It takes 80 pallets of dry materials to make a run of 4000 boxes of M370
M370 is now our second-most-popular clay body. Are you curious of the recipe? It is really quite simple. It has enough silica to resist crazing. It has enough Nepheline Syenite to mature to 1% porosity at cone 6. The rest is a mix of #6Tile kaolin and M23 ball clay with a little added bentonite to push up plasticity.
G3806C has been our recommended base recipe for reactive glazes (by the addition of colorants and opacifiers). It excels for copper blues, for example. But its thermal expansion is high enough that it crazes on some bodies (e.g. Plainsman P300). To adjust it (via glaze chemistry) I introduced some low expansion Li2O (from Spodumene) at the expense of high expansion KNaO, this dropped the calculated COE from 7.1 to 6.6. The melt fluidity, shown here at cone 6 (its most important feature), is exactly the same. The color is bluer. But not as dark, so copper oxide might be better. Or a higher percentage of copper carbonate. The base recipe (without the copper and tin) is potentially very valuable to create other reactive effects that depend on melt mobility. Why? Because it is very difficult to create a high gloss melt fluid glaze that also has a low thermal expansion.
Supercharge the plasticity of cone 6 reclaimed clay
If your reclaim is short and non-plastic you can make it better-than-new by using an additive of 50% ball clay and 50% bentonite. While only a few percent bentonite supercharges the plasticity of any clay body it is almost impossible to get it to mix into a wet slurry or plastic clay. But thoroughly shaking it together with ball clay (in a plastic bag) separates the super-tiny particles of bentonite between the almost-as-tiny particles of ball clay, that new powder will easily mix with water. And it fires to a tan-buff stoneware at cone 6 so it won't change the fired appearance of most buff or brown cone 6 stoneware bodies.
It is impossible to dry this clay. Yet I did it. How?
These are made from a 50:50 mix of bentonite and ball clay! The drying shrinkage is 14%, more than double that of normal pottery clay. It should be impossible to dry them, the most bentonite bodies can normally tolerate is 5%. Yet notice that the handle joins with the walls are flawless, not even a hairline crack (but the base has cracked a little). Remember that the better the mixing and wedging, the smaller the piece, the thinner the walls, the better the joins, the more even the water content is throughout the piece during the entire drying cycle and the more damp of a climate you live in the better your drying success will be. What did it take to dry these: 1 month under cloth and plastic! I changed the cloth every couple of days. So by implementing these same principles you will have better drying success.
Brushing glazes can go on unevenly for more than one reason
Both of these were glazed by brushing. The inside transparent and white glazes are fairly easy to apply evenly but the bright color on the outside left one certainly is not. The problem is a combination of things. It is difficult to apply it evenly with a brush. It is difficult to get it on thick enough. And this commercial glaze does not contain enough of the purple stain (so I added 6 grams of Mason 6304 Violet stain powder to the 2/3 of a jar I had left, and mixed thoroughly). That, more careful brushing, and an extra layer produced the piece on the right!
Partially and fully opacified cone 6 G1214Z matte glaze
This is a calcium matte base (as opposed to the magnesia matte G2934). The clay is Plainsman M390. 5% Zircopax was added on the left (normally 10% or more is needed to get full opacity, the partially opaque effect highlight contours well). 5% tin oxide was added to the one on the right (tin is a more effective, albeit expensive opacifier in oxidation). The PLC6DS firing schedule was used.
Body made from Plainsman Fire-Red, ball clay and feldspar
Fire-Red is an unusual material for several reasons. It has a high iron content yet is a fireclay (the iron percentage is so high that it fires black at cone 10R). It is also non-plastic. Most important, it is not ground to 200 mesh like industrial materials. This body demonstrates it well: 42.5% Fire-Red, 42.5% ball clay and 15% feldspar. All that ball clay gives it awesome plasticity. The feldspar gives control of the degree of vitrification (just raise or lower it for more or less). This recipe produces good density and strength yet still exhibits deep red color! Look closely at the surface: It is covered by thousands of tiny iron eruptions, these will bleed through an over glaze to give "speck-city" like no other!
Orange is a very difficult color in ceramics. Inclusion stains are the only reliable method and universally used in industry. But you could ignore that and try a bunch of recipes online, buying exotic materials to complete each one. Maybe one will be orange enough, but will it craze or run or blister or leach or cutlery mark or crawl? Or you could put an orange stain into a transparent glaze you already know works on your clay. Or, how about trying a premixed orange at low fire? Ware can be amazingly functional and there are so many other bright colours available.
When the cone does this I need to adjust the program
This is a cone 04. It is bent too much, the kiln has over-fired a little (cone 03 was also bent somewhat). The built-in firing schedule goes to 1945, that would be much more over-fired than this was (and the built-in ones do not soak, drop-and-soak or slow cool). It only takes a minute to edit the program I made, all I have done is drop the step-three temperature to 1930 (it was 1935). I adjust my schedule fire-up-to temperature as needed, I cannot imagine not doing this.
This is a small cup-sized object made from Plainsman P600 (simply composed of Tile #6 kaolin, nepheline syenite and quartz). It is valued as a product-of-the-process piece, consigned to the "kiln God" as unglazed. It exhibits carbon-trap, soda glaze deposition and flashing. The soda-vapour atmosphere of the kiln glazed one side of the vessel early enough in the firing to trap carbon under a crystal-clear glass. Often such glazes are crazed, but this one likely is not because the body contains 25% quartz, giving it a high thermal expansion. The other side of the piece exhibits tones of red, brown and yellow on the bare, vitreous porcelain surface - this is characteristic of "flashing".
