Simply put: Glaze misfit. The glaze is under compression and it is pushing outward. That compression was created as these terra cotta pieces cooled in the kiln. After the glaze solidified, somewhere above red heat, it became a glass and began to contract. The body, to which that glaze is attached by a glass bond, had its own higher rate of contraction. The glaze has some advantages in this battle. Its thick application gives it extra power to assert its thermal expansion. The body is over-fired and has become brittle. The unglazed outsides, incised designs and varying thickness provide points of weakness where cracks can start. The body resists the relentless force from inside but the odds were stacked against it and the pieces do not even make it out of the kiln. Of course, the glaze could be applied thinner, ware could be fired lower, it could have a more even cross-section and the outsides could be glazed. All will help, but increasing the thermal expansion of the glaze (by increasing KNaO at the expense of other fluxes), is one change that would fix this issue.

Lilly will take you step-by-step through the 3D design process of drawing a propeller. We tried many methods of doing this to finally arrive at a simple procedure that produces a flexible parametric design. Follow the full transcript as you watch. You can use the same process to create one in this or other CAD software. Our design has only nine steps yet is flexible enough to accomodate a different number of blades, changes in the blade shape, angle, thickness and size and different heights and diameters for the hub and hole. If you would like this 3D file in Fusion 360 format, it is available in the Files manager in your Insight-live.com account.

In the past, we have used Adobe Premiere for making videos. This video marks our transition to using KDenlive instead (please be patient with the rough edges until we learn this better). We are using it on Linux! It is amazing that a tool this powerful exists as free software (although they accept donations).

Traditional 50:50 kaolin:silica kiln wash can be a real bummer to use. It flakes, both on drying and after every firing. Pieces of it stick to the feet of ware (plucking). It is not refractory enough either. Shelves need cleaning and rewashing often. Three outside-the-box ideas make this a better recipe.

#1 No raw clay! Strangely, calcined kaolin is better than raw kaolin, it imparts multiple advantages.
#2 No silica. We use zircon or alumina instead, they are more refractory. And we use 80%, not 50%.
#3 We add CMC gum. It is the hardener, it enables a high specific gravity, imparts awesome brushing properties and slows down drying on cordierite and alumina shelves.

The low water requirement and slow drying make this behave more like paint. It can be applied by roller or brush. Coverage is much more even and it does not shrink and crack on drying. Normally the raw kaolin in 50:50 kiln wash suspends the slurry, makes it brushable and hardens it on drying. But CMC gum is way better for the latter two. It is so nice to be able to apply a thin layer of wash even on highly porous shelves (like these alumina ones we make ourselves). Unfortunately, we can't have everything - a down side of this recipe is settling (more information on the recipe page linked below). Fortunately, if used every few days it won't be a problem.

This is not available as a product, we just like it so much we made a label for it!

Dipping glazes can, in very controlled circumstances, be multi-layered. If you have done it for some time, with success, you may have been just lucky. These pieces demonstrate one of many factors that can produce failure: The top glaze contains 7% bentonite and 5% zinc oxide - that is 12% hyper-fine particles, perfect to create the drying shrinkage to make this happen. The recipe author must have reasoned that it could "pinch hit" for the inadequate clay content. But 7% bentonite in any glaze is highly unusual. And, it is actually not even necessary here. Why? The high percentage of Ferro Frit 3124 is sourcing needless Al2O3 (alumina), that should be coming from kaolin or ball clay instead. Frit 3134 is the perfect stand-in, it contains almost no Al2O3, but otherwise is quite similar. The equivalent recipe we calculated on the right has the same chemistry, but does pass a sanity check. It is not guaranteed to work, but has a better chance than this one. For assurance of success, the base should be mixed as a base-coat dipping glaze.

