cbarnes Posted June 3, 2016 Report Share Posted June 3, 2016 I'm making bowls & platters using a mid fire porcelain clay labeled as 04-6, painting with various underglazes then overglaze with Amaco clear HF9. then firing to cone 5 or 6. I believe this all to be food safe. what I'm wondering is if I can tell people its ok to put these in the oven and in the microwave? dishwasher? I realize it went into a very hot kiln so the oven is nothing compared to that but I wasn't sure if they hold up to repeated use. I will be selling at a farmers market this weekend and don't want to steer anyone wrong. any advice? also, I was told by Seattle Pottery where I buy supplies that at that temperature it is still food safe even if it crazes, probably wouldn't sell them but I'm curious. would you agree? thank you some samples Pottery Page 2.pdf Pottery Page 1.pdf Pottery Page 2.pdf Pottery Page 1.pdf Link to comment Share on other sites More sharing options...
Min Posted June 3, 2016 Report Share Posted June 3, 2016 Definite “it depends†on all your questions. Just because you fired the clay to over 2000F degrees it doesn’t mean it’s oven safe. How long did it take to go from room temp to 500F? A lot longer than the 10 minutes or so an oven takes. If your clay is sold as porcelain and fired to maturity it’s going to be pretty tight, I’ve found tight bodies and large flat surfaces don’t do well in the oven. The outside edges heat up before the middle part of the pot and crack. Shape makes a big difference with ovenware, no tight corners, rounded bottom, even walled thickness, well fitting glaze covering all but the foot ring if possible. If you do think it’s okay to put your pots in the oven then covering the base with food, putting pot in cold oven then turn oven on so pot and oven heats up together, don’t add liquid to a hot pot. I gave up making oven ware about 10 years ago. Whatever you say or print on care cards doesn’t matter, a lot of people think all pottery / ceramics is the same. If they can put their terracotta or stoneware pot in a very hot oven oven then why not a porcelain platter? I’ve had past customers think they can put pots under the broiler too, and one who put a frozen puff pastry covered brie in one of my pots in a 450 oven and complain that it cracked. As for the microwave what is the definition of microwave safe? does it mean it won’t break or crack in the microwave? does it mean it won’t leach harmful materials when used in the microwave? does it mean the pot won’t get too hot to pick up when used in the microwave (like terracotta mugs handles do)? or does it mean the glaze won’t spark in the microwave? all of the above? If you have used your pots in the microwave and didn't have any of those problems then I would say they are okay. Food safe? Probably okay but don’t know for 100% sure unless you get them lab tested. Cadmium is turning up in more and more underglazes (and glazes), there is going to be some chemical interaction between the underglaze and the glaze. Probably a negligible amount but again have to test to know for sure. I'm guessing your red, bright yellow and probably the purple and orange have cadmium. Different opinions on crazed ware being sanitary. Some say if the clay is vitrified and good housekeeping is applied then no worries. Others say the crazing can harbour nasty bugaboos therefore it’s not foodsafe. To test for glaze durabilty in the dishwasher you can run a trial of 2 pieces. keep one in the dishwasher and leave it go through about 40 cycles then compare to the unwashed piece for any loss of gloss or colour. Another way is to simmer a piece in a 5% soda ash solution for 6 hours in a stainless pot, then rinsed, dried and compared to untreated piece. Really the best tests would be to use the pots for a few months in your own home and see how they stand-up. Good luck at your Farmers Market (good weather this weekend on the west coast) edit: some of the people working at SPS know their stuff, others not so much. Tacoma Clay Art makes decent clay bodies and the owner there is very knowledgeable. Link to comment Share on other sites More sharing options...
