If I may oversimplify, clay melts if you get it hot enough. Kaolins are typically pure enough that they melt only at very high temperatures, but many darker clays have enough iron and other easily-melted materials in them that "hot enough" is well within the pottery range.
These clays make eminently satisfactory glazes either by themselves or mixed with the right Other Things. What those Other Things are, of course, depends upon the temperature at which you are cooking your pots. (In my case, because I work at 1300 celsius or so, I can add nothing; feldspar; silica; iron oxide and/or other colorants; other clays; wood ash, either natural or synthetic; other glazes; and so on.)
Perhaps the most well-known clay that was used primarily in glazes was something called Albany Slip, which was dug at or near Albany, New York. If you've seen old brown insulators either on guy-wires or telephone poles, chances are that they were covered with an Albany Slip glaze. Albany Slip was used by potters, too, and many of them loved it. You can still find lots of old glaze recipes that call for it.
...But it's gone.
Because Albany Slip is no longer being dug commercially, people have made various equivalents by starting with other clay and making judicious amendments to it. The object is to get something that you can substitute straight across, so you don't have to develop the glaze recipe all over again from scratch with different materials. In order for this to fly, you either have to have rock-solid (if you will excuse the expression) analyses of Albany Slip and your starting material, or exquisite examples of Albany Slip glazes, the recipes for them, and detailed information about how they were fired... and in either case, lots of patience is recommended.
Some people think it's no good to attempt to replicate materials that have gone missing. It's better, they argue, to use what's available, and do what you can with it. I have no particular axe to grind, myself -- if someone wants to use "Seattle Slip", "Alberta Slip", or some other approximation of Albany Slip, more power to them.
Here, however, I must in good conscience admit that I have a very special reason for not worrying about this issue: I've got something that gives me glazes I love so much that I couldn't care less about Albany Slip itself, to say nothing of its imitations.
You see, I have a friend who lives on top of millions of tons of clay. (I've mentioned this before, I think.) This clay is relatively ordinary stuff, as far as one can tell by looking at it.
On the left is a wee lump of the stuff, as dug. I broke it off a much larger lump. On the right is what it looks like after I run it through a 10-mesh sieve and then a 40-mesh sieve. To the naked eye, it closely resembles powdered chocolate milk drink mix. (I actually saw a jar of Albany Slip clay once, btw, and it, too looked just about like powdered chocolate milk.)
In what you are going to see below (as I get this page
built, which may take me a little while), I use, for
the most part, four ingredients: clay, ash, silica,
and red iron oxide. There will probably be a few tests
in which I use just clay and ash, or just clay and red
iron oxide, and if I have time I may put up one or two
in which I use feldspar or petalite in place of the ash,
but under ordinary conditions it's those four things.
Further info, in case you give a hoot:
The numbering system I'm using divides the total contents into 20 parts, so a change of 1 is a change of 5%. Thus, "XX00" (I'm using "X" for "10") is 50% clay and 50% ash. I haven't actually made that mix, but I did make 17.300. (You'll see it, below.)
If I get a chance to make tests with natural ash, I'll
probably precede the numbers with "N". Otherwise, I'm
using a synthetic ash that I bought at Seattle Pottery
Supply. It is apparently intended to be similar to a
generalized unwashed wood ash.
Here are some tests I've made:
One other thing: I started out used clay that had been sifted only through a 10-mesh geological sieve. When I mix the glazes that way and then run them through a 60-mesh sieve I lose a lot of material, most of which appears to be sand. (No big surprise there.) For the recent synthetic ash tests, however, I've been running the dry clay through a 40-mesh sieve. Unfortunately, I failed to do that on the 17.300 test, so it is slightly suspect. When I get to the east coast I will probably acquire a large 60-mesh sieve, and I'll run a lot of the dry clay through that.
Fireplace ash, of course, contains lots of large particles
even after I put it through the 10-mesh geological sieve,
and I lose quite a bit of that, too, when I run the glaze
through a 60-mesh sieve after I mix it. My admittedly
limited experience is that if I just run the ash through
the coarse sieve, and do not run the glaze itself through
the fine sieve, I don't like the way it looks after I
fire it. As you'll see in a moment, however, it looks just
fine if I add clay and iron oxide and then don't sift it.
Here (please excuse the pebble under the jar -- I should have hidden it), until I get a chance to do more page-building, is a piece with the usual four-ingredient glaze (clay, fireplace ash, silica, and red iron oxide). It is in the collection of D. Potter, and shows the glaze just before I added more clay and some red iron oxide, a couple weeks ago. When I did that, I got this back as my first glaze test. Fortunately, I hadn't had time to run the result through a sieve after I added the stuff, and I was very happily surprised by the little dots and splashes of color.
I'm probably going to run the glaze through a sieve, now that I've added a quantity of ash to it, because every time I dipped another piece into the glaze, I had to stir it up. Had I been able to dip three or four in one session things might have been different, but the stirring eventually broke up the clumps of iron oxide and of clay, so the splashes of color mostly went away. Such is life. On the other hand, I'm certainly going to check a glaze test before I go running the stuff through any sieve!
The fireplace-ash glaze has gone through various changes since I first mixed it. My first teapot is an example, as is the teacup on that same page. Both of those, in addition to the glaze itself, make use of an overglaze wash consisting of equal amounts of ceramic rutile and gerstley borate. On some glazes, such a wash is scut-awful, but on many tenmoku glazes and similar things, it is quite pleasant. (I've also had mild but happy results from a wash consisting of equal amounts of ceramic rutile and wollastonite, applied over a kaki glaze. I believe it was the old standard Ohata version, which you can find in various books.)
I should probably mention the fact that natural plant ash is insanely variable in composition. I typically burn one particular brand of fake fireplace "logs"; these are made of California cedar, I think, and give an ash that is relatively uniform. I'm not enough of a purist, however, to worry about bits of paper or modest quantities of other kinds of wood that may get into the mix.
I have been fortunate enough to get an analysis of a sample of this clay. I haven't fully chewed the results yet, so it isn't as cut-and-dried as saying "63.4% silica", but I should eventually be able to derive that sort of detail from it.
(Note, added somewhat later: I have since gotten a
regular analysis of this clay, which turns out to be
just about 75% silica, actually a fairly high value.)
Pseudo-mailto: jon [at] bazilians [put it here] org
Last modified: Mon Jan 29 20:07:45 PST 2001
