Glazes Are Crazing on This Casting Porcelain

The casting process enables a unique and effective fix

This potter almost has the casting process working, making these beautiful porcelain mugs. They are fired at cone 6 using a transparent glaze over underglaze decoration. But the devil is in the details. Look closer to see it: Crazing. Why? The reason is evident on the SDS for the body. Notice it has 10.5-15.8% crystalline quartz (or silica). This is not enough to prevent crazing in typical glazes. L

Almost always, the solution is to find or formulate a clear glaze having a lower thermal expansion (in this case, a lot lower). But with casting bodies we have another option: Mix our own. Unlike glazes, porcelain recipes are typically just three materials: kaolin, feldspar and silica. The starting percentages are simple for cone 6: 30% feldspar to vitrify. And 25% silica to fit the glaze. That leaves 45% kaolin. It is that easy! Of course, you need to choose which feldspar and kaolin. Start with the L3778G recipe. Its information page (the preceding link) is also a launching pad having dozens of links, enabling you to dig as deep a desired into understanding casting slips (the materials are cheap so losing a few batches while you learn deflocculation is not a problem).

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Generated with a prompt for a crazed glaze on a slip cast mug

Related Pictures

v3 Shelled AI Mug Using OnShape CAD

A great on-ramp to learning slip casting

Available on the Downloads page

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Follow the video, step-by-step, to create this yourself. Don’t expect perfection the first pour, we did this dozens of times to get to this point. You’ll still make a few mistakes before getting it right, that is what DIY is about.
This design differs from the v2 drawing in that the plaster pouring cavity is formed by shelling (hollowing) the back side (top right). And this is designed for removal using a heat gun. This method has several advantages:
-No sketch offset or extrusion was needed to make the outer wall.
-Because the first 3D action is to extrude everything as one solid mass, corners of the outer 3D perimeter can be chamfered (but don’t do it so much that that there is insufficient thickness for the natches).
-The mug's geometry (offset inward by 0.8mm) is revolved, swept and bevelled by cutting into the block. Shelling to the same 0.8mm wall thickness, from the backside of the block, produces the cavity needed (top right).
-The last steps, after shelling, are chamfering the outside inner corner, cutting the holes for the natches and revolving the pouring spout (as a separate body).
-Our v3 natch system continues to work well with this (lower left).
-Printing artifacts are not a problem for prototype molds (visible op right). Production is asking me to enhance these (a freshly cast mug is shown lower right).
-The plaster mold is of stunning quality (the PLA 3D print was softened using a heat gun and carefully removed using needle-nose pliers).
-Slip cast mugs most often have poor-quality and oval lips. This one stays round because of the outward flare and the quality is better because the 3D printed pouring spout also acts as a cutting guide at the pre-removal stage.
-The PLA pouring spout is deep and absorbs no water. Thus, the slip level does not need to be topped up during casting, the slip surface stays flatter (not developing a bowl shape) so pour-out time can be accurately gauged by its slip level.

There are many casting body recipes that would work with this. DIY CAD skills will enable you to follow me into another exciting world: Low-cost 3D printing of the clay itself! Coming soon.

This is how much casting slip 10,000 grams of powder makes

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There are 8.8 liters of slip in this 2 imperial gallon bucket. The cone 10 stoneware slurry was propeller mixed in a larger bucket. First I stirred about 3/4 of the projected 44g of Darvan into 4000g of water. Then I dumped in 10,000g of the powder (shaken in a plastic bag) and let it sit to slake as much as possible. Then I used a high-energy propeller mixer, and to finish, trickling in extra Darvan until the rheology was right. The slip itself thus weighs 14 kg (31 lb) and has a specific gravity of ~1.75. It has sat overnight and formed a film on the top, but has not settled (indicating that it likely is not over deflocculated). The casting process enables even a hobbyist to make his own custom recipes and tune them over time. Would you like to develop a recipe? An account at Insight-live.com is the first step, that’s where you’ll keep all the development notes, pictures and data.

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