Dick White Posted June 3, 2016 Report Share Posted June 3, 2016 Just my opinion on a rainy Friday afternoon, but before you buy any more supplies or clay from Seattle Pottery, you probably should have a sit-down with whoever runs the place and explain to them the difference between cone 04 and cone 4. If they don't understand, then you should take your business where that fundamental detail is understood. The very notion of a mid-fire clay body having an acceptable maturity range of 04 through 6, a range of 10 cones or ~300F, is absurd. While there is considerable debate amongst the cognoscenti about clay bodies with manufacturer-stated maturities ranging over several cones (i.e., there is concern that such clay bodies are not fully vitrified at the lower end of the stated range), the industry seems to operate on a general consensus that low fire clay bodies are marketed as in the cone 06-04 range (though sometimes as high as cone 1), mid-fire clay bodies are marketed as in the cone 4-6 range (though sometimes as high as cone 8), and high-fire clay bodies are marketed in the cone 6-10 range. As for whether that clay body is food-safe when fired to cone 5-6ish, that depends on the absorbency. The Seattle Pottery website gives information on shrinkage of their "cone 04-6" products at cone 5, but fails to give any absorbency information. If the clay body fired to a stated cone has an absorbency of less than 2%, then it could be regarded as food-safe (though that's a dicey term to use, too many different conflicting interpretations of what those words mean). However, even if the body is fully vitrified and watertight, crazing in the glaze surface can give lots of growing space for fuzzy grey things that even if not fatal, might become unsightly. Again, that's my opinion, YMMV. Link to comment Share on other sites More sharing options...
Mark C. Posted June 4, 2016 Report Share Posted June 4, 2016 That clay has to large a temp range-04-6. Without testing your fired work I would not say its microwave and oven proof-food safe is unknown to me as I do not know or use any commercial glazes. I think you should not tell any customers this weekend that all these are true points as they are not tested yet. More testing is needed as you are selling unknown results . Link to comment Share on other sites More sharing options...
Pres Posted June 4, 2016 Report Share Posted June 4, 2016 Years ago, before I started to sell at shows, I used a white stoneware clay that had a wide range of maturity. I also was making up a Bristol type glaze for over the body with underglaze decoration applied over the glaze as in-glaze. Over the years of use, we found that the clay body absorbed water from the dishwasher as the bottoms felt damp, the Bristol glaze crazed, and the surfaces got dull. Mind you I did not use any of the underglazes inside of the containers or near the rims, but those things were highly noticeable. I determined that the wide maturity range was partly at fault, that the Bristol glaze had issues as formulated, and that there were other problems with my method of decoration. Sometimes you get stuck in a rut admiring what you think is really nice, good work, and find that your perceptions are flawed not by the work itself, but the attributes inherent in the work from materials. When using commercial glazes or underglazes you depend on the suppliers/manufacturers integrity, as you are when you use clay bodies from a commercial manufacturer. Best to research your materials, test their integrity, and have chemical analysis done when in doubt. I do preliminary testing on most every thing anymore with vinegar, and dishwasher detergent baths on test tiles of new glazes and clay bodies. If they fail this, then no need to send them out for further analysis. best, Pres Link to comment Share on other sites More sharing options...
LeeU Posted June 5, 2016 Report Share Posted June 5, 2016 Such good info---I would not even attempt making anything one would put in an oven or microwave. I'll leave that to the pros LOL Frankly, I'd be less concerned with any actual harm to fish or fowl or person than the whole opening-of-the-door to liability enticements. Link to comment Share on other sites More sharing options...
cbarnes Posted June 6, 2016 Author Report Share Posted June 6, 2016 Thank you so much for the info. I decided to tell everyone that asked "that I don't recommend oven or microwave, and that the dishwasher is up to them but I hand wash mine"... although I have a few pet dishes I've made that we wash daily for a couple years now and they've held up fine. My first Saturday market went pretty good. sold 12 pieces. but more important I got a lot of great feedback from lookers and interested people. yay... I might actually start to believe I'm a real potter now . Oh, and I'm also going to do the vinegar/lemon testing on some pieces to see what happens. has anyone tried using the lead testing kits you can buy? or is the vinegar good enough. I feel pretty confident with the commercial glazes that I'm not mixing, but I have layered a couple glazes and not sure of the reaction on those. thanks!!! by the way... Seattle pottery told me that its a mid range clay 4-6, I was getting crazing results at low temp. I have no idea why the label says 04-6. Link to comment Share on other sites More sharing options...
