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Greetings!

    I'm a fairly novice potter who has made figurative pottery with some functionality using Amaco Low Fire White, which fires to 04. These are mostly ring dishes and candle holders. 

I've recently been looking to make food safe pottery such as plates, teapots, and pie plates, but have been given conflicting reports on how to make them fully food safe.
I know that low fire clay is somewhat porous, but would a good quality glaze make them 100% food safe?
I also have heard that pie plates and other bakeware may be prone to thermal shock if just stuck into a hot oven, and then contradicting information that well glazed, low fire pots tend to have less of this issue than say, stoneware.
  I'm not sure, and would love any insight! Thank you!

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Hi and welcome to the forum! First off, is this the Amaco White clay you are using? If so the first thing I would suggest trying is firing it hotter than cone 04, Amaco lists this clay as having 13.5% absorption at cone 04 which is high but typical for lowfire bodies, absorption will be less if you can fire higher. Amaco states this clay can go to cone 3. Which leads to a second point, your glazes. Some glazes can fire hotter than what they are rated for, but not all can. If you can do some tests to see how hot you can push your glaze(s) in order to tighten the body up from what it is at cone 04 that would be the first thing I would look at, it's still going to be porous but less so. (BTW It’s much easier to make non leaking pots in the mid and high fire ranges than lowfire)

Regarding your question about "glaze quality and 100% food safe”. Are you using commercial glazes? If so you can look up the Safety Data Sheet for the glaze(s). If it is a clear or white glaze then the odds are better that it doesn’t contain materials that will cause possible harm if the glaze leaches, as opposed to if it’s a metallic or heavily pigmented glaze. Gloss glazes are preferable for liner glazes rather than matte glazes. If you line the pots with a clear or white gloss glaze and don’t underfire the glaze then many of the “food safe” issues are mitigated. There is no legal definition for “food safe”, durable would be a more apt description. For earthenware especially you need to have a really well fitting glaze, no crazing. It is what is stopping moisture from being absorbed into the body versus a tight stoneware or porcelain body. Water that soaks into the claybody, while doing the washing up etc, will cause the pots to get hot in the microwave. Even a little moisture in the clay can cause the pot to get very hot to the touch from the steam within the claybody when microwaved. Crazed glazes can also lead to oils etc going rancid in the pot,  good housekeeping practice can help avoid this.

Insofar as ovenware goes, in theory the body should be a low expansion one.  When a pot is placed in the oven it likely won’t heat up evenly from side to side and top to bottom. In a kiln the pots heat up much slower than the typical kitchen oven, this gives the pot time to heat up more evenly. You want the clay to expand as little as possible and as evenly as possible to avoid a stress crack from uneven heating. Putting the pot in the middle of the oven, covering the base of the pot with food, preheat oven and pot together, and not putting a hot pot on a cold surface when removing from the oven are all going to help. Design of the pot is another consideration. Wide flat bottoms and sharp corners are more likely to have issues than a pot without those characteristics. You also want to have an even thickness throughout the pot, avoid thick bottoms and thin walls, again this is to avoid uneven heating. Since many potters make ovenware without knowing if the clay they are using is a low expansion one or not the design and glazing of the pot is very important. 

 

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Hello Min!

    Wow, thank you for taking the time to answer me in such a thorough, thoughtful manner.  I HAD heard that the Amaco White clay (which I AM using) goes up to cone 3, but was told by another ceramic artist that it would just likely be better to go ahead and purchase a stoneware clay or clay body that goes up to cone 6 is this is the standard for less porous clay?  So I never bothered to try to go up to cone 3 with the Amaco. I don't know whether it would just make more sense to purchase the "proper" clay instead?

I tend to underglaze painted art on my pottery work (Duncan CoverCoat, which fires at 06) and then glaze with Duncan Envision Clear glaze, using two coats. I'm not sure how well that might mitigate any leaching of water, and of course, I know that crazing would be a problem. I've been lucky to avoid that so far, but I'm not sure what other techniques to use to better the process.

Thanks again, and hoping to hear any and all additional advice for anyone who would be so kind to offer it!

 

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Min touched on all the points I would have mentioned too. The only other thing I will add, is that most pots that potters make, tend to have unglazed bottoms/feet. This is of course to prevent sticking to a kiln shelf, but it is an area on low fire bodies that have high absorption rates, where water/soap can be absorbed. Because of this, it is better if you trim a foot where the majority of the bottom of the pot can be glazed, and even better if you can glaze the entirety and fire on pins/stilts.

