Porcelain Clay Body

Back in the late 80s I was working with Miller 550 porcelain and it worked very well for me, except one little thing. It's really a cone 9 clay and if I trimmed too thinly near the bottom of a medium sized piece, it would sag if I fired it hot, which I usually did. 

I contacted Karl Miller, who was at that time involved in the daily operation of Miller Clay, and he suggested Miller 570, a clay with higher firing temperature but still based on Miller 550. It was a step in the right direction, just not a large enough step. 

So I asked Karl to help me design a porcelain formula which could be fired to cone 12 without sagging, but still based loosely on 550, and this is my final iteration, some 20 years later. 

Porcelain Formula

Grolleg  55

Custer Spar 15

200m Silica 28

Bentolite L 1.5

VeeGum T 1.5

Plus add 44 grams of Epsom Salts dissolved in hot water for each 100 pounds of dry mix. 

I do not know the amount of water that is added to the formula, but if I were making it myself, I would determine the correct amount of water. And it would be pure water. 

Notice how this clay differs from the so called "Equal Parts Body", which is 25% each of Kaolin, Ball Clay, Silica, and Feldspar. It has way more clay, more silica, and less feldspar. 

My celadon glaze, the 4, 3, 2, 1 glaze from Leach, does not craze or shiver on this body.

Possible Fronts for Holiday Show Invitation

Here are 4 possible fronts for my holiday show invitation this year. I would like to know which one you think I should send out.

Thanks.

Lead Based Crystalline Mattes

Here are some of the lead based tiles I got out of the kiln yesterday. I plan to also post fire these in the large gas kiln in the next week or so. 

To me, at this point, the glazes do not have enough contrast between the light and dark and hopefully this post fire reduction will help them along a bit. 

Right now I am working on 2 different kinds of matte crystalline glazes. These, which feature a lead based frit, and the satin mattes, which are softer and more sensual to the touch. I really like them both. 

Glaze Micrometer in Action

Here is a photo of the glaze micrometer being used to measure the glaze on a test tile and giving a reading of almost .0017, or seventeen thousandths of an inch. 

You can see how the plunger rests in the pad of my finger - It's right up above the 25 on the gauge - I'm using pressure to hold the needle against the clay body as the shoe rests on the top of the glaze. 

The knurled knob that you can see closer to my palm, is used to set where the needle is with respect to the background. As the needle wears down you have to make this adjustment. 

I have had this thing apart to work on it and it is really a very nice machine. Nothing about it is cheaply done.

Glaze Micrometer Revisited

I would like to address some of the comments on the original Glaze Micrometer post.

First of all, when you are doing crystalline glazes, or any other type of glazes, the thickness is really important, but particularly for crystalline and other "art" glazes. 

You can buy a Starrett 643JZ at MSC Industrial for 319.97 and there are 35 in stock. You will have to grind the shoe down to a size so that it sets on top of the glaze -- mine is 3/16" x 7/8" and Bill Boyd's is 1/2" x 1/4" so it is not absolutely set in stone what size the base of the shoe needs to be. Mine does not work well on the inside of bowls and Bill has fashioned his for that purpose. 

You have to keep pressure on the needle while you read the thickness on the pot or tile. I had hoped to get a picture of this today but no one came into the shop to visit -- and press the button on the camera while I make the measurement. I will try to get a photo tomorrow. I do have a way to let the top "thingy" that you press down rest in the middle knuckle of my index finger so that it keeps the pressure on naturally. 

You also need to get a reading just after the glaze is dry enough so that it won't come off. You can wait 5 minutes but if you wait an hour, the reading will be off because the glaze will have dried, and it will be thinner. 

This tool is nothing like a hydrometer. If I understand correctly, a hydrometer is a tool that you put into a solution and it measures the specific gravity. The problem is that glazes are not solutions, they are mixtures of chemicals and water, and maybe electrolytes too. So while the hydrometer can be helpful in that you can, if you use no electrolytes, measure the amount of water to the amount of chemicals and get that right every time, what if your water changes? Say all of a sudden your well water has lots of sodium. Your hydrometer may not give you the answer you want.

Most glazes that are used by crystalline potters need to be adjusted with either CMC, a flocculant, or both.  The hydrometer does not seem like a good tool for this purpose. 

There is another way to measure the amount of glaze on a pot and it is used by industry a lot. They just measure the surface area of a pot and put on a specified amount of glaze, so you might see the amount of glaze expressed in grams/square inch. My friend Phil Hamling, uses this method and you can see some of his efforts Here.

If Burlon Craig could read the glaze thickness from a shard, and I'm sure you could get that good, he would also have to make the interpolation from a wet glaze to a dry one. The shard is dry and the glazes are wet. The early folk potters were really in tune with their materials. 

My friend Darrell Adams, who is the grandson of Bill Gordy, told me that he used to go out to find materials with his uncle DX and DX would tell him about the spirits in the rocks. I bet DX did not have a glaze micrometer.

However, that doesn't mean that I don't need one. It's been a very powerful tool for me. 

Crystalline Matte Test Tiles

Here are a couple of crystalline matte test tiles which just came out of the kiln this week. I plan to refire them in a post fire reduction in my large gas kiln but wanted to get a nice photo of them as they are before post fire reduction. 

Just recently, I've been labeling my glazes with a number and a letter so that glazes are called 0-A, 0-B and so forth. It would be more efficient to use letters for both -- A-A, A-B, ... but I started this way and was too far along to change. Instead of running out at 256 combinations, you would have, by using all letters, 676 combinations before you have to add a digit.

The mixing notes are carefully kept in order so that all I need to write on the bottom of a tile is the number-letter and the thickness of the glaze. So a tile might say 1-R   15, which means it is glaze 1-R located in order in the notebook, and the thickness is .0015, as measured by the glaze micrometer. After the tile is fired, I write the firing it was in on the bottom in permanent marker. The kilns are hooked up to my computer by a program called KISS, and so the firing cycles are named and duplicable. And I keep the cones in a ziplock bag for reference. 

Probably waaaay more than you want to know.

26th Annual Holiday Show, Dec 6th and 7th 2008

Our 26th Annual Holiday Show is just 5 weeks away and I am busy making some utilitarian stoneware. While I mostly concentrate on porcelain and crystalline glazes, making stoneware is an opportunity to go back to my roots and make some things for customers who may not appreciate the porcelains. Stoneware clay is also a fine break from porcelain.

Here are some bisqued mugs and bowls -- and some porcelains in the background. I really have a lot of throwing to still do and the time is short, as always.