Dick White Posted June 6, 2016 Report Share Posted June 6, 2016 Glad to hear the sales and feedback were good. And yes, you are a real potter. The lead testing kits are intended to determine if there is ages old lead-based paint on the woodwork and walls in your house. If so, you will have a more complicated time repainting. For us potters, lead typically has not been in commercially-available glazes for a long time, and if so, it must be boldly labeled. That's no longer the issue for hand-made pottery. The issue now is more subtle. If the glaze is not a stable glass (properly balanced chemically and fired to maturity), the copper, cobalt, manganese, and other metal oxide colorants might leach out of the weakened glass when acidic foods (tomatoes, some fruit juices) are placed in them, or when the glass begins to break down in highly alkaline dishwasher detergents. The vinegar/lemon slice test will begin to show any weakness, and put one of a matching pair of pieces in the dishwasher for a month or two and then compare the washed and unwashed pieces to see if there is any degradation of the gloss or color. As for your friends at Seattle Pottery, they may have now told you the clay is mid-fire ^4-6, which is highly probably correct, but then their label is wrong as is their entire website, listing these clays as ^04-6. Whoever is in charge there needs to get their act together. When you, the innocent potter, follows the incorrect label, then it is their fault your work isn't as it should be. JMO, a friendly but firm chat with the boss is in order. Link to comment Share on other sites More sharing options...
cbarnes Posted June 6, 2016 Author Report Share Posted June 6, 2016 As for your friends at Seattle Pottery, they may have now told you the clay is mid-fire ^4-6, which is highly probably correct, but then their label is wrong as is their entire website, listing these clays as ^04-6. Whoever is in charge there needs to get their act together. When you, the innocent potter, follows the incorrect label, then it is their fault your work isn't as it should be. JMO, a friendly but firm chat with the boss is in order. I don't disagree... for a new potter it was frustrating on the learning curve until I figured this one out. we have enough to learn without being mislead. Link to comment Share on other sites More sharing options...
glazenerd Posted June 7, 2016 Report Share Posted June 7, 2016 Absolutely agree with the comments about extended firing range(s), but probably more critical. 2 to 3 cones is the maximum lead way I would give a clay body. However, I would like to address a different issue: thermal expansion. If you wish to pursue oven ware pieces then I would be looking for a porcelain body that has a pyrophyllite/silica substitution. This type of clay body would be much more suitable for this purpose. In general terms; a porcelain body would have 25% silica typically. For thermal expansion, pyrophyllite/silica would be added in equal proportions to reduce thermal shock. You would have to do some research to find out which bodies have this composition, and some clay makers will refuse to divulge this information. Nerd Do you have a scale that weighs to 100ths?, to do your own absorbency tests? Link to comment Share on other sites More sharing options...
Min Posted June 7, 2016 Report Share Posted June 7, 2016 As well as absorbency/porosity tests like Nerd brought up I'm wondering how people do tests for crazing (if that's an issue for them). I tend to be brutal with my craze testing. 3 cycles of oven to room temp water, 310F, then 315, then 320 or 325 oven to water. Test pieces then brushed with calligraphy ink and rinsed off and checked for crazing. If the glaze is on the low expansion side then I do shivering tests by freezing overnight then filling with boiling water. (into a thin walled glazed inside only cylinder) I tried the boiling water to ice water test but found it wasn't as accurate at predicting crazing, I guess because the temperature difference isn't as great. Link to comment Share on other sites More sharing options...
Pres Posted June 7, 2016 Report Share Posted June 7, 2016 I have some recent bowls that have been used by my wife to bake simple things like buffalo chicken dip, a Susie special that involves squashes, salsa, and cheese, and a few other things. We follow the cold oven rule always, but I have been watching the glazes for crazing and other defects. Happily I can say that after several of these simple dishes there is no softening of gloss, no crazing or other apparent problems. These are thrown with the SC Hazelnut clay body. I have also heated things in the microwave with these bowls and found no problem there either. I have always told customers, never in a hot oven, not from freezer into oven, and never on a stove top. Surprising how so many folks ask if their new teapot from me is good on a stove top! best, Pres Link to comment Share on other sites More sharing options...
LeeU Posted June 7, 2016 Report Share Posted June 7, 2016 The Susie special, huh? Hmmmm...maybe a new forum, for recipes, is needed! Link to comment Share on other sites More sharing options...
Pugaboo Posted June 7, 2016 Report Share Posted June 7, 2016 What's a good test for microwave safe? T Link to comment Share on other sites More sharing options...
Joseph Fireborn Posted June 7, 2016 Report Share Posted June 7, 2016 What's a good test for microwave safe? T I just put two mugs in the microwave, one with water and one without. If the one without is hot when I take it out of the microwave then it isn't microwave safe(and probably should be looked at for vitrification issues). The other one should be warm, but the handle should be able to be held. There are probably better test, but I have commercial dishes that I heat for 45 seconds and have to use a pot holder to take it out of the microwave or I get burnt, and on the bottom they say: microwave safe. I suppose they have microcrazing that I can't see and have taken on a lot of water of the years. from big ceramics store: To test microwave safety, take a piece (such as a mug or bowl) and immerse in a pan of water. Bring the water to a boil, then simmer for a few hours. This will allow the piece to absorb water. Then put the piece in the microwave. (The piece should be empty, and you should also put a separate mug of water in the microwave to protect the microwave.) Heat the microwave on high in 10 second increments. After each 10 seconds, carefully touch the piece to see if it is hot. If it has absorbed water, it will heat up. This tells you the piece is not dishwasher safe. You can stop the test when the water in the second mug is boiling. Link to comment Share on other sites More sharing options...
Pugaboo Posted June 7, 2016 Report Share Posted June 7, 2016 Thanks! On a kind of side note: I've only been using handmade mugs for decades (made by other potters at festivals I've worked) have only had a few that one day the handle just cracks off, it's not dropped or anything you just go to pull it out of the microwave or dishwasher and the handle is damaged....is this caused by the piece absorbing water over the years and weakening the handle? Link to comment Share on other sites More sharing options...
Pres Posted June 7, 2016 Report Share Posted June 7, 2016 Yep, I think you have hit the nail on the head. However, there are times that construction is at fault. Often a poor join will have poor glaze coverage/fit. This will allow over time the moisture to do as your describe. best, Pres Link to comment Share on other sites More sharing options...
Pugaboo Posted June 8, 2016 Report Share Posted June 8, 2016 Thanks Pres! I wondered what was going on for years, and bad mother that I am even asked my daighter if she dropped it and didn't want to tell me! GROAN, the things we could change if we only knew then what we know now. T Link to comment Share on other sites More sharing options...
Guest JBaymore Posted June 8, 2016 Report Share Posted June 8, 2016 What's a good test for microwave safe? T Unlike "food safe"....... there IS a testing standard in the USA for that term: https://www.astm.org/Standards/C1607.htm best, ................john Link to comment Share on other sites More sharing options...
Dick White Posted June 8, 2016 Report Share Posted June 8, 2016 Thanks John, that is interesting. Do you know what the testing method and standard is without having to buy the document for $39? And the abstract refers to another testing standard for crazing, but that would require another $39. Link to comment Share on other sites More sharing options...
Min Posted June 8, 2016 Report Share Posted June 8, 2016 Doesn't industry standard test for crazing use an autoclave? Link to comment Share on other sites More sharing options...
Magnolia Mud Research Posted June 8, 2016 Report Share Posted June 8, 2016 ASTM C554 - 93(2011) Standard Test Method for Crazing Resistance of Fired Glazed Ceramic Whitewares by a Thermal Shock Method ASTM C424 - 93(2012) Standard Test Method for Crazing Resistance of Fired Glazed Whitewares by Autoclave Treatment https://www.astm.org/Standards/C554.htmhttp://www.astm.org/Standards/C424.htm LT Link to comment Share on other sites More sharing options...
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