[pent19]

I purchased a kiln this spring (Skutt KM 1027) and am in the process of making work to sell. I throw functional pieces (vases, pots, mugs, bowls) and do some handbuilding. I have been using Laguna 66 clay and reasearched whether or not I could list my work as dishwasher and oven safe. (i currently don't make oven pieces, but I want people to able to use my pieces for food and mircowaving if needed) I discovered I need to fire my glaze work to cone 6 to make that happen, I have been glaze firing to cone 06 and using commercial food safe 05-06 glazes. I bisque to 04.
What would others recommend-changing over to cone 6 and purchasing all new glazes etc? or continuing to work in cone 05-06? I also know that mid-firing and high-firing puts more wear and tear on my new kiln. I don't want to false advertise my work and want my customers to able to wash my work in the dishwasher and heat it as neccessary.
Any feedback, advice comments would be greatly appreciated!
Thanks

Possible 06->6 convert

[Marcia Selsor]

Higher firings do cause more wear and tear on kilns but that is the price of doing business. The question should be more of what do you want to do.
Lisa Naples, for example, makes beautiful functional earthenware fires to ^2 or 02, I don't remember. She just said she fired her earthenware to vitrification so it is hard and durable. She learned that from a ceramic engineer who took her class.. I met Lisa at the show after NCECA. She really does lovely work. I would say that most earthenware fired to ^06 is not vitrified , will collect moisture, will be punky, and may have steam escape if used in a microwave which might explode. Majolica fired onto earthenware is usually bisqued to ^04 as you are doing, or higher ^02 sometimes. It is also glazed all over stilts in firing to avoid moisture absorption which can cause the glaze to flake off. That is why you often find the three marks from stilts on the bottom of majolica. So you really need to decide what you want to do. But some of your reasoning seems a little misinformed.
Check out Lisa's work. She is an inspiration for earthernware.

[bciskepottery]

If you want to continue using Laguna/Miller 66 clay, then you should shift to glazing at ^6. At ^05-06, your clay will not vitrify and it will absorb water/moisture from a dishwasher that could turn to steam in a microwave; your vases could also be leaving moisture rings on table tops. If you want to continue glazing at ^05-06, then you need to change clay bodies. I also make functional ware, both wheel and handbuildt, and fire to ^6 in an electric kiln. I carefully choose my clay bodies for functional ware to vitrify at ^6 and to have an absorption rate of 1 to 2%. Those two factors, along with using stable and durable glazes, allow me to be comfortable in telling customers that the wares are food and microwave safe.

[Jane]

Marcia is right. Vitrification happens at cone 6 and higher or it is not oven safe. Low fire glazes are safe to eat from if they do not contain lead, but you cannot cook with them in the oven or continually put them in the dishwasher without damaging the glazes eventually. Once the glaze is damaged, the vessel will absorb water and can explode when heated if moisture is stored. Also, germs can live within the clay body and be passed on to the user. Stoneware is much better suited for cooking with and will last a lot longer.

[Marcia Selsor]

Vitrification happens when the clay vitirfies which can happen at ^02 if the clay fluxes enough. I did not say clay has to fire as high as ^6. I said just the opposite. Lisa naples fires her earthenware to vitrification..and I don't remember if she said ^02 or ^2 but it certainly isn't ^6 to vitrify earthenware.

[hansen]

Dishwasher safe and microwave safe can easily be tested using the dishwasher and microwave. I have pieces that have been in and out of both for about 20 years. That information will also tell you about the durablity of the glaze, something you did not mention. Oven safe is a similar question, with more extreme parameters. But again you can test it yourself.

I would hesitate to research a commercial product and make claims based on what the vendor says, since you will be liable for what you say, not the vendor.

Your whole question falls into the category of "standing by" and "wait for it" - in other words you don't actually know until you actually know.
- h a n s e n -

[hansen]

p.s. I have some fine pieces of lowfire white, either 05 or 06 which do just well in microwave and dishwasher. Note; they were glazed 100% and fired while setting on triangle stilts.

[Marcia Selsor]

That makes sense if the pieces are glazed all over. I have Spanish majolica glazed all over that I have used in the microwave over the past 20 or so years as well. It is when it isn't glazed all over that moisture can be absorbed and create problems.

[pent19]

Thanks for all the tips! I appreciate the feedback from everyone! I have gone ahead and order ^6 glazes that are food safe and will continue using my laguna 66 clay for my pieces that will be used with food and re-heating. I will also be doing some heat tests in my oven and see what happens before i sell them.
I will switch clay bodies for my 05-06 work and continue to use my older 05-06 glazes for this.
once again thanks for answering and sharing your expertise!

