Example of a glaze (G1916J) shivering on the rim of a mug. But the situation is not as it might appear. This glaze fits some bodies, crazes on others and shivers on yet others! This is a testament to how tricky it can be to fit glaze at low temperatures. But this is not a white glaze, it is a transparent over a white slip (or engobe). Alone on this red body this glaze appears to fit OK, but here it is exploiting the weaker body-engobe interface - this is where the release is taking place. This failure turned out to be a blessing, alerting us to the need to increase the expansion of the glaze a little to fit this body better (and enable its use with this engobe). This being said, the engobe likewise may be under too much compression, it may not contain enough silica.

This is part of a test hybrid case mold for a Medalta Potteries ball pitcher. The bottom plate is 3D printed and the top form is solid plaster. Recessed holes in the back of the plate enable securely inserting screws into threaded anchors embedded into the back of the plaster form. The upper plaster surface has artifacts (stair-casing remnants) from the 3D printed shell used to cast it. While these could be sanded out, we find that a flexible metal rib works much better. Even better than that is this custom 3D printed rib that I made, the edges are sharp and precise. I designed it with contours to match the belly, neck and rim of this piece. Using this, it takes minutes to smooth out the surface.

I have had a dream of being able to slip-cast jars with this type of fastener. Until this week, I did not even know what they are called or anything about their history. Now I do.

The first step was to design and 3D print a prototype. After three iterations, I found the best measurements for these bails, for use without a silicone or rubber sealing ring, to be 19.5mm vertically between the rings, a lid ring seat diameter of 79mm and a rim ring seat diameter of 87.5mm. It is going to be a challenge to create slip casting test molds to make jars out of ceramic instead of glass - the latter does not shrink out of the mold while clay has both drying and firing shrinkage. That being said, 3D printing enables quickly changing a mold design and size, reprinting in PLA and remaking a plaster mold. 3D printing also enables making holders, cutters and jigs to tool the diameters precisely at the dry stage. After that, precise firing along with monitoring of clay firing shrinkage should enable success. After that: Embossed designs cast into the flat walls.

G2917, which I mixed as a brushing glaze, is on the right. This is not sold in jars, I make my own labels as part of the demonstration that it is possible to make your own brushing glazes (ink-jetted onto regular paper, cut 62mm wide (2 7/16") and held securely on with 2 7/8" transparent packing tape). This glaze is less runny but lacks some of the floating white colouration. But that can be achieved using Alberta Slip floating blue L4655, it employs titanium instead of rutile (and relies on the rutile/iron mechanism for the blue color).

This spoon was dipped into a ceramic dipping engobe, L3954B. It contains no CMC gum, it was only flocculated using powdered Epsom salts. Without the Epsom salts, the engobe runs off, leaving only a film. But, when turned into a thixotropic slurry, it stays on the spoon in an even layer (as a gel), then hardens as it dewaters (left) and finally dries completely (right). With no cracks! It also fires to cone 03 with no cracks. Of course, if this were fired high enough, it would begin to shrink, crack, crawl, melt and then craze, ceasing to be an engobe. Of course, special low-expansion frits and additives and mixing, preparation and application techniques make enamels, which do melt, possible for metals.

This is M340 stoneware fired to cone 6 using the C6DHSC schedule. The L3954B engobe fires deep black (it has 10% Mason 6600 black stain). The engobe was applied by pouring and dipping at leather hard stage (inside and partway down the outside). After bisque firing, the piece was glazed inside using the base GA6-B Alberta Slip amber base. The outside glaze is Alberta Slip Rutile Blue GA6-C (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers).

This is 50g of sodium bentonite in 1 gallon of water (~0.5% concentration). Sodium bentonite creates a gel that resists mixing. With a better propeller, faster mixer, hot water, slow addition, etc. we might be able to achieve 1% (or ~100g/gallon). Mud-men (drilling mud mixers) claim 4% is possible using shear pumps, ribbon blenders, or paddle mixers - that seems impossible to me! Why does all this matter? Some say that a gel like this can be added to a settling glaze to fix it. Is that true? Not really. Here is why. Our typical imperial gallon of dipping glaze contains 3500g powder (US gallon 2800g). At least 1% bentonite, or 35g, is needed to make any difference. The more typical 2% would be 70g. If you have a great mixer and can make a slurry having double the concentration of this one, then to deliver the minimum bentonite for a gallon of the glaze would require 1/3 gallon of this! While the added bentonite might help somewhat, the effects of all that extra water will cancel the benefits.

This award-winning couple are-all in on crystal glazes. They have learned that success is about data. A lot of data. Thousands of pictures, hundreds of firing schedules, hundreds of recipes, endless notes all come together in the growth of crystals like these! Notice the clear background, no micro-crystals fogging it up. Notice that two fundamentally different types of crystals are being grown. Not to be ignored is the throwing skill it takes to make a porcelain piece like this, these are not small pieces.

The blue line is a crystal glaze firing schedule. While it reaches the same temperature as a typical glaze firing (purple line) it is different in how it does so. Notice key differences (while cone 10 is most common for this type of glaze, we will discuss theoretical differences in a cone 6 version):
-The steep climb: Crystallization needs a clean bubble-free melt, no lingering in temperature zones where they might start prematurely.
-The steep drop to 2000F: Crystals typically grow during a long soak in the 1900–2000°F nucleation zone.
-If the temperature is simply held steady at 1950°F only one type and size of crystal would form, likely smaller and crowding out others. The ups and downs are about manipulating the thermodynamics and kinetics of crystal formation — nudging new crystals to form or existing ones to grow differently.
-Cooling and then raising the temperature in the nucleation zone can re-dissolve smaller crystals or unstable nuclei. Then, cooling again encourages new crystal nucleation, rejuvenates existing ones or even changes the pattern of their growth.
-In the upper range of the nucleation zone, faster diffusion produces larger, more spread-out crystals. In the lower range, slower diffusion produces smaller, tighter crystals or detail-rich growth.
-Crash-cool to finish: Drop melt viscosity quickly to halt all crystal formation - this preserves a clean background and prevents blurring of crystal edges.
Crystalline firings are about precision and timing: Get in fast, melt everything, play within the range where crystals want to grow to get the type, distribution and size you want - and then get out. It is not difficult to see why crystal glazers may do thousands of test firings to discover the curve that produces what they want. The nucleation zone depends on firing temperature and glaze chemistry, testing is likewise required to discover it. Meticulous record keeping is critical to success; not surprisingly, many crystal glazes do it in an account at insight-live.com.

The evolution of the quality and aesthetics of your work, and even your ability to cut costs, are stunted when you depend too much on others (e.g. for firing, for premixed glazes). This mug is a good example of tests I need to do. This is G3933, made by adding iron oxide, rutile and tin oxide to a 75:25 blend of our base matte and glossy glazes (G2926B and G2934).
-It is crawling at some of the sharp angles of the incised decoration, would a little CMC gum fix this?
-Would an 80:20 blend of the two glazes give a little more matteness?
-Our red-burning body gives better color at cone 5, would this glaze be richer and more matte on it in the C5DHSC slow cool schedule?
-I want to test increases in the rutile (for variegation), iron (for better color) and granular manganese (for more speckle).
-Would a Ravenscrag Slip base glaze be a better host for the rutile, iron and tin?
Having a small test kiln puts all of these changes on my radar. An account at insight-live.com to document everything well brings it all together.