These two glazes look the same, they are both cone 6 satin mattes. On the same porcelain. But the matteness "mechanism" of the one on the left is a low Si:Al ratio melted by zinc and sodium. The mechanism of the one on the right is high MgO melted by boron (with the same Si:Al ratio). The "baggage" of the mechanism on the left is high thermal expansion and crazing (drastically reducing strength and providing a haven for bacteria). The glaze is "stretched" on the clay (because it has a higher thermal contraction). When the lines are close together like this it is more serious (I have highlighted them with dye). If the effect is intended, it is called "crackle" (but no one would intend this on functional ware). The glaze on the left calculates to a high thermal expansion so the crazing is not a surprise.

This bowl, although only 33cm (13in) in diameter weighs 6kg (13lb). The walls are an inch thick. It would very likely not be possible to fire this up and down through quartz inversion sitting on a kiln shelf, almost certainly a significant temperature gradient would develop between the outer walls and the base. And when a gradient enters into the zone of quartz inversion, especially on the way down, it is trouble. I have already made the situation better by making the wall and base thicknesses as even as possible. This refractory setter the next step. I made it using the L4543C recipe, it is wonderfully plastic and it was easy to throw on the potters wheel. This setter enables radiant heat and airflow (what little there is in an electric kiln) access to the base, reducing thermal differences between the walls and the center. An additional benefit: It is so refractory that cone 6 does not even touch it, the platform stays absolutely flat, thus the bowl stays round.

This cookie cutter can both cut and stamp the piece (notice the 3D render in the centre, the logo is 2mm lower than that cutter around the outside). We make them by rolling a slab to 3.2mm (1/8in) thick, applying stretch wrap over it and then pressing the cutter/stamper into it (using a wood block). Then just peel away the plastic and the outer waste clay and a perfect crest is left. This method enables using clay of almost any stiffness. We find that softer clay works best, just peel it up from the board, apply slip to the back, position it on the side of the leather hard ware and press it down (from the centre outwards). On the lower right is a crested mug that has just been glazed. Upper right is a crest that has been glazed and fired. About the cookie cutter: We create them with 0.8mm wall thickness (twice the width of the 0.4mm extruder on the 3D printer). We export the vector image (made in Illustrator) into Fusion 360 and then add elements to stabilize and hold the profile in place. This cutter is 8mm tall and the stamp lines are 5mm tall. The crest is 52mm (2 in) wide. This whole process may sound a little intimidating to you - but we are working on a step-by-step video.

This eight-step firing to cone 6 took 33 hours, that is our conservative effort to assure this test bowl with inch-thick walls and base would not crack. It is true that the speed of firing is not the determining factor, the key is the avoidance of uneven distribution of temperature across the piece at all stages of firing, especially near quartz inversion. But achieving that in an electric kiln having no airflow and radiant heat is tricky. Two things helped assure success: The custom setter on which this was fired and the °/hr being controlled both upward and downward. To be even safer I slowed to 75°F/hr between 950 and 1150°F. A 30-minute hold at the beginning and end of each of these slow-downs added further insurance against any temperature gradients developing in the piece.

The problem occurred with standard Plainsman M340, M390, M350, M370 and P300. The stonewares have porosities of 2-3%, the M370 1% and the P300 0.5%. Thus, all of these have comfortable margins for overfiring. The G2926B glaze, used on all of them, does seal the surface pretty early so it can contribute to over-fired ware bloating sooner than typical. The problem here is the cone-fire modes on kiln controllers. For this kiln, the cone 6 program goes to 2236F. That's cone 7. Adding the error of the thermocouple and the misinformation from poorly set cones the temperature overshoot could be more. Manually programming your kilns in consort with calibration using self supporting cones, that is the way to get control. Then you will benefit from the drop-and-soak PLC6DS and slow-cool C6DHSC firing schedules.

This wheel-thrown bowl is large, heavy and thick walled. There are pouring techniques to apply different colored dipping glazes to the inside and out but they are cumbersome and slow. It turns out that the hobbyist way of glazing is actually better - by brushing. I make these 500ml jars of brushing glaze and tune the percentage of gums to get the painting consistency and drying time best for me. Applying multiple coats by brush took only a few minutes and no wax resist or any other tools were needed. And it was easy to meet the two glazes in a straight line at the rim using the brush. The inside glaze is G2936B and outside is G2926B.

The glaze thickness is also the same. The firing schedule is critical with iron reds, we tried both the C6DHSC and C6IRED schedules, both of which normally produce the result on the left (with this G3948A recipe). But in both firings the result is now like the piece on the right. The one difference is this: The new batch was ball milled. Although all materials in the glaze are processed to 200 mesh, the iron oxide particles agglomerate. Neither our propeller mixer or blender remove these agglomerates - but the ball mill does. That means their presence is important to achieving the red, those agglomerates must be either seeding the crystals or facilitating their growth.

The old one inside is in bad condition (a new one is sitting on top ready to install). In 2022 these cost about $35 CDN. The temperature-measuring part of a thermocouple is the join of two dissimilar metal wires, these are 8 gauge. The junction produces a temperature-dependent voltage that a pyrometer or controller can convert to a reading. Thermocouples can degrade into pretty poor condition yet still work, notice the one in this kiln is separating in two. Thermocouples generally need replacement more often than elements, they generally last about 150 firings (cone 04-06) and 50 firings (cone 6). Replacing these does not require electrical expertise.

In bisque kilns ware is being fired for the first time. If pieces are thick and the rate-of-rise is too fast then water (which is turning to steam) cannot escape fast enough. The internal pressure will fracture a piece like this that has not gone through a thermal drier. The schedule to fire this test brick was 150F/hr to 250 and hold 90, 200F/hr to 1640 no hold, 120F/hr to 1888 no hold. The fracture likely happened because the kiln schedule did not allocate enough hold time at 250°F or proceeded too fast to 1000°F. 250°F might sound like too high a temperature to do water smoking at because it is over boiling point, but in practice it does work in industrial kilns and driers with good airflow (which this kiln does not have).

It is often said: "No good deed goes unpunished"! This can happen when doing a clay session with children. So there is a need to enter, with "both eyes open" to avoid a negative experience. If you can succeed they will get a unique tactile experience in a social setting. And they will experience the anticipation of waiting to see what the kiln will do with their creation. If the kiln gifts them a brightly colored durable piece, especially one that they can drink or eat out of, the experience and the object can stay with them for a lifetime. If you are an instructor inheriting a kiln and clay program for kids it can seem overwhelming, especially if your technical knowledge is limited. But there are some simple things you can do to assess the condition of the kiln in your facility and whether it is practical to attempt some clay sessions with the children in your trust. Click the link below to learn more.