[hansen]

Okay - so for your cone 6 wares you might not need to glaze 100% fully all surfaces. You will want to get a nice tight well-vitrified firing, so that the clay will be less absorbent form the sink, dishwashing machine, etc. You'll have unfired areas along the foot and the bottom that this is an area to watch.

To fire the cone 05-06, what ever the more popular cone is (?) - those pieces need to be fully immersed in glaze before firing, and set up on stilts. You can google stilts for ceramic applications easy enough to see what I'm talking about. The commercial low fire glazes rival the higher fired ones for hardness and durability, surprisingly enough.

I want to say that the same qualities that make clay bodies thermal shock resistant as for example raku firing would also work for oven ware, for example, extra grog, coarse fireclay, kyanite, pyrophilite, but I'm not real sure about this. Often in more primitive settings, underfired earthenware serves the purpose very well. Out of curiositry I bought some generic oven-ware from the grocery store. This is an interesting topic and I wish i knew more.

h a n s e n

[pent19]

i stilt my cone 05/06 work since i glaze completley, however i was under the impression that you couldn't use stilts at cone 6 since deformities can occur. I prefer to glaze my pieces entirely, any feedback on this? would having a thinner foot help? cone 6 is a new step for me so any advice is appreciated!

[Marcia Selsor]

pent19, on 16 July 2010 - 08:10 PM, said:

i stilt my cone 05/06 work since i glaze completley, however i was under the impression that you couldn't use stilts at cone 6 since deformities can occur. I prefer to glaze my pieces entirely, any feedback on this? would having a thinner foot help? cone 6 is a new step for me so any advice is appreciated!

Why would someone stilt at ^6. The point of stilting ^06 is to seal nonvitrified clay. ^6 clay should be vitrifying unless you are using ^10 clay and only taking it to ^6. You can stilt clay if you bisque fire to the same temperature but then adhering glazes can be tricky. Originally crystalline glazes were bisqued to ^10 , then fitted to a kiln washed soft brick and refired to ^10 with the glaze. I believe this practice was followed in the early 1900s at Champaign -Urbana IU Department of ceramics/Ceramics engineering. I did a lot of research there in the 1970s for my thesis on crystalline glazes.
The point is...if you use a stilt, the clay needs to be fired only as high as previously fired or shrinkage on three points during the firing will created deforming as you called it.

[JBaymore]

Leading off..............

Marcia is dead on correct when she says that vitrification is not dependant on the firing cone.

It is the glassification of the clay body that matters. Lower temperature clays can be made that become vitreous by adding fluxing materials that promote melting of the glassy matrix of the body. The caveat here is that as the cone end point goes down, it gets a bit harder to do that withouty affecting the handling characteristics of the clay for typical "handcraft potter" type forming methods.


The next thought,.....

There is pretty much no such thing as a "cone 6 to cone 10 clay body". Manufacturers make such bodies because to do otherwise would require too many stock clay bodies sitting around the warehouse. If the body actually is well designed to be fully vitrious at cone 6....... it is overfired at cone 10. And if it is really vitreous at cone 10... then it is underfired at cone 6. Now it is possible that the "6-10 body" is actually deliberately optimal at cone 8....right in the middle.... and is "acceptable" or "OK" at either 6 or 10. How one defines "acceptable" or "OK" is the potential issue.

For many things ... "OK" is just fine. For others, it is not. You have to figure out if "OK" actually fits your needs.. or if you need to have a more tightly controlled, specifically formulated clay body.

For legal and insurance purposes, to formally label something "Microwave Safe" or "Dishwasher Safe", you'd best have the data to prove that you have complied with at least one of the international testing organizations standards for determining such things. There are a number of people out in the world publishing standards for ceramic objects... the most common in the USA is ASTM (American Society for Testing and Materials)....but there are others.

For example .................. "Microwave Safe" = http://www.astm.org/...dards/C1607.htm

As hansen pointed out already....... you are not indemnified by the manufacturer of the products you use in the manufacture of your products when it comes to potential legal claims. Just because they say their product is "food safe" does not protect you from an "it was not food safe" claim. And most cetramic suppliers have a pretty good release of liability statement on their advertising / websites that says they are not responsible for how you use the products they sell.

So it pays to "do your homework" technically and to know your materials and how your products stand up. If you are not willing to go thru the necessary background work, then maybe just tell people that your claywork is special and prescious and deserves the care of not placing it in the dishwasher or microwave.


best,

.........................john

[Marcia Selsor]

Thanks John!
Marcia

[Deb Evans]

Just for clarification.... the low fire glazes that have the label "food safe" mean 2 things : 1) that there is no lead or other heavy metals as flux or colorents in the wet glaze and 2) if fired to the correct temperature , the glaze is stable and will not leach into food on the ware.
I use to fire earthenware clay to ^1 and applie mason stain slip under thinned ^04 clear glaze> vitrivied clay body and food safe ware.