Differences Between Stoneware And Earthenware Body When Firing To 06

[hanee]

Hi there,

 

I'm a figure sculptor working in clay and firing to around cone 06. I'm getting ready to order a new batch of clay and am having some difficulty in understanding the differences between earthenware and stoneware bodies when firing to 06 (such that neither one will be vitrified). I'm interested both in the differences in the raw state (i.e. plasticity, wet-to-dry shrinkage, etc) but also the differences when firing (strength, resistance to thermal shock, ability to tolerate thick sections) -- in all of these I mean the difference independent of any grog/silica-sand added.

 

I'm confused because from my research Earthenware seems to be defined purely by the act of low-firing, for example here's the definition I see often:

 

"Earthenware is the term for pottery that has not been fired to the point of vitrification and is thus porous."

 

Based on this definition an "earthenware" body that is pushed to 02 and vitrified is no longer earthenware and a "stoneware" body which is fired to only 06 and not vitrified would now be called earthenware. Am I the only one that finds this sort of definition, well, sort of useless? What I would like to know is what are the intrinsic differences between these two bodies when BOTH are fired to the same non-vitrified state at the same cone.

 

So, what are the real differences in terms of:

1.) Working properties like plasticity

2.) General rules about shrinkage, strength, thermal shock, etc.

3.) Final strength, texture, etc.

 

(please clarify if you use the word maturity to describe the differences, too, since that seems to be defined in a rather circular way too roughly as "when a clay gains the properties you want it to have")

[Chris Campbell]

Welcome to the forum!

There are others who can give better technical advice on clay, but I just wanted to let you know that I think your work is amazing!

[Cavy Fire Studios]

LMAOOOOOOOO HAHAHA

Dude.

That definition just killed me.

That definition is also BISSSSSQUE....geez! Whoever wrote that is a dummy.

I work in lowfire and my stuff is vitreous, because I fire it at ^03 come glaze time. I fire to ^04 for bisque. Never once had a problem with leakage.

Thing is, stoneware is a butt-ugly bandaid color when fired to ^06, because the poor thing isn't "done" yet. Earthenware now...it will look just fine at that temp. I would suggest going to ^04 though.

 

1. About the same. I find terracotta to be a lot more buttery, though. 'Tis nice!

2. Eghhh...thermal shock with earthenware ain't so great. I am always careful to make my pieces "room temperature" before adfibg really hot or cold stuff to avoid cracking. There was this one redart from Seattle Pottery that gave so a ton of grief before I told it to kick rocks. I still use it for slip on my whiteware (my glazes like iron a lot, whiteware has none, so I slather on RA slip), because it is so richly red, but I use another amazing clay body for my throwing that is much stronger. Great for handbuilding, too. Shrinkage varies from 8-15%, depending on the clay you use. Porcelain is the worst for shrinkage--my college had one that shrank 18%, hehe.

3. Stoneware is stronger and more durable, no question.

 

Oh! Welcome to the forums!

[hanee]

Hi Chris, thank you for looking at my work! It's hard to get my work seen because I'm more of the "cabin in the woods" type artist and am not that great at socializing/networking. I love it when new people get to see my work; makes me happy...

 

@TheGuineaPotter - So would you say stoneware is more durable than terracotta when BOTH are fired to only 06 (pre-vitrification)? I'm currently using a "raku" body of unspecified contents from Laguna/Miller and it fires to an old-bubblegum color at 06; not sure whether it contains a stoneware or earthenware clay in it (though it seems targeted at 06 firing or so). The raku body I use is plenty strong enough for my taste. Sculpture has no need to be vitrified...

[Tyler Miller]

Hanee,

 

earthenware and stoneware are indeed useless terms without a little more information.  It's best to think of the issue in terms of two different definitions rather than just one.  First is the temperature at which maturation occurs: "high fire"  "Mid fire" "low fire."  The second is vitrification--earthenware is very porous (above 10% water absorption in the standard tests or so--some wild clays I've fired are over 20%), stoneware less porous, and porcelain no porosity or negligible.  You can have a low fire porcelain (porcelain dental work is like a cone 016 body, and most industrial porcelain is like cone 02), or a high fire "earthenware" (though you don't hear it called that very often).

 

Working properties aren't really all that different, IMO.  Others have said porcelain and a fine grained terracotta work very similarly, I don't quite agree, but there's a truth to it.  Smooth, grogless bodies throw/work more alike than bodies of the same maturation temperature, but different clay, flux, grog, and silica content.

