|Monthly Tech-Tip |
If you are a potter and want to make restaurant ware, read this. Many of the things you already think you know will mislead you in this type of venture.
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Ceramics (mostly plates, cups, saucers) used in restaurants has, as its most important properties durability. The experience of trying to break a piece of this type of ware helps one appreciate how incredibly strong it can be. Restaurant ware is also easy-to-clean, non-leachable and has defect-free surfaces. Durable means that ware not only withstands rough handling but also sudden temperature change. And it does not scratch or cutlery mark. All possible clay surfaces are covered with glaze leaving only a foot-ring exposed. The body is dense enough not to absorb water where exposed. The glaze surface of leach-resistant ware does not dull on exposure to thousands of wash cycles. Restaurant ware is also also of consistent size and shape, it stacks and fits into dish washers well.
To be non-leachable and resistant to cutlery marking means that the suite of suitable pottery glazes you might have is much reduced. Common pottery glazes are often just too finicky for consistent results. Surfaces are often too matte and stain easily. Colored glazes often do not withstand the caustic environment in a dish washer. Just because a jar of commercial glaze says it is "food safe" does not mean it is "restaurant safe". This is not to say that only glossy glazes are suitable. While many matte glazes used in pottery stain easily and craze, it is quite possible to make very durable silky matte surfaces (examples are the G2934Y and G2571A recipes).
Using stoneware or porcelain, or firing to cone 6 or 10, does not automatically mean that you are making ware suitable for a restaurant. These terms have a different meaning in the pottery community than they do in a restaurant. In restaurants, "stoneware" is ware that is as strong as a stone. Potters think white-firing ware is porcelain, restaurants see it as white firing stoneware. The term "vitrification" is a process in pottery, "vitrified" is a term people see on webpages, there is generally no data to back it up. But a dishwasher in a restaurant can tell you immediately which ware is really vitrified.
If you want to make truly durable ware, an important tool you will need is a hammer. Break ware at every opportunity. And pay attention to what it does in comparison to commercial products. When ware breaks easily, be suspicious about how vitrified it really is. We do an SHAB test on all clays to get a picture of their density across a range of firing temperatures. And learn how to do the IWCT test (or something similar). Glazes must fit, with no tendency to craze or shiver. If they are under too much compression or tension, even vitreous ware will be weak and break-prone.
While durable ware can be made at lower temperatures, typically potters to not have the technical know how for this. It is best to fire higher and get the built-in benefits (easier glaze fit, glazes of higher SiO2 and Al2O3, bodies with a well developed mullite matrix). It is also best to employ a limited suite of base glazes, known to be balanced and durable. Add your colors, opacifiers and variegators to these. When other glazes catch your eye, identify the mechanism of their appearances and transplant that into your own base glazes.
Durable ware has smooth contours (no abrupt angles). Wall thickness are even across the pieces. Ware is dried and fired evenly so as not to build in stresses. Lips are rounded and not too thin.
Developing durable ware takes time. Slow evolution of firing techniques, glaze composition and application, body makeup and fabrication methods. You do not need fancy equipment. Strong and durable ware can be developed by anyone willing to do simple testing and development, by people willing to question what they are told, do the testing and get meaningful data. While some art can be a part of it, it is much more about craft and technical issues. Automation is your friend, because it can help maintain consistency, that is a big part of durability. And automation enables you to step back a little from personal involvement in the "art" to assess the "craft". Jiggering plates, for example, not only creates a more even cross section, but the increased production makes you more willing to smash a few to see how strong they are! And to think about vitrification and durability instead of just artistic merit. Casting mugs (instead of throwing) enables you to make your own clay body much more easily, a body you can learn to understand and tweak over time to improve it.
3D printing can also be your friend to make restaurant ware. It encourages you to carefully catalog cross sections of ware, classify them, develop them to work well together and be easier to make. 3D printing also makes it much easier to make molds, templates, tools and other things that aid production.
This is M340 casting, L3798H. The handle was just glued on with slip. I had questions about this new body. Are my glazes under compression? How well is the handle stuck on? Because I have broken so many pieces I know how they break. It is a good sign when there are no fractures along the handle joins. And when the handle is the last to go. And when the item breaks into large pieces rather than shattering into small ones. Try doing this on some of your pieces and you might be surprised. Either by how strong they are. Or by how easily they break.
The porcelain on the right (Plainsman Polar Ice), breaks much more like glass, to razor sharp-knife edges. But it is not a glass, it is a zero-porosity, dense matrix of glass-bonded crystals. The whiteware on the left (Plainsman M370) has 1% porosity, and although vitreous and quite strong, clearly it is not in the same class as the one on the right.
I use a nylon hammer, and glasses of course. I just filled two five-gallon pails and three boxes. Every type of clay and glaze I currently use. Every temperature. I started with a commercial Denby stoneware piece to get a feel for how quality ware should break. It becomes immediately evident which pieces are weak by the way they shatter. Breaks with knife-like edges indicate strong body/glaze combos. Strong ware breaks into fewer pieces. Crazed ware is weak. Low fire vitrified ware can be very strong. High-fire ware can be weak (e.g. iron stonewares having high porosities). Give attention to this, make quality ware.
Although not clearly visible in this photo, the unglazed body surface on both of these MNP (Plainsman 3D + 10% feldspar) pieces has a sheen - because they are super-vitreous. And broken surfaces appear like glass (as can be seen in the broken chips on the right). Because the foot-ring of the piece on the right is too angular it is plucking all the way around. This is not happening on the piece on the left for two reasons: The angle was rounded and it was fired on silica sand. Because it is so fine-grained (wet screen to 325 mesh) and contaminant-free the body tolerates firing well past the point of vitrification - not only resisting bloating and warping but giving something else: Incredible fired strength.
The unglazed surface of the left piece has a sheen, it is a product of glass development during firing to cone 6. That body is a 50:50 mix of a cone 8 stoneware and a low fire earthenware red (a material that would normally be melted by this temperature). Together they produce this dense, almost zero-porosity ceramic. The unglazed surface on the right looks more like plaster, and it is absorbent, about 5% porosity. It is a mix of the same stoneware but with 50% ball clay. The refractory ball clay assures that the stoneware, which was already inadequately vitreous, is even more so. As you can imagine, the left piece is far stronger.
Ceramics, by their brittle nature, have high compressive strength. But in functional ceramics we are more concerned about the tensile strength as this relates better to serviceability.
In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
Ceramic glazes that mark from cutlery are either not properly melted (lack flux), melted too much (lacking SiO2 and Al2O3), or have a micro-abrasive surface that abrades metal from cutlery.
Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue
Concentrate on One Good Glaze
It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
2019 Jiggering-Casting Project
|By Tony Hansen|
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