Jump to content
Liam V

Pit-firing pottery and bisqueing

Recommended Posts

Hi,

I've been teaching myself throwing with a stoneware body. I understand that this needs higher temperatures to be bisqued and I will need to use an electric kiln (which I currently don't have access to).

As such, I'm planning on buying an earthenware clay so that I may pit-fire it. Here are some questions I thought I'd ask the professionals:

Do I need to bisque these pieces beforehand? I've been watching videos of pit-fired pottery and I'm not sure if I need to bisque it and then pit-fire it to add colours with things like banana peels and seaweed.

If I need to bisque it, can earthenwares be bisque fired with a pit-fire? (Essentially, I would need to pit-fire it twice?)

Can pit-fired pottery somehow be made food safe?

Can pit-fired pottery hold water? I'm assuming an earthenware body will vitrify in a pit-fire; it should then be able to be used as a vase to hold water and plants? Or even as a pot to hold earth and plants for long periods of time?

 

Thanks for your time!

 

Share this post


Link to post
Share on other sites

Clays bisque to the same temperatures.  I've never pit fired but I've seen other people do it and I highly doubt you'd ever reach temperatures required to form glass or vitrify.  I've only ever seen people put bisqued pieces in pits as well, I'm assuming that greenware is just too fragile for the process.  As far as eating and drinking from it... That's up to you, I wouldn't personally 

Share this post


Link to post
Share on other sites

A lot of pit fired work is bisqued to a low temperature such as 017. I've bisqued to 04, then pit-fired the pottery and am still happy with my results - its all trial and error to see what you like. 

Forgot to mention - my stuff isn't vitrified - just decorative. 

Edited by terrim8

Share this post


Link to post
Share on other sites

I have used mica clay or micaceous clay to coil construct things. It is pit fired and can be relatively water proof as long as it is hand compressed and burnished a bunch.  Google  ShumaKolowa, as I recall  functional Pueblo pottery.

Fun to try, fun to learn, and lots of mica everywhere! Seriously it was a good experience and pit firing finish is the norm.

Edited by Bill Kielb

Share this post


Link to post
Share on other sites

No, you will not get either an earthenware or stoneware body to reach vitrifaction which would make the CLAY body food/drink safe, in a pit fire. Likewise, it is very difficult, if not nearly impossible to get a high iron clay body like an earthenware to vitrify to make it food safe, no matter what firing method you choose. Iron acts as a flux in high percentages, and in high temperatures; if you tried to fire a high iron body, like an earthenware(typically low fire), past its recommended cone, in an effort to vitrify it, it will begin to bloat and melt due to the extra fluxing action caused by the iron. If you could vitrify the body, it will likely be very fragile/brittle due to the over vitrification. High iron clay bodies, that are made into functional ware need to be used in specific manners to make them food safe; either they rely on a well fitting glaze (difficult due to high expansion) that covers nearly every part of the finished pot (to avoid not only food/bacteria being absorbed, but also water/soap from washing being absorbed also) or they dont come into contact with food (i.e. evaporative cooling wine chillers), and are cleaned with very mild soaps and water (not in the dishwasher).

Some earthenware bodies( containing high amounts of mica, etc) make for wonderful cookware, able to withstand thermal shock much, much better than stoneware/porcelain clay bodies because they are specifically unvitrified  and can "move" without cracking. 

Not only will you not be able to vitrify your clay body, but any "coloring" method you choose that are traditionally done in a pit firing (stains, bannana peels, oxide, etc, etc, etc) will not be durable for utilitarian use. 

Pit, saggar, raku, etc are almost exclusively decorative pots! A recommendation; if you make both stoneware/porcelain table ware, but also decorative ware (raku, pit, etc..) you should stamp the bottom of your wet pots with NFS (NOT FOOD SAFE), or at the very least a sticker after they are fired. The customer generally has no idea the differences in the clay bodies/finishes, and will be highly upset if they take your pit fired yunomi home to find it absorbs & expels moisture like a sponge, which ruined their grandmamas antique coffee table. (let alone, they could be poisoning themselves with chemicals or bacteria!).

Low fire functional ware also needs to be treated with more skill, care, and precision when made as it too generally isnt vitrified or food safe. I see plenty of potters, selling gorgeous work, made from incorrect clay bodies, that are just selling stuff that will not last, or make their customer happy in the long run.

Pit fires can reach 04 temps, and hotter given the right design of the pit, combustibles used, etc. You can pit fire either bone dry pots, or already bisqued pots. Look into information about traditional native american southwestern pottery; practically all of this was made from earthenware, or micaceous clay, and almost exclusively pit fired (traditionally). You will lose some pots if you are firing them bone dry vs bisqued, but thats kind of "part of the deal".

