High Fire, Then Pit Fire? Food Safe?

[karinagenevieve]

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!

[curt]

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?

[neilestrick]

Nope, can't be done. If you high fire it first, the piece will be vitrified and won't take in the carbon to give the effects of pit firing. Plus it will most likely crack.

[karinagenevieve]

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?

[karinagenevieve]

I mean food safe-- they will be used for drinking water from.

[neilestrick]

There's nothing that will seal porous pottery and make it food safe other than glaze.

[karinagenevieve]

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!

[Dick White]

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.

[karinagenevieve]

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. 

[LeeU]

Nope-don't do it. There is no easier, softer way. You cannot offer those pieces for food/water.

[Dick White]

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.

[Mark C.]

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.

[Dick White]

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...

[Min]

There was an interesting thread a few years ago about using milk. 

[Mark C.]

The reason I like the pointy bottoms is they also make good spears when the neighboring villagers attack the castle.

[Up in Smoke Pottery]

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.

[Marcia Selsor]

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