There are vitreous slips which will become more vitrified than the clay body, but less "sticky" than a glaze during firing. The slip could be used to coat the underside of your pots, and when glazed, some wax with alumina will minimize the sticking action to your shelves. A little touch up with sand paper after firing will remove any rough stuck on bits. This would be my secondary option as opposed to glazing the entirety with a well fitting glaze.

Is there a reason that you want to stick with low fire clay/glazes? If you are not bound by your kiln's temp, then maybe just choosing a clay body/glazes which will be better suited to the utilitarian duties of functional pots would be easier. If you are trying to match a look of lowfire surfaces, then maybe developing a slip/glazes which imitate the low fire look, but at mid-high fire ranges.

Another thing to consider with pie plates, bakers, etc. If you dont glaze the underside of a large flat object, but glaze the inside, if your glaze has a high coefficient of expansion, then you could have uneven stresses placed upon the clay body, which will lead to the integrity of the pot failing.

Certain clay bodies handle thermal shock better than others; unglazed earthenware has a little better ability to "move" when heating/cooling, hence why you may have some confusion about what clay bodies are better for cooking. Clays high in Mica, or flameware clay bodies handle thermal shock REALLY well, but are difficult to fit glazes to.

Edit;

One other thing to consider when making utilitarian wares; not all wares are the same, and arent meant to be used the same. Flameware works magnificently on the stove top, is made from clay, just as my stoneware pots are. However, I would never put my pots on the stovetop. I inform my customers of how to use the pots; not all pots are alike. Some need handwashing only, some can do the oven, some can do the microwave, some can do the stove top, some can do the broiler. Know what your work can and cant do, and inform your customers of this, otherwise, you'll be replacing a lot of pots when they destroy them unintentionally. Not saying that this is an excuse for making poor fitting glazes, or using improper clay bodies, but your utilitarian work doesnt have to be the same utility as anothers' pots.

Edited by hitchmss

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Hello Sam!

   Thank you, as well for this additional information!  Yes, of the plates, buttons, and decorative spoons I have made, I have glazed the ENTIRETY of the piece, inside and out, including the bottoms, and fire them on stilts. 

My understanding, then, is that stoneware is more vitrified and so would be fine if the bottoms were unglazed (such as the bottom of a pie dish)? Wow, I really cannot see myself making something for the stove top, that sounds like quite a feat! I'm thinking ovenware: pie plates, small casserole dishes. And then platters and chargers, and teapots and mugs.

I am not adverse at all to changing clays or glazes, I just started with low fire clays and have an abundance of the low fire clay, underglazes and glazes and was hoping to utilize them somehow. If it definitely makes more sense for stoneware clay and midfire glazes, I can try, though now that I think of it, I guess it's not possible to work low fire underglazes on higher fire clay (yikes, sorry to throw on another question to this thread). 

I certainly appreciate this guidance. To give you some idea of my work, here is a picture of some hand-scupted buttons. I mostly sculpt my wares, or throw and and add a scuptural element to the piece. They are all painted in fairy tale style with underglazing, and then glazed.  This is true of all of the work I do, so not sure if this would add a complication in stoneware? I'm sure those would all need to be glazed to protect the underglaze in any case, but I'm guessing the tighter midfire clay would still be a better choice for foodware.

Buttony.jpg

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Vagabond-

Many folks use commercial underglazes at cone 6, and some use them at come 10. However, they have done testing to ascertain whether they hold their color and/or are compatible with the clay and glazes that they are using.  As in many/most topics in ceramics, “it depends “.

Try them, test them. Only way to find out whether they will work for you and what you wish to make. Your mileage may vary, as is often said. Good luck and please return to let us know your results. People are interested.

Regards,

Fred

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1 hour ago, BeyondVagabond said:

My understanding, then, is that stoneware is more vitrified and so would be fine if the bottoms were unglazed (such as the bottom of a pie dish)?

Mid and high fire is far more vitrified than lowfire (with the exception of lowfire fritware but those bodies are not common). There is a range in the absorption with mid and high fire claybodies. Have a look at the absorption figure for the clay you are thinking of using, look for one about 1 1/2%. There is a fudge factor in published figures so do your own absorption tests in your kiln with your firing methods. Yes, you leave the bottoms unglazed for mid and high fire.

How to do an absorption test here, about 1/2 way down the article.

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Thank you again, Min! This is invaluable :)   

I wonder: what say you about low fire under glazes on mid fire clay? I'm guessing it is a matter of testing them on sample shards in kiln. Of course, I can also buy mid-fire underglazes, I'm just not very familiar with which might be a superior brand to consider.

Thank you again!

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