Jump to content

Recommended Posts

Hello! Is it possible to high fire a glaze on the interior of a cup and around its lip, and to then pit fire? I'm trying to find a way to make pit-fired cups food/water-safe! I'm assuming that this process wouldn't affect the high-fire glaze, but perhaps the outside (unglazed) body would no longer be able to take in the marvelous colors produced in a pit fire? I'm new to this process, but will be doing a ton of experimenting over the coming months with local blue clay that I've begun harvesting. Thanks for any insights!

Share this post


Link to post
Share on other sites

High firing the clay will tighten up and quite possibly vitrify the clay you are using. I doubt that any effects produced in a pit firing process after that will be permanent (particularly if the object is in functional use, eg dishwashers, etc.), but actual pit-firers reading this can confirm or not. However, I would suggest that the first step would be to find out more about your found blue clay. Can it even take a high firing, and more generally when does it mature? Have you tested it yet?

Share this post


Link to post
Share on other sites

I have not tested the blue clay yet and will need to do that as soon as its dry enough to work with.. Thanks for the help! I also reached out to another professional ceramicist who had this to say: 

 

I never make any guarantees about predicting what clay may or may not do. It continues to surprise me all the time and I encourage experimenting. Presumably if you are firing a stoneware clay to high temperatures (i.e. Cone 8-10) then you are vitrifying the clay and it isn't going to change significantly in a lower temperature firing environment. That being said, a pit fire may be a reduction atmosphere and cause the clay to go black or purple. Also, any soluble salts that could be introduced due to the fuel or soil might cause further color changes. Keep in mind if a clay body is vitrified then it can be food safe without a glaze and that just because a glaze is high fire doesn't necessarily mean it is food safe.

 

Also:: I'm wondering if it might be possible to just seal the pit-fired cups with something to make them water-safe..? Was looking into Tung Oil, but ideally it would be something that doesn't wash off easily.. Thoughts?

Share this post


Link to post
Share on other sites

Also:: I'm wondering if it might be possible to just seal the pit-fired cups with something to make them water-safe..? Was looking into Tung Oil, but ideally it would be something that doesn't wash off easily.. Thoughts?

 

What do you mean by "water-safe" ?  There are a number of things you can do to seal the surface, and make a pot hold water - but there's a big difference between water-tight and food-safe.  (A lead cup will hold water -  but that doesn't make it safe to drink.)

Share this post


Link to post
Share on other sites

Ok, so I have an earthenware glaze that is food safe and low fire (cone 5 or 6) so I'm wondering:: If I bisque at cone 4, then pit fire, then glaze and re-fire in the kiln at cone 5, will it still keep the coloring from a pit-fire?  Thanks for help!

Share this post


Link to post
Share on other sites

No. If you bisque it to cone 4, then it will be near vitrification, and, as Curt and Neil pointed out, it will not absorb coloration from the pit firing. If it were bisqued to 04 (the zero in front of the number is important! Specifically, your statement using the words "earthenware", "low fire" and "cone 5 or 6" in the same sentence is nonsense gibberish as they are completely inconsistent terms.), then it could still be porous enough to absorb coloration from the pit firing. Or it might be mature at cone 04 if that is the nature of that particular local clay. Until you test the characteristics of the clay, you won't know. The technical matters that you need to incorporate into your thought process are: 1) A vitrified clay body will not absorb the smoke and coloration of a pit firing. The clay body must still be immature and porous. 2) Much of the smoke and coloration that is imparted to the immature and porous clay body in a pit firing will burn off if you fire it again for any reason - whether to just bisque/low-fire temperature again or to an even higher mid- or high-fire temperature for the purpose of glazing or maturing the body.

 

In summary, pit firing and food safety do not live on the same planet, much less in the same piece of pottery.

Share this post


Link to post
Share on other sites

Thanks for the help and clarifications-- I do mean cone 04/05/06 (would bisque at cone 04, then pit-fire, then apply a food safe low fire glaze and fire in a kiln at cone 05)

 

I will experiment using this process and several types of clay (one that I've harvested and another standard grey clay) and post the results. I do not expect most of the coloration from pit-firing to stay, but am curious to see what the results are anyways. 

Share this post


Link to post
Share on other sites

The notion of of a "food safe low fire glaze" is meaningful only if said glaze is applied to a properly matured low-fire clay body, and then only if it completely covers the body with as little exposed surface as possible (and best if fired on stilts so there is no exposed surface). The misused term "food safe" only means that the glaze, when properly fired, will not leach toxic chemicals into acidic or alkaline foods that might come in contact with it. Food safety is totally unrelated to any notion of containing a liquid without leaking. Leaching and leaking are two different concepts. Leaking is prevented when a clay body is mature, vitrified, and non-absorbent, and the glaze properly fits without crazing or shivering. Clay bodies fired to only 06-04 range are typically not vitrified, even those bodies designated as earthenware and intended for that firing range. That is why low-fire functional ware must be properly glazed inside and out. Clay that can be fired to a higher cone must be fired to maturity before it will be watertight. Putting a pretty low-fire glaze on an immature clay body does not magically make it functional.

Share this post


Link to post
Share on other sites

Dick does that mean my play dough pots cannot work well in daily kitchen use??I was going to use deck sealer to make them water/food safe.

The Greeks made those water amphora pots for water without glaze -I made my play dough pots in the same shapes.Drought years are a coming.

Share this post


Link to post
Share on other sites

The classical Greek amphora shape has a pointy bottom so they could set them in a hole in the dirt floor of their huts. Probably didn't matter it they leaked a bit. They didn't have finely finished mahogany and walnut dining tables either. But deck sealer might work - if you put several coats only on the interiors and leave them outside upside down on the deck to dry for several days during the rainy season...

Chilly likes this

Share this post


Link to post
Share on other sites

I agree with what has been said.  pit fire is not food safe!  there are many chemicals that can make it less porous, but those are almost worse in the level of toxicity. The milk firing can effectively seal the vessel, much like Obvara, but both of those can wear through with use requiring the vessel to re-fired.

Share this post


Link to post
Share on other sites

Thanks for the help and clarifications-- I do mean cone 04/05/06 (would bisque at cone 04, then pit-fire, then apply a food safe low fire glaze and fire in a kiln at cone 05)

 

I will experiment using this process and several types of clay (one that I've harvested and another standard grey clay) and post the results. I do not expect most of the coloration from pit-firing to stay, but am curious to see what the results are anyways. 

If you fire AFTER pit firing , you would erase  or burn off the fire markings the pit fire created. Possibly use a low fire glaze before pit firing. It would most likely craze and then not be food safe.

 

Marcia

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×