suziewyllie Posted October 2, 2015 Report Share Posted October 2, 2015 HI there wise wizards of the ceramic world! I am so grateful for this discussion group. I have very little experience and am learning as i go so find this resource so valuable. Thanks in advance for your advice and guidance. I am working with southern ice to make accessories (beads and pendants) as well as small to medium sized homewares (small dishes, bowls, spoons, plates etc) So far i have been working with cobalt oxide of the dry porcelain and then firing to stoneware temps. I'm enjoying the raw matte finish but am wondering what i need to do to make the homeware items functional and safe for food? I have purchased a clear satin glaze but am wanting to work with a one firing system if possible, as i live an hour and a half from my community firing service. So thats the back story.... the question is: is porcelain hand painted in cobalt oxide and fired to stoneware temps safe for food and drinks? or do i need to seal with a clear glaze? I am also aware that items might stain easier if not glazed? My main concern is for a small collection of tea strainers that i am wanting to make. Is it possible to use oxide and clear glaze and avoid bisque firing and go straight to stoneware? Any advice would be so helpful. I really appreciate your time and apologise for the silly beginner questions! Link to comment Share on other sites More sharing options...
jolieo Posted October 2, 2015 Report Share Posted October 2, 2015 Hi Suziewiyllie, welcome to the forum and pottery, Here is the kind of short answer: anything toxic is hard to prove it had been made food safe. So your cobalt needs to be sealed, and more than likely the clear glaze would probably make it food safe, but w/o testing , know one knows. So if one is a production potter wanting to use something iffy, maybe they would have the overgraze or colored glaze tested. But glazes are dependent on the environment of the kiln and the clay that they are on , so change anything and all bets are off. Also acids in food can weaken a glaze and make it suceptable to leaching. So the toxic stuff is put where it can't be leaching out into food, perhaps on the outside of the cup. But even then it can travel through the clay to the inside of the cup., so I am going to stay away from toxic on food wares . Perhaps one day I will be able to judge what is ok, but until then I will stick w non toxic. There is all kinds of info for safety data available online and on the websites of the stores that sell the raw ingredients and the premade stains and glazes. I didn't know what I was getting into, then I got a bit of an inkling coming to this forum( I am a beginner). I was quite intimidated for quite awhile because then is toxicity and then a lot of the raw materials are not toxic once made into clay pots that have been fired, are toxic if inhaled or ingested in their raw state. I am by no means an expert, but I am calming down because I read a lot here, and online, and I am getting a feel of how to handle all these chemicals. Definately better to err on the side of caution! Link to comment Share on other sites More sharing options...
Joseph Fireborn Posted October 2, 2015 Report Share Posted October 2, 2015 Here is a quick place to start to get some definitions of things so that you can understand what others will post on this thread. http://digitalfire.com/4sight/education/are_your_glazes_food_safe_or_are_they_leachable_12.html Link to comment Share on other sites More sharing options...
bciskepottery Posted October 2, 2015 Report Share Posted October 2, 2015 is porcelain hand painted in cobalt oxide and fired to stoneware temps safe for food and drinks? or do i need to seal with a clear glaze? Cobalt can leech into food items. So, a clear glaze is needed for food ware. I am also aware that items might stain easier if not glazed? My main concern is for a small collection of tea strainers that i am wanting to make. Non-glaze clay is likely to stain if used for food serving. Tea would especially stain . . . tea even stains glazed wares. Is it possible to use oxide and clear glaze and avoid bisque firing and go straight to stoneware? Yes. Single firing is possible but you might want to use a different firing schedule than for a plain bisque or glaze load. If you are firing at a community studio, you will want to talk with the kiln operators to see if they would accept once-fired items. You can apply glaze with a spray gun, dip, or brush . . . but you will have to find out when is the best time to apply glaze (leatherhard, bone dry, etc). Some one-fire potters also adjust their glaze recipes to have more clay to prevent the glaze from shrinking and cracking when drying. Link to comment Share on other sites More sharing options...
Rae Reich Posted October 2, 2015 Report Share Posted October 2, 2015 Just on the practical side, Suzi, I think you surely need glaze on the insides of food containers, and that can be single fired as above. For your tea strainers, glaze could/will give you problems by filling up small holes. IMO, they should be left as white, unglazed porcelain which is expected to take on coloration from the teas used. You can advise customers to clean them with denture cleaning tabs if they get smelly or take on unwanted flavors. Link to comment Share on other sites More sharing options...
Tim T Posted October 4, 2015 Report Share Posted October 4, 2015 The short answer is it's complicated! I don't use porcelain, but if it is fired towards the top of its range so it is fully vitrified then it is probably OK, though you will probably get more staining than glazed ware due to the rougher surface trapping more of the tea. If you glaze it, it has to be the right glaze - if it is underfired or badly formed then the glaze will leach its contents, and quite probably also the clay and its colourants. If your tea strainers are for good quality, large leafed tea then you can probably glaze them with a thinn coat of glaze, but if it is for everyday tea you'd probaly have to leave them unglazed - or use a metal sieve instead of a ceramic one! Tim Link to comment Share on other sites More sharing options...
suziewyllie Posted October 7, 2015 Author Report Share Posted October 7, 2015 Thank you so much for taking the time to answer my question. I really appreciate it and am slowly coming to terms with just how complicated it all is. I will have to set aside far more time for reading so that i can continue to make more confidently. I will be sure to seek advice here again. Thanks again! From an overwhelmed but determined newbie! Link to comment Share on other sites More sharing options...
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