I need to take certain things for granted here, so I'm
going to refer you to a few references in case this is
unfamiliar territory.
If I say anything so abstruse that it defies comprehension, you can probably figure it out by looking at one or more of the books listed above. That said, let's have some fun with technoid scunge, shall we?
As you can tell if you read the pottery screed on my homepage, I've been after a good satin matte glaze from the beginning of my involvement in pottery. It has proved slightly elusive. My first attempt, taken from Warren Chappell's book (I think it's "The Potters' Complete Book of Clay and Glazes", but don't quote me), led me to a nice blocky crackle clear and eventually to a warm buttery clear for my translucent ware, but not to a satin matte. That original glaze, which I never mixed exactly to formula, was something like 84% colemanite, 10% whiting, and 6% magnesium carbonate. It is supposed to be a cone 10 translucent matte.
The fact that Larry Bruning fires his kiln rather hot may have something to do with the fact that my test didn't matte out, but I suspect that it's more likely my use of Custer feldspar instead of colemanite.
Just by the bye, if you look at the photo of the three cones on the Bruning Pottery page, you'll see that all three are bent right over. I believe that when a cone has bent to 90 degrees it is at its designated heat level, so it appears that the set you can see in the photo has been taken to at least 11.5, if not 12. The original formula from the Chappell book was listed as being a cone 10 glaze. Sometimes glazes will work well over a long range, and sometimes they won't. Usually, at least at high fire, they change substantially as the heat increases. At least, that's my experience with a lot of the ones I've played with.
To make a long story short, while I was playing with the clear glazes I set the satin mattes aside for a while. Eventually, however, I worked up a flameproof porcelain, and when I tried to glaze it I got what seemed like a nice satin matte. What it actually was, as far as I can tell, is mostly a glaze that is so intensely crackled that it looks and feels like a matte. (There are also, I think, some odd crystals in the surface, which contribute to the effect.)
When I saw and felt this glaze I got excited again, and began to attempt to duplicate it on regular porcelain. This proved to be possible, but the formulation I came up with had a problem: it contained 32% clay. There is a reason why we try to avoid using more than about 15% clay in ordinary glazes. Clay loses water when it gets to be approximately red hot, and when it loses the water, it shrinks by more than 10%. This can lead to crawling, a condition where the pots end up with bald spots.
In addition, the satin matte glaze seems to be very stiff, not as fluid as a clear glaze, and I believe that it has a high surface tension. As a result, if it dries with cracks in it (like a dry lakebed or the Bonneville Salt Flats), those cracks get wider when the piece is fired. Perhaps because of the clay content, it does tend to dry with cracks, and even if I spray it carefully onto greenware (which also shrinks when it gets to be red hot, and which partially compensates for the shrinkage of the glaze) it is (ahem) ill-behaved.
The obvious solution is to pre-shrink the clay, and that is what I've done. Unfortunately, I'm a bit of an idiot, and I didn't bother to break up the lumps before I put the clay into the crucible. The lumps are very soft when the clay is still clay, but when I calcine it they become very hard.
It became clear that I needed to mill the stuff to take out the lumps, so I acquired a jarmill. If you want a jarmill or other used lab-type equipment, have a look at LabX. I tend to describe it as "eBay for nerds", but in fact I like the way LabX auctions work a lot better than the way eBay auctions worked when I looked at them a couple months back.
When the jarmill arrived, I put my plastic jar full of marbles and calcined kaolin lumps on it, thinking that surely an hour or two would reduce the stuff to powder. I was wrong. It turns out that ball-milling is something of an art, and I'm going to have to learn it. By running the thing overnight, however, I did get enough powder to make some glaze tests, which I expect to get back tomorrow morning. I hope to be able to extend this entry after that happens. In the meanwhile, I got very slightly smarter, and milled some of the raw kaolin before I put it into the crucible this time. If it all works out reasonably well I should be able to make a modest batch of a well-behaved satin matte within a few days.
It is now the 21st of July, and I got the first
glaze test back today. Sure enough, it didn't crawl.
Unfortunately, it's not quite a satin matte, so I
was obliged to reformulate the glaze and try again.
(If you're interested, the glaze formulas are as follows:
OLD:
This may work reasonably well at, say, cone 10.
NEW:
[It is my considerable pleasure to note that this research is being supported by The Banzai Institute and the Santa-Cruz Institute, thus justifying the title of this Web page. I should note that both of those are real, at least to first order. I should further note that this research is only partly being performed by JoJo Bazilian's Regrettably Destructive Testing Service, which (inasmuch as it is a figment of my fevered imagination) is not real, even to first order. It's me, though, so it's entirely capable of performing research. As a sometime victim of this ability, I suggest that you refrain from using JoJo's services.]
I'm currently working up some fluorescent glazes that are activated by rare-earth elements. I have only a first test, so nothing is at all optimized, but it begins to look like Dysprosium gives an orange color, Terbium is yellow-green, and Thulium is blue. None of these is particularly unexpected, though the fact that they work in a glassy environment is certainly pleasant, and I'm pleased that long-wave UV suffices because short-wave is more dangerous and more expensive. Also, the glazes are not visibly colored; they look like the plain clear glaze that they are based on. I'm jazzed.
(Now I want some Europium.)
I almost think I see some green emission from the Erbium test, also expected, but it's not very bright, and the orangy background fluorescence of the base glaze largely swamps it out. It is possible, by the way, that the background is being activated by copper that the glaze has picked up from the kiln gases. It's about the right color, and this glaze is very likely to trap copper. Interestingly, the sample that I put Manganese Dioxide into exhibits the background fluorescence as well, which I did not expect. I'd have thought the Mn would quench this emission. (So much for my level of expertise, eh?)
Ahem.
My next effort in this direction is to make a set
of concentration tests in as clean a clear glaze as
I can produce (the quick-and-dirty clear that I
used for the first test is itself fluorescent,
and may be quenching some of the other rare earths),
and also a set of crystalline glazes to see whether
a more ordered environment is more effective, as
Scott Scidmore suggests it may be.
I recently acquired a very small 1200-degree-celsius furnace. I thought I'd be using it as a cone 5 or 6 test kiln, but it needed repair, and I was not in a particularly repairing mode, so I passed it along to the aforementioned Scott Scidmore. (Yes, that's the same family as the Skidmores. No, he doesn't pronounce the "c".) When Scott has a Web page, I'll put a link to it here. In the meanwhile you should know that he's one of my advisors, particularly in matters chemical. (I have a BS in chemistry too, but Scott is more active, so his information is fresher than mine.)
In the meanwhile, I fired an initial gold-based
glaze test in my enameling kiln, at 900 degrees c.
That isn't actually hot enough, but it's a start.
At some point I'll try to scan the result and
put it here. (I may have it fired to cone 5 first,
if that proves to be easy to arrange.)
Pseudo-mailto: jon (you know) bazilians (punkt) org Last modified: Mon Jan 29 20:05:38 PST 2001