Typical zero-boron high-temperature glazes will not soften in a 1500F decal firing. But low-temperature glazes will (especially those high in boron). Even middle-temperature ones, especially those having significant B2O3, can soften. G3806C (right), for example, is reactive and fluid, it certainly will. Even G2926B, which has high Al2O3 and SiO2, has tiny pits (because of the amount of B2O3 in contains). In serious cases, they can bubble like the mug on the right. What happened to this one? Steam. It was in use and had been absorbing water in the months since it was first glaze-fired at cone 03. The one on the left was not used, but it did have some time to absorb water from the air, it is showing tiny pits in the surface. Even if moisture is not present, on refire low fire bodies continue to generate gases of decomposition that affect glazes. Each decal manufacturer has a recommended firing temperature, that is for their decals, not your glaze.

This is a buff stoneware body, Plainsman M340. A L3954F black engobe was applied inside and upper outside at leather hard. The piece was fired at cone 6 using the PLC6DS schedule. The inside, totally clouded glaze is G2926B. Outside is GA6-B Alberta Slip amber transparent. Normally this inside glaze is crystal-clear on other bodies (and on this one without the black engobe). Clearly, the black stain in the engobe is generating tiny gas bubbles at the exact wrong time during the firing and the melt is unable to pass them. The outside glaze on on the same engobe, but the GA6-B glaze is demonstrating its ability to clear the micro-bubble clouding. It contains a lot of Alberta Slip, a material that is not finely ground like others. Particles across the range from 60-200 mesh are present, some of them appear to be acting as a fining agent to clear the bubbles.

This is a cone 6 porcelain mug with G2934 matte glaze (with 6% black stain added). We get this satin matte effect in our test kilns using the PLC6DS schedule. Larger kilns cool slower so this glaze turns out too matte in them, we deal with that by increasing the percentage of glossy base (this is a 15:85 blend of G2926B glossy and G2934 matte). The gold decal is from Sanbao Studio. On the left, it has just been applied, other than the glossy finish revealing its location, no gold design is visible. But, after the decal firing, using the MDDCL schedule, we get the result on the right.

The G2934 base matte recipe is good for decals because it has a very low B2O3 content (unlike high boron glazes that can begin to melt very early, even in a decal firing, and alter their degree of matteness or even produce tiny pinholes or blisters). G2934 can tolerate some high-boron G2926B glossy, enough to de-matte it, and still work well with decals.

This is G1947U clear glaze with 8% Mason 6021 encapsulated red stain added. The body is P700, a Grolleg kaolin porcelain. The one on the right, having significantly reduced clouding within, has one tiny addition: 2% Zircopax. It is acting as a micro-bubble fining agent, producing a brighter color and smoother surface. But there is a possible problem: These stains are not recommended for use above 2300F. Even though the color is very good, cone 10 is just on the edge of the limit temperature, so suitability for food surfaces would require careful testing for leaching cadmium.

Drying cracks are opportunistic, especially in highly plastic or fine-particled clays. They like to initiate inside sharp acute angles. The sharper the angle the greater the chance of crack. By doing this procedure before the clay gets too stiff (in the leather-hard stage) you will deny a crack a place to start. Of course, even drying is still important, the water content of a handle should now be allowed to get too far ahead of that of the main body of the mug at any time. In the pictures on the right, two tools are being used to compress and round the angle at which the handle meets the wall of the mug.

These are whiteware mugs (Plainsman M370) with tissue transfer designs that were applied at the leather hard stage. The pieces were then dried, bisque firing, clear glazed and then fired to cone 6. Tissue transfers are sold online in a wide range of designs (in full color also). They can also be made at home by silk screening the appropriate type of ink onto the tissue paper (you can make your own ink using ceramic pigments). These overglaze transparent recipes are G2934 matte (left) and G2926B glossy (right). The matte glaze softens the edges of the design. The pig is a good demonstration of how crisp the edges of lines can be (once the application techniques are mastered). This method of decorating is far less expensive than decals. And does not require an extra firing. While the ink is somewhat powdery, it can be stabilized with spray starch if pieces need to be transported for firing.

Not to be ignored are the two transparent glazes. Control of thickness is important. Too thick and they will go cloudy. Too think and they won't fire smooth. For use as a dipping glaze the slurry should be thixotropic. Using them as a brushing glaze, while taking longer to apply, does enable tighter control of thickness.