 

That's the real difference, maturation temperature.  Because a cone 10 or cone 6 clay has less flux/melter in it, it won't be as vitrified at cone 06 as a low fire body meant to "mature" at cone 06.  Under fired clay bodies are always far weaker than clays fired to proper maturity.  Low fire clays are much stronger than bisqued high/mid fire stoneware.

 

As Chris stated, your work is amazing.

[Benzine]

Tyler put it very well.

 

I agree with the others, great work! Since you are currently using a Raku clay, do you, or have you thought about, Raku firing your work? Those figures would look excellent with some of the effects Raku creates.

[hanee]

Tyler,

 

Thanks for your clear response. My takeaway is (1) a clay designed to reach "maturity" at 06 will most definitely have higher strength than one sold as a cone 6 clay but fired to 06 and (2) this is due to the relative distance from the vitrification point, so even though a low-fire earthenware fired to 06 might not truly vitrify until 02, it's going to be *closer* to that point than the cone 6 clay fired to 06. Is this a reasonable way to look at it?

 

I have a small follow up question: I have read, perhaps incorrectly, that all (earthenware) clays that mature at low-fire, whether directly sourced from nature or mixed, will fire to a range of orange-to-red (except some concocted whites that people have recipes for).

 

The reason I'm looking for a clay body right now is that the raku body I was using fired to a pinkish-white and though I'm *okay* with the full on orangy-red colors that I've gotten from standard terracotta preparations, I would *ideally* like to have something that comes out more in the buff-to-brown range. There seems to be quite a bit of terracotta figures coming out of paris that have that sort of color range (for example these two canova bozzetti), whereas the classic italian terracotta sketches and even a large subset of parisian terracottas tend to be a definite and distinctive orange-to-red (though many were coated with a slip after firing to subdue the tone, something I'd like to avoid getting into the hassle of).

 

So, what gives -- are the parisian buff terracottas naturally so or were perhaps certain oxides added or some other modification to natural clay deposits to move them towards that color? Or is it jus that terracotta has become a color-term and most people looking to do low-fire expect and what that color and so all the commercial moist clays tend towards the orangy-red through the inclusion of redart, etc?

[hanee]

@Benzine, thanks! As for raku-effects: as a sculptor working with the figure I try to have as little emphasis on the surface qualities of the clay and as much as possible on the figure itself. A little bit of natural patination is nice -- after all skin is not a single tone or texture -- but I'm not a big fan of patina-effects that distract from an understanding of the figure itself. Also, not sure if they'd survive raku firing!

 

-- I'm trying in fact, to move away from high-grog clays. I love the strength and robustness of the formulations but sometimes I have to fight with the grog too much on small details. There's a long tradition of firing even such crazy things as SOLID heads with no grog. Though I'm a bit scared to try.

[Tyler Miller]

hanee,

 

you're looking at it exactly right.

 

Earthenwares do vary from a buff to orange to red to brown naturally, the main variable being the iron content.  It all depends on where it's dug.  The natural clay in my locality is a pale yellow colour when fired, but 50-100 km in any direction from London, Ontario, and it turns bright red--the difference is iron content.  An article on the history of bricks around here: http://www.walkervilletimes.com/37/yellow-brick.html Terracotta does seem to be a colour definition.

 

 You can get rich brown from a commercial terracotta if you go above cone 03, but it can be a risky proposition, since some terracottas have razor thin vitrification points, and you can end up with slumped pieces or vitrified puddles.  See my gallery here for the colour:  http://community.ceramicartsdaily.org/gallery/image/4187-4-cups/

 

You can't really take iron out of a clay body without some fancy chemistry, so you can't turn an orange clay buff, but there are definitely cone 06 buff clays on the market like Laguna's EM-214 a buff with sand (though the sand may not be so desirable for you). 

 

I hope that helps.

[hanee]

Thank you again, Tyler, your information is invaluable.

 

I looked through some of your gallery photos and like your aesthetic, it is both earthy and refined.

 

Unfortunately I only have access to the "Northeast" clays from Laguna (the ones they got when they bought Miller): http://www.lagunaclay.com/clays/northeastern/cone06.php-- EM-214 is a western clay in their line-up. The "Northeast" low fires seem to be the most atrophied selection.

