High Bridge Pottery Posted February 20, 2017 Report Share Posted February 20, 2017 I have been working in a studio that sells tableware to restaurants mostly and some big platters fail under their heat lamps. My initial thoughts were maybe they have high absorption and are cracking because of that. Let a fired bowl soak in water for 3 days, 369g dry and 369.7g after 3 days. Not the most accurate but shows there was hardly any? Should probably redo. Happened to be speaking to a clay chap on the phone and he said I wanted more absorption and my clay was too closed up. Anybody else come across this problem? Is there a balance to be struck between the ability to be waterproof and good thermal shock. I always aimed for the lowest absorption possible but was that the wrong idea? Link to comment Share on other sites More sharing options...
bciskepottery Posted February 21, 2017 Report Share Posted February 21, 2017 Might be more to do with thermal-shock value than absorption -- sitting under a heat-lamp for xx hours a day (followed by a somewhat quick cooling when the heat is turned off or it is put in water for cleaning) is likely stressing the clay body. You might need to go to a flameware clay body or a clay body with kyanite or some similar material to increase its thermal shock capacity. Link to comment Share on other sites More sharing options...
glazenerd Posted February 21, 2017 Report Share Posted February 21, 2017 Yes Link to comment Share on other sites More sharing options...
neilestrick Posted February 21, 2017 Report Share Posted February 21, 2017 Heat lamps only heat the top of the plate/platter. Very uneven heating, and that is what's probably causing the cracks. Here in the US, restaurants are required by law to use only 'vitrified' ceramics. They don't give an actual number as to how low the absorption rate must be, but I would assume that it has to be tight enough that it doesn't weep during normal use. Yes, a less tight body would handle the heat lamps better, but a less-than-vitrified body can have all sort of issues in a setting where food safety and durability is of utmost importance. They need to find a body that can handle the uneven heating of the heat lamp but is still vitrified. Link to comment Share on other sites More sharing options...
glazenerd Posted February 21, 2017 Report Share Posted February 21, 2017 Is there a balance to be struck between the ability to be waterproof and good thermal shock. Pyrophyllite is commonly used to increase the thermal shock capabilities in clay, while still maintaining lower absorption values. Swap out half the silica, if you are using a porcelain recipe. If you are using stoneware, use 10% additions only: with no further silica additions. Stoneware bodies supply a great deal of silica, so silica additions are not required above the pyrophyllite. Increasing the ultra fine (sub-micron) ball clay will also close up the body; thereby lowering absorption. The other alternative is to reformulate to a flame-ware body. Flame-ware has a much lower COE than porcelain or stoneware. The easiest way to produce a flame way body is to remove all feldspar additions, and replace them spodumene. Run your current formulation on a glaze calc, and simply replace feldspars with spod- you will see the immediate difference. However, that also means you will have to adjust your glazes to fit that much lower number. Nerd Link to comment Share on other sites More sharing options...
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