Too much frit in an engobe and it will lose opacity and whiteness. The white slip on the left is an adjustment to the popular "Fish Sauce" slip recipe (L3685A: 8% Frit 3110 replaces 8% Pyrax to make it harder and fire-bond to the body better). The one on the right, L3685C, has 15% frit. Although applied at the same thickness, it is becoming translucent, moving it into glaze territory. That means it will have a far higher firing shrinkage than the body (a common cause of shivering at lips and contour changes). This slip is basically a very plastic white body. Since white burning slips are made from refractory materials they are not nearly as vitreous as red ones, at low fire they need help to mature and a frit is the natural answer. With the right amount of frit the fired shrinkage of body and slip can be matched and the slip will be opaque. This underscores the need to tune the maturity of an engobe to the body and temperature. Although zircon could be added to the one on the right to opacify and whiten it, that would not fix the mismatch in fired shrinkage between it and the body. And it would increase the price.

This is part of a project to create a new mold. I have to make various iterations to arrive at a final design where rubber will be used to make the case mold. But until then I will 3D print the case mold directly. Here are some features that make this super cool:
-The center section is the jug being cast (two pour spouts will be glued on). It is hollow and will be split horizontally in the slicer so the two halves can be printed with open side up and then glued together (with the aid of printed inner hoops to align them).
-The mold seams, where the two halves mate, is printed as a 0.4mm membrane connected to the model and running vertically down its center.
-The membrane has 9.4mm holes for the insertion of standard mold natch pairs. During plaster pour the membrane will flex somewhat, this will be a benefit to aligning the halves.
-The outer shell halves have no angles steeper than 45 degrees and thus print without supports. The flanges were drawn using our rotation technique (see link below).
-The round flat base will be glued onto the bottom disk.
-Any rough surface sections of the model (printed on top of support), will be coated with a fluid epoxy to smooth them.
-A separate handle mold will be made.

This is an example of how a glaze that contains too much plastic clay has been applied too thick. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and roasted clay. Enough raw clay is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. Perhaps you have been using a glaze having a high percentage of clay and this does not happen - the reason is likely that the clay is not highly plastic.

This is the same piece in sunlight and a fluorescent flood lamp. The picture was taken by an iPhone and then cropped to a white background using the path tool in Photoshop. The whole mug is glazed with GA10-B. The inside and upper outside of the leather hard piece was engobed using L3954N.

This is a GLFL test comparing the melt flow of the three materials at 1800F. Frit 3124 is barely out of the starting gate and the other two have crossed the finish line! With frits chemistry is a big deal, they are all about supplying oxides to the melt. Frit 3134 is low-alumina/high-boron, 3124 is medium-alumina/low-boron and 3195 is medium-alumina/high-boron. Boron is the melter. Alumina thickens the melt and hardens the glass. Just from this it appears that Frit 3195 is a better starting point for calculations to replace frit 3134.

We want to achieve embossed lettering inside the foot rings of slip-cast ceramic pieces by using a standard consumer 3D printer with PLA filament. There are plenty of obstacles to overcome in doing this. Since plaster does not release well at all from lettering having sharp corners, bevelling has to be done. However, our CAD software has problems rounding the corners of many fonts, I had to search for one having no variations in stroke width. Then it was a matter of discovering how much to extrude and how much bevelling it would tolerate - this one permits a 1 mm extrusion with a 0.6 radius bevel.
Left front: A 1mm extrude without bevelling and cold release produced very poor results.
Centre: 1mm extrude, 0.6 radius bevelling, 0.2mm rise of background to minimize verticals and cold release - better but still problems.
Right: Same as center but released with heat gun - almost perfect.

This custom sample board was made by Solange Roy, it is great evidence of her meticulous and thorough approach to ceramics. This picture is taken at an angle to show the surface finish and character. These are Stroke & Coat ® underglazes, they are made by Mayco and widely used and praised. Although used as such by countless potters, they are not traditional underglazes (like Mayco Fundamentals or Amaco Velvets). They are heavily pigmented and have a highly controlled viscous melt fluidity. Applied by brush they give watercolour-like effects in thin applications and increasingly opaque coverage with each added layer. These products give evidence of meticulous lab work (like this board done by Solange), technicians would have had to do melt fluidity tests of each color and fine-tune the base recipe to get the desired degree of melt fluidity (compensating for the unique effect on melting of each stain type and percentage needed).

This handle mold is for v.5 of our 3D mold-making (and discovery) project for the ball pitcher. The process to make the 3D drawing is quite simple: Cut it out of the model (top left), draw and extrude side walls (top right) and slice off and remove the pointy parts (a step-by-step video coming soon). Bottom left: A ready-to-use mold. Notice how it fits perfectly onto the side of the pitcher form (bottom right). Because of the good fit, attaching these is just a matter of using some casting slip as the glue. Casting this handle separately affords multiple benefits: It simplifies making the mold of the pitcher itself, of extracting pieces after casting and it produces a more professional-looking product (without holes inside where the handles join). And, handles can be stockpiled in a damp box, ready to use when needed.

The cracks appear to have happened on heat-up (because they have widened). Bisque firing was done around cone 04. Issue 1: The cone 10 electric firing was up-ramped at 400F/hr to 2330F (so it whizzed pass quartz inversion on the way!). Issue 2: Wall thickness variations in the pieces, they produce temperature gradients that widen as firing proceeds. Issue 3: Abrupt contour changes and sharp corners, especially when coincident with thickness variations, provide failure points that rapid temperature changes exploit. Issue 4: This new body is more plastic than the previous Grolleg porcelain used, that was likely an enabler to making these thin wall sections even thinner. But remember, practically any piece (unless it has huge in-stresses from uneven drying) can exit a kiln crack-free if firing is done evenly and slowly enough. Results of past firings are the main guide to know what to do in future ones, this is now a "past firing". So the first obvious fix here is slower heat-up, especially around quartz inversion (1000-1100F). Second: more even wall thickness.

This problem typically happens after some years of use. Here are some questions to answer:
A glaze may look smooth visually, but is it really? If the surface has micro-cavities and surface disruptions this could give organics a place to attach and build up. Firing curves need to take into account the LOI of glaze materials (which can affect the microsurface), materials like calcium and barium carbonate, dolomite, talc and even clay.
Has the mirco-surface of the glaze changed? Many labs offer surface analysis services so it is possible to have a surface compared with one known to be good.
Is the glaze subject to leaching? This could happen if the fired matrix has excessive phase changes, possibly due to poor mixing, frit quality issues or materials of excessive particle size (e.g. use 400 mesh silica instead of 200 or 300). A change in the flow of a routine melt fluidity test can alert you to changes in materials or slurry preparation.
Zircon is implicated in cutlery marking, if your glaze is marking this puts suspicion on the zircon as at least part of the problem. What grade of zircon do you use? It should be 5 micron. Are you using high-energy mixing to separate the zircon agglomerates?
Is your glaze fired at the temperature to produce optimum hardness and durability? Fire it higher and lower than the production temperature and test surface properties and melt flow. If you determine, for example, that it is under-firing, then add flux to melt it a little more. If it is under-fired, add SiO2 or Al2O3 or reduce flux (KNaO, MgO or CaO).