These two Plainsman M370 test mugs were fired at cone 6, the left one with G2934 matte glaze, the right one with G2934Y4 matte. They look and feel identical in the hand. The two glazes have the same chemistry. But they employ different materials to source that chemistry. The secret of of the matteness is high MgO (magnesia content). In the glaze on the left MgO is sourced by dolomite, a lot of it. The glaze on the right sources it from a special frit, Ferro 3249. The impact of this difference is visible in the melt fluidity tester, the fritted one is melting and flowing much better. On other clays, especially stonewares, the G2934 can have a dry surface that cutlery marks. Thicker applications make it worse. But the Y version exhibits no such issues. Its mattness, durability, cleanability and hardness are so good that it is being used in floor tile.

These cone 6 glazes are the same (G3806G), except the one on the right has 3.5% copper carbonate added. Copper is commonly fluxes glazes, making them melt more. But in this case it is not, the clear base is running just as much as the stained one. Either the percentage is not high enough or the host transparent glaze is resistant. Another observation: I was suspicious that the micro-bubbles in the glass matrix were coming from the copper carbonate gassing during firing. But not so, as you can see on this melt fluidity tester, the flow on the left has many more (it appears less melted because of this). In this specific glaze it seems probable that the copper bubbles (generated as it decomposes) act as a fining agent to coagulate and help clear the others.

Here is why the stair-casing artifacts are not the problem many people think. These are stonewares fired at cone 6 oxidation. The dark one is M370C with 10% added raw umber. The other is M370C. Both are glazed using GA6-B Alberta Slip amber transparent. The wood-grain texture on the right is an artifact of 3D-printing - the case mold was printed flat rather than upright. Strangely, that is the bottle people want! But the production prototype bottle is the one on the left and the stair casing is barely visible. Additionally, these are prototypes, the production molds would either be made by printing the model upright or by casting a plaster model of a bottle half, smoothing and soaping it, attaching it to a clamping baseplate and then setting up 3D printed railing around it.

Same clay body: Plainsman Coffee Clay. Same glaze: MA6-C rutile blue. But the mug on the left was fired in the PLC6DS schedule (normally that one does not produce this much blue, but the heavily pigmented clay brings it out). The one on the right was fired in the C6DHSC schedule. That schedule also improves the gloss and surface quality of the inside GA6-B liner glaze.

It is a clay, a very non-plastic one. These are fired SHAB test bars of Barnard Slip going from cone 04 (bottom) to cone 6 (top, where it is melting). Porosity is under 3% and the fired shrinkage above 15% from cone 1 upward (second from bottom). Drying shrinkage is 4% at 25% water (it is very non-plastic). The darkness of the fired color suggests higher MnO than our published chemistry shows (and also higher iron). The white areas on the lower temperature bars are soluble salts.

Since this is a fine particled material, it could likely be made plastic with a bentonite addition, likely 5% or more would be needed. Solubles could be precipitated using barium carbonate.

Long after installation, handmade clay tiles can be susceptible to chipping at the corners and edges. This is more of an issue when the tile is glazed. As the temperature of the tile increases from the heat of the sun, the dimensions increase and they can begin to press upon each other. This can create high compressive stresses at the bearing points. If the gap between the tiles is not sufficient, the stress at the bearing point can continue to build until a piece cracks out of the corner like this. Terracotta tiles are most susceptible because they have much lower strength than vitrified ones. Since such handmade tiles have been in use since ancient times makers have always needed to compensate for this issue.

Of course the installers of the tile bear the main burden of avoiding this problem. However, in the manufacture of the tile itself, various things can be done to minimize the issue. For example, if an engobe is used, pay attention to its compatibility to the body (e.g. it needs a similar firing shrinkage and thermal expansion). Likewise with glazes, they should fit (not be under excessive compression or tension). Rounded corners are better than angular ones. Another option is to fire the tile to a higher temperature to get more strength - with caution since this can significantly increase firing shrinkage.

Once you try these you will never go back to making molds without them. Unfortunately, these are not easy to get in North America. Or even online. But you can 3D print them yourself (we use PLA filament). This design interlocks with standard 3/8" natches used in industry. There are more aspects to printing and using these than meets the eye, here are some aspects to know:

-The base can be widened for sticking on the build plate better. If you need to print large numbers it might be advisable to use a glued plate to make sure they stick well.
-The inner edge is chamfered to ensure better insertion of the nipple.
-Print without infill for better strength.
-These are hollow, no support is needed.
-The bottom can be widened to stick better to the build plate.
-The ribs can be moved.
-A 9.8 mm hole is needed in the mold.

Plainsman Clays extracts 6 different sedimentary clays from this quarry (Mel knows where the layers separate). The dried test bars on the right show them (top to bottom). The range of properties exhibited is astounding. The top-most layer, A1, is the most plastic and has the most iron concretion particles (used in our most speckled reduction bodies). The bottom one, 3D, is the least plastic and most silty (the base for Ravenscrag Slip), it is also the cleanest and most vitreous. The middle two, A3 and 3B, are complete buff stonewares (e.g. M340 and H550). A2, the second one down, is a ball clay (similar to commercial products), it is refractory and the base for Plainsman Fireclay. The second from the bottom, 3C. fires the whitest and is the most refractory (it is the base for H441G).

Reactive glazes don't melt into a homogeneous melt and they don't freeze as a typical glass. The physical nature of the material powders (e.g. their particle size and the individual nature of how they respond to heat, soften, melt and interact with their own kind and others) create a melt that does not solidify into a homogeneous glass. These glazes are said to be dynamic. And unpredictable effects often occur during firing, like color variegation, speckles, streaks, mottled and flowing textures, crystallization, pooling, etc. The outcome is influenced by factors such as the materials chosen to source the needed oxides, firing schedule, kiln atmosphere, cooling or heating cycle, etc. These glazes are at their best when each piece has a unique, artistic character. But, this is also their worst feature, making them "tipping point glazes", ones whose visual character is a product of fragile and not well understood features of the materials and process. Small changes typically produce big changes in fired appearance (often to the chagrin of the potter).

Here GR6-M it is fired to cone 8 where the melt obviously has much more melt fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course, it is running alot more, so caution will be needed.