The glaze is G1214Z cone 6 base calcium matte. 5% titanium dioxide has been added. This Plainsman M390 tile was fired at cone 6 using the PLC6DS firing schedule. Titanium can create reactive glazes, like rutile, even with matte surfaces (provided the glaze has good melt fluidity). Calcium mattes host crystallization and work particularly well. Because titanium dioxide does not contain iron oxide lighter colors and better blues are possible than with rutile. Like rutile, the effects are dependent on the cooling rate of the firing, faster cools produce less reactivity.
Wowzers! These are actually hand-made, not thrown on the potter's wheel. You can see the vertical join by the handle as it rotates. Her's is a simple concept: A red clay (M390) with a thin application of partially opacified matte glaze. She flaunts a bare red clay base, polishing it. You can find her easily on Instagram and google.
How to give children a good experience in working with clay
Teachers who have never worked with clay face a formidable challenge with this. This read-in-three-minutes page is a complete beginners crash course in what you need to know. It explains what clay is, the advantages of working at lower temperatures, how to plan and inspire the children before starting, how to join and dry pieces, what is glaze and how to use it and how to fire the ware. The page explains things with an objective that the reader understand the basic whats, hows and whys of ceramics and pottery.
Most low-fire bodies contain talc. It is added for the express purpose of increasing thermal expansion. The natural quartz present does the same. These are good for glaze fit but bad for ware like this. You could fiddle with the clay recipe or change bodies, but better to change the firing schedule. While stoneware dunting happens between 950-1150F on the way down, this could be happening anywhere. A simple fix is to slow down the entire cooling cycle. Learn to program your kiln. Use a conservative cooling rate of about 200F/hr (even slower between 950 to 1150). No electronic controller? Learn a switch-setting-schedule to approximate this down-ramp (buy a pyrometer if needed).
Wanna throw porcelain plates with thick bottoms and thin rims?
Then they may need a week to dry! This plate had a one-inch-thick base (while the rim is a quarter of that). During the first few hours a thin rim like this will dry quickly, leaving the base far behind. But as soon as it would support the weight of a cover-cloth I put it into a garbage bag and sealed and left it for several days. Even after that it did not detach easily, even though the bat had been dry. The base was still quite soft but the rim was stiff enough to enable turning it over and trimming it (I endeavoured to create a cross section of even thickness). Then I dried it under layers of cloth for several more days. It took at least a week. Had I allowed the rim to dry out during the first few hours it would likely have cracked later on.
GA6A Alberta Slip base using Frit 3124, 3249 and 3195 on dark body
The body is dark brown burning Plainsman M390 (cone 6). The amber colored glaze is 80% Alberta Slip (raw:calcine mix) with 20% of each frit. The white engobe on the inside of two of the mugs is L3954A (those mugs are glazed inside using transparent G2926B). The Alberta Slip amber gloss glaze produces an ultra-gloss surface of high quality on mugs 2 and 3 (Frit 3249 and 3195). On the outside we see it this glaze on the white slip until midway down, then on the bare red clay. The amber glaze on the first mug (with Frit 3124) has a pebbly surface. These are fired using a drop-and-soak firing schedule. Some caution is required with the 3249 version, it has low thermal expansion (that is good on bodies that normally craze glazes, but risks shivering on ones that do not).
The matteness this glaze develops is dependant on the cooling rate
This is the G2934Y matte cone 6 recipe with a red stain (Mason 6021). The one on the left was fired using the C6DHSC slow-cool schedule. The one on the right was fired using the drop-and-soakPLC6DS schedule. The only difference in the two schedules is what happens after 2100F on the way down (the slow-cool drops at 150F/hr and the other free-falls). For this glaze, the fast cool is much better, producing a silky pleasant surface rather than a dry matte.
Functional ware at low fire! Don't dismiss it just yet.
These were only fired at cone 04, but they are durable enough to last some time with normal use. The insides have a transparent glaze (Spectrum 700), it is leadless and completely safe. These are great insulators, they keep coffee warm longer than porcelain or stoneware. The feet are glazed so they are fine for the dish washer. They are super-light, the body is made from ball clay and talc and throws really well. These shrink very little on drying (in this case less than 2% compared to porcelains which can be 8% or more). These are inexpensive to fire, only four hours to cone 04. They withstand impacts better than you think (a thin porcelain mug propagates cracks and can shatter). Colour, glorious color! These are Spectrum Opaque low fire glazes, dozens of wild colors are available.
Make some adjustments and it is usable. First, it is very quiet and has lots of power. The plastic sliders ride smoothly and provide precise adjustability (but the plastic threads might not last). The vertical shaft is stainless steel and the cast iron base is heavy, sturdy, practical. The motor-to-shaft mounting collar is good quality (but must be tightened with a tool). It has a stepper motor that runs less than 300 rpm (not the 3000 advertised)! The timer switch will not likely last, better to leave it on and use on/off. It does not turn off completely on zero-speed setting. The propeller shaft is too short and the flapper on the end is useless in ceramic slurries. The shaft rotates opposite-to-normal direction. You have to 3D-print a large propellor (we can help you if needed), with that it will easily mix 2 gallons of thick, high-specific-gravity slurry (we replaced the 8" shaft with a 12" one).