Black-burning bodies are popular with many potters. This one is stained by adding 10% raw umber to a buff-burning stoneware. Umbers are powerful natural clay colorants, they have high iron and also contain some manganese oxide. Could a white engobe produce a porcelain-like surface on such a clay body? Yes. L3954B engobe was applied during leather-hard stage to this Plainsman Coffee Clay mug (on the inside and partway down the outside). After bisque, transparent G2926B glaze was applied inside and GA6-B outside. Notice the GA6-B over the engobe fires amber but over the black it produces a deep glossy brown. The engobe was mixed into a thixotropic slurry, as explained on the page at PlainsmanClays.com (see link below), and applied in a relatively thin layer. This porcelain-like result is a testament to the covering power of a true engobe. It is no wonder they are so popular in the ceramic tile industry - a red burning body can be turned white as a porcelain, that enables all the marvellous glazing and decorating they can do.

Yes. If it is a true engobe. This is L3954B fired at cone 6 on Plainsman M340S, it is fire-shrinkage-fitted to this clay body and opacified with Zircopax. The cover glaze is G2926B transparent. The opacity that this engobe is able to achieve here is because it is vitrifying to the same degree as the body, no melting is occurring and that is why it is completely opaque (even though it is applied as a very thin layer at the leather hard stage). This same performance could be expected in reduction firings to block the iron speckle (using the L3954N and variations recipes).

This is a cone 10 glossy glaze. It has the chemistry that suggests it should be crystal clear and smooth. But there are multiple issues with the materials supplying that chemistry: Strontium carbonate, talc and calcium carbonate. Each has a significant LOI and produces gases decomposition. When the gases need to come out at the wrong time it turns the glaze into a Swiss cheeze of micro-bubbles. A study to isolate which of these three materials is the problem might make it possible to adjust the firing to accommodate it. But probably not. The most obvious solution is to just use non-gassing sources MgO, SrO, CaO and BaO (which will require some calculation). There is a good reason to do this: The glaze contains some boron frit, that is likely kick-starting melting much earlier than a standard raw-material-only cone 10 glaze. That fluid melt may not only be trapping gases from the body but creating a perfect environment to trap all the bubbles coming out of those carbonates and talc. All of this being said, a drop and hold firing schedule could also smooth it out a lot.

This was done in Fusion 360.
1: A make a sketch of a box, around the handle, on the XY plane. Offset that outward by 1.2mm (my printer prints 0.4mm wide, three passes give good strength).
2: Extrude to create box 1: The base backward by 1mm and the sides forward by 20mm.
3: Use five sides of the box as cutting planes to slice it out of the mug.

At this point I could print this in PLA filament, pour plaster into and then use a hair drier to peel it off. But let’s make rubber molds instead.

4: Move the box-with-handle away from the mug. Pull the four sides out by 5mm to thicken them.
5 & 6: Create box 2 around the outside of it, as a new body, 1.2mm wider and taller, 1mm more frontward and 1mm less backward.
7: Use box 1 as a cutter to remove material from box 2 and then pull the outer 1.2mm sides 5mm backward.
8: Shell out the back side to 1.2 wall thickness and make two 9.4mm holes (to accommodate natch clips).

To make side 2 mirror-image a new body using the front or back as the reflexion plane. The back side is then filled with PMC-746 rubber to make the block mold. Plaster is poured into that to make each working mold.

The crawling is happening at sharp concave surface contours (e.g. inside bottom corners on mugs) and on pieces like this. G2934 has good melt fluidity so that is not the issue. Here are some questions to consider:
-Did the glaze crack on drying? That is a sure sign it will crawl. Did you use calcined kaolin as specified in the recipe? That helps reduce the shrinkage as it dries (to eliminate the cracking).
-This looks like it is going on pretty thick - was it correctly mixed as a thixotropic slurry (high water content, 1.44 specific gravity, and gelled)? Or was it mixed with low water content (e.g. 1.55 specific gravity).
-These crawled areas were disturbed during spout forming, what can be done to repair the surface then?
-This is firing pretty glossy so kiln cooling is likely pretty fast. A drop and hold firing can help (as long as the glaze is not too thickly applied).
-Slower cooling means it will likely fire too matte, that is why we make an 80:20 blend of G2934 and G2926B glossy - to enable tuning matteness to any firing schedule.
-Mix up some of it as a brushing glaze. Apply a thin layer onto the bisque in the areas likely to crawl. Then dip the whole piece in the dipping glaze version.
-Mix the batch as a base coat dipping glaze, then it will adhere better to the bisque.
-Reduce the amount of calcined kaolin somewhat in favor of raw kaolin (perhaps 5%) - that may produce a slurry with better coverage and adherence.

The machine is being cleaned in preparation for a porcelain run (after producing stoneware clays). The pugmill has been stripped down completely - all parts in the main chamber, vacuum chamber and in the nose). The casings, augers and other components are washed, dried and inspected. Clean-downs and associated maintenance like this are costly but necessary to ensure quality porcelain. Each time we do this we are reminded how amazing this machine is - it can premix, pug and de-air even the most difficult of all clay bodies, Polar Ice. That body can be thrown straight out of the machine as plastic as it would be if aged for a year. This machine routinely produces 600+ boxes per day, turning powder into pugged in a continuous operation.

The problem is a combination of the shape and the degree of vitrification this body reaches. Polar Ice porcelain has to vitrify enough to achieve translucency, that means it literally softens - not enough to fall down but enough to warp out of shape given the opportunity. A sagging kiln shelf, for example, will produce a "rocking chair bowl". A non-stable shape will do the same thing. This piece was likely made by rolling a plastic clay slab and draping it down over a bowl-form, adding a foot ring, allowing it to stiffen and then uprighting it to dry. In this case the foot ring was too small creating an extreme overhang. Had the foot ring been wider and deeper it would have enabled the rounded inside contour, provided support for the outer section and minimized the overhang. If a small foot is really needed then pieces would have to be supported by donut-shaped setters sized and positioned correctly (and the outside would have to be unglazed). Or, it would have to be bisque fired, and supported, at cone 6 and then glazed at low temperature.

Left are case molds, they are made by 3D printing the positive profile on a backplate (with holes for natches). These are secured into slotted rails. Right is a block mold, it is made by 3D printing the profile of a working mold with integrated rails. This one is printed vertically in four pieces. It is held together and straight with printed brackets. We pour rubber into these to make case molds. Each method has advantages and issues.
-Case: Faster to print. Easier to draw. Joins cast as easily removed bumps on the working molds. This is only suitable for prototyping, making one working mold.
-Block: Much more attention is needed in printing, there are more issues with orientation of print, infill, support, multi-piecing, fit and seam-filling. 3D drawing of these is more difficult. And block molds are bigger because they are molds of molds. They also need to be more precise to merit the cost of the rubber.

This is housed on the lower floor of the Medicine Hat public library. It is a testimony to the skill Luke Lindoe, this heavy piece has no visible cracks. Made at the time Plainsman Clays was just starting. Luke did not have a high-temperature red burning clay to be able to make the warm colors of H440, for example (this employs the clays available in the I-XL Brick suite of raw materials). He did have materials mined around Elkwater, two of them he could have used were 45R, a low-fire red similar to BGP, and 45D, a medium-fire plastic material. These types of materials would have limited firing temperature to around cone 2. But Luke would also have been aware of the more refractory clays around Ravenscrag, Sask and could have been processing them himself - that would have enabled a high-temperature body for gas firing in a reduction atmosphere.

Ceramic glazes are actually just glass. But they are not like bottle glass. The latter is formulated to work well in forming machines (harden quickly), melt and stiffen quickly, have low melt viscosity and resist milkiness and crystallization on solidification. The chemistries to accomplish this have adequate resistance to leaching and adequate durability for a single or few uses. A stoneware glaze melt needs to be much more viscous (to stay put on vertical surfaces). And, it must have a much lower thermal expansion (to match common clay bodies). And, it must resist crystallization more much (since it cools slowly). Fortunately, meeting these needs brings along big benefits: Greater durability, hardness and resistance to leaching. Common target formulas express typical oxide formulas of glazes. Stoneware glazes and bottle glass share a common trait: They have about the same amount of SiO2. But the similarity ends there, stoneware glazes have:

-High Al2O3. Three to five times more! It is the key oxide to producing durable glass. And it stiffens the melt (that disqualifies high levels from bottle glass).
-The same fluxes (CaO, MgO, K2O, Na2O). But they distribute very differently (half the CaO, half to one third the KNaO, much more MgO). Other fluxes like SrO, Li2O are also common.
-Low KNaO (which they call R2O). In glazes it produces crazing, 5% is a typical maximum. But bottle glass can have double or triple that (the high thermal expansion is not an issue and its cheap source materials supply lots of melting power).
-B2O3 melter. It is expensive but can be justified because the glaze is just a thin layer. Glazes at the low end of the stoneware range have 5% or more boron.

The ceramic bottles shown here are made from a dark burning stoneware, the glaze is GA6-B. On the left is the same glaze on a porcelain mug. For the above reasons this glaze is more durable and leach resistant that regular bottle glass.