Even as flower pots, your pit fired pots will not last more than 5 or so years; check out the terra cotta flower pots on your porch/deck. After a couple years of service you will notice they are "softer" than newly made, and will begin to crumble. This is caused by the constant expansion/contraction due to the pot absorbing moisture; eventually they will fall apart.

Sorry to be a downer, but until you can find access to traditionally accepted firing methods for utilitarian ceramics, you're gonna be making pretty to look at (only) pots. Doesnt mean pit firing is bad, just bad for eating from! Some of my favorite pots in our collection have been a combination of alternative firing techniques (alcohol redux, pit fired, saggar, naked raku, etc, etc...)

Share this post


Link to post
Share on other sites

All very true as @hitchmss says. That said, there is an interesting story and limited functionality to some of this early crudely crafted pottery. Today we would not consider it functional or durable nor vitrified but artistically very interesting with an even more interesting functional past. Many of my more technical friends enjoy the craftsmanship and art even though they are modern day fully vitrified folks now. If you have the chance to explore, I highly recommend as an important part of culture, art, and the evolution of pottery. This is one Website link I was able to find, I am sure there are many others.

 

3761E7E7-3431-4019-8F89-5950BE73889F.png.7c226a160b2617246bbe84928220a98f.png

Share this post


Link to post
Share on other sites

Google Maria Martinez.  Nit pit fired butt definitely functional as well as decorative pieces.

Rapid temp changes are the danger so modern potters bisque.

Beautiful subtle cplours achievable when "stuff" is used to fire next to pots. 

Google Rod Pedlar

Share this post


Link to post
Share on other sites

It depends on how you do your pit, and what you define as functional.If you're adding copper-based materials, salts, etc. to the pit, or using scrap woods from pallets or plywood, then you definitely don't want to eat or drink from them. If you're just using wood, then they could very well be functional. Is it the best way to make functional work? Not at all. It will be porous, which means it will have a lot of hygiene issues, since liquids will be able to soak into the wall of the pot where they can't be cleaned out well. If you only use the pieces for water or tea, or for roasting food in the oven, you'll probably be fine. I wouldn't drink orange juice from it, or anything else that could go rancid in the walls of the pots. Personally, I would never use pit fired work of any sort for food, because there are much better (safer) options out there. I have enough digestion issues without adding that risk to the mix.:D

Bisque firing will make the work much more likely to survive the pit. The pit heats quickly and unevenly, so greenware is more likely to crack (or explode) than bisque. For decorative work you can use your stoneware clays. If you're going to make functional work, then I would go with an earthenware.

Share this post


Link to post
Share on other sites

Our Explorer Scouts dug clay from the campsite,  and made pots with it.

When it was dry, we fired it inside a fire.  Not a pit, but we used an altar fire, laid the pots in it, then lited it onto an already lit fire, and piled wood on top.  The bars across the top protected the greenware from falling wood or pressure of building a fire around them.  We fired (a huge fire) for about 4 hours until we (two leaders)  ran out of time.

When we unloaded, the pots were fired, but still quite soft.  When we broke some open the inside was black.

I later bisque fired the same clay in an electric kiln, which cured the black centre, and vastly improved the strength of the clay.

Pit-firing is able to produce useable ware, it was the only method available in the early days of pottery, and is still the method used in many countries.

Give it a try, and let us know how you get on.

Share this post


Link to post
Share on other sites
2 hours ago, Chilly said:

Pit-firing is able to produce useable ware, it was the only method available in the early days of pottery, and is still the method used in many countries

"usable" is a relative term. Like neil said; water or tea is one thing....milk or orange juice..... The clay will still absorb water, so the way those pots are used have to be different too; if you used a pit fired mug, drank water from it, and then washed it, stuck it in the cupboard, you'd come back to find the wood in your cupboard shelves have swollen due to the moisture being expelled from the mug, which will take some time to fully air dry again. Run that mug through the dishwasher, and everything you drink from then on will taste like the highly alkaline dishwasher soap.

I once came across an etsy story, for a potter from Ukraine who was making and selling yunomis. His experience level was clear as his listing information/terminology was all mismatched/incorrect. He was advertising them as functional stoneware vessels, however he was pit firing them, and after the firing he was washing them in water to remove ash, etc, then boxing them up in some kind of straw/grass and shipping them in cardboard boxes. A number of his customers were reviewing them and complaining of the smell of rotting organic materials when they opened the box. While yes, the grasses he used could have still be wet, more than likely, the moisture from the mug, packed into absorbent materials, without air flow, was causing the grasses to decay. Some of his customers said they tried to wash the smell off, but it wouldnt go away, and some complained that there was a bad taste in the water they drank from the cups. Case and point IMO.