 

Otherwise I've got Sheffield, Tucker, and Standard-clay-co available. Sheffield has a brown-black that's 05 to 2... probably too dark, but maybe it would work. The Laguna/Miller supplier has the great advantage of only being 1 1/2 hours a way, whereas the rest are 4 hours away...

 

I've got a neighbor who has a ton of clay on their property that in raw state is a rather dark gray and wonderfully plastic. Winter is setting in so I can't dig it out until spring but maybe I'll get lucky on that one and it will fire to something more towards brown and less towards red... I feel very strongly that it's a bit absurd to dig clay from Pennsylvania or Tennessee and have it mixed with other ingredients from all over then ship it to northern Vermont when there's gobs of clay locally, even if it may need a few modifications for my purposes... the french and italian sculptors sometimes ordered clay from a few towns over but it was like Tuscany to Florence, not Tennessee to Vermont...

 

Anyhow, I'm happy to accept if orange-red is the most realistic considering my local suppliers -- reality is easier to accept when it's backed up by a bit of knowledge as it now is. Sheffield seems to be the only place nearby that has slightly different formulas and descriptions on all their terracottas, since they have their own deposit of a native clay that's going into the mix.

 

As for pushing the red darker via higher firing, I'm pretty cautious about pushing too far towards vitrification, not just because of the possibility of melting but I know shrink rates increase and increase as you go up. If a figure with a wide variety of thicknesses survives 06, could I expect it to survive, say, 04, since it's already passed quartz inversion and all that, or will it always be more strain and likelihood of cracking due to increased shrinkage? The few figure sculptors I know of that work in clay fire to 06, so I've defaulted to that...

[Chris Campbell]

I could be wrong here ...BUT ... here goes ...

 

If you are going to low fire the work and are not considering installing them outdoors then you can go with pretty well any clay body that suits your color/carving needs. In fact higher firing clay bodies will survive lower temp firings with ease and since no one is using them for food why care about anything other than color and texture??

[hanee]

Chris,

 

That was my original thinking if the strength was in the same ballpark at least. But these do have to survive shipping and sometimes have very thin sections (like a finger or hand). My current clay, a very highly grogged raku, fired to cone 06, is PLENTY strong in my book, but I do know Raku, though having stoneware in the mix, is usually formualted to be fired to 06. I'm not very familiar with raku-body details, but as I said, the Raku I just had fired was substantially stronger than I would have expected based on firing terracotta and raku bodies in the past.

 

Though, I would say, I'm a bit eccentric and like to be as close to historical norms and local possibilities. If stonewares weren't used for this type of work in any of the periods that I find value in, I see no reason to use them; and similarly, if all of the clay I'm likely to be able to hand-dig in the surrounding 500 mile readius is going to be earthenware then I'd feel a bit shamefully exotic or bourgeois, making my whole process dependent on stoneware or ball clay... Of course I may be misinformed on the rarity of stoneware in easily-dug northeastern/northamerican deposits. And, naturally, I'm letting myself take that moral extravagance only because it sounds like earthenware will be 15 degrees within the range of perfect already.

[Chris Campbell]

Well ... NCECA is going to be somewhat near you in March and all the vendors will be there ... just waiting to talk clay.

[bciskepottery]

I work in lowfire and my stuff is vitreous, because I fire it at ^03 come glaze time. I fire to ^04 for bisque. Never once had a problem with leakage.

GP -- what earthenware are you using that vitrifies at cone 03/04? Most earthenwares I am familiar with are still rather porous at those temperatures. A vitrified clay body should be non-absorbent without any glazes.

[Benzine]

Hanee, I fully understand, why you don't to Raku your work. I do believe they would survive however. If you look in my gallery, I have a two piece figural sculpture I made, and Raku fired. I had zero issues. No cracks at all.

Also, even though it is not vitrified, it is plenty strong, for it's purpose.

[Tyler Miller]

 If a figure with a wide variety of thicknesses survives 06, could I expect it to survive, say, 04, since it's already passed quartz inversion and all that, or will it always be more strain and likelihood of cracking due to increased shrinkage? The few figure sculptors I know of that work in clay fire to 06, so I've defaulted to that...

 

I don't see why it wouldn't survive to cone 04, I've never noticed an increase in failure rates between cone 06 and cone 04.

 

Sheffield clay is good stuff, I've not used it myself, but people I greatly respect and admire have used it or have recommended the use of their terracottas.

 

Keep us updated on how using your