middle were fired in oxidation, the two on the edges were fired in reduction. I may have reduced a bit too much; the tiles that were fired in reduction are considerably darker than I would expect.
Oddly, though it may be difficult to see in this photo, the tiles that were fired in reduction show the opposite effect: the ones that contain lanthanum are actually darker than the ones that do not.
I don’t have an explanation for this yet, but I may be obliged to work out what’s going on here.
In the meanwhile, I am testing other possible additives, including (at Dr. Carty’s suggestion) barium carbonate.
More as it transpires...
My email address is a@b.com, where a is my first name (just jon, only 3 letters, no “h”), and b is joss.
My phone number is +1 240 604 4495.
Last modified: Tue Dec 25 21:17:16 EST 2007