Non vitrified wares have their place in the kitchen, but it should not be considered synonymous with vitrified stoneware/porcelain wares.

The black cores to your pots were caused by a lack of oxygen in the firing; the organic materials which are burned off during a firing, did not have enough oxygen to completely combust and burn away completely. The bisque firing in the electric following the pit provided enough oxygen to rid you of this coring. It could also be caused by an excessively high amount of iron in your clay body, but more than likely a lack of O2.

Edited by hitchmss

Share this post


Link to post
Share on other sites

Mark into clay as suggested above if marketing to folk who "expect " dishwasher tolerant , modern type pottery.

As with my granny's china ,I use it but wouldnt place it in a dishwasher but treat it with respect and enjoy my morning cuppa from it. .

Got a number of primitive pots which are well sealed with age and use....an old lady here so havent been bowled over yet with this habit .

Tell your clients by stamping ware.

 

 

Share this post


Link to post
Share on other sites
On 1/5/2019 at 2:50 AM, Stephen said:

are you going this route because you can't afford a kiln or because that's where your interest lies?  

I'm definitely more interested in making things that I can eat or drink from. I'm planning on paying to use someone elses kiln as I can't afford a new one or find a second hand one for sale.

As I am just in the beginning stages of learning throwing, I wanted a method in which I could create close to finished pieces without the hassle of getting my poorly made pots professionally fired. 

I thought pit-firing would be a fun way to move my bone dry pots into a more permanently hardened state and judging by all the feedback I've been reading so far, I think this may be a good experiment for making planters. I'll be able to practice throwing different forms and potentially be able to keep some pots for my plants. From the answers I've read, it seems like pit-firing bone-dry pieces results in higher pottery casualties but is still feasible.

Everyone's answers so far have been amazing, I am loving the little lessons that I can garnish from all these responses!

(The next step is working out the legalities and safety measures involved in starting a pit-fire in a suburban neighbourhood)

Share this post


Link to post
Share on other sites

A friend Brought back a mug from Mexico for me as a gift-its raku-with a very thin clear glaze on the inside. Natural unglazed outside. Its not waterproof and will weep rather badly-I tested it for lead as I thought for sure it would have a lead liner glaze but it came out clean. I choose not to use it as it will absorb any build thats is in it. Functional well kinda of??but not really.

Share this post


Link to post
Share on other sites
On 1/6/2019 at 7:09 AM, Liam V said:

I'm definitely more interested in making things that I can eat or drink from. I'm planning on paying to use someone elses kiln as I can't afford a new one or find a second hand one for sale.

As I am just in the beginning stages of learning throwing, I wanted a method in which I could create close to finished pieces without the hassle of getting my poorly made pots professionally fired. 

I thought pit-firing would be a fun way to move my bone dry pots into a more permanently hardened state and judging by all the feedback I've been reading so far, I think this may be a good experiment for making planters. I'll be able to practice throwing different forms and potentially be able to keep some pots for my plants. From the answers I've read, it seems like pit-firing bone-dry pieces results in higher pottery casualties but is still feasible.

Everyone's answers so far have been amazing, I am loving the little lessons that I can garnish from all these responses!

(The next step is working out the legalities and safety measures involved in starting a pit-fire in a suburban neighbourhood)

you find something like this a little better and is pretty cheap:

https://ceramic.school/diy-raku-kiln/

Share this post


Link to post
Share on other sites

Liam V.  Pit firing can be very fun and challenging, been doing for 10 years now.  We do bisque 1st, but can be done from a green state just need to go much slower and start with the pots outside the fire and slowly move them towards the fire as time goes by and turning them to heat/dry evenly.   They will never be considered "food safe".  Obvara is another fun technique, it will produce items that are sealed and useable for short periods of time.  Water or other fluids will turn the obvara mixture to paste.  It will not prevent chemicals from leaching through though.

As for pit firing in your suburban neighborhood, I've done small firings in my backyard fire pit, I just dig down a bit to give it more depth and break my own rules of adding more wood as it burns. 

Attached is one of our pit fired pieces.

Good luck,

Chad

pit.jpg

Edited by Up in Smoke Pottery
typo

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

×

Important Information

By using this site, you agree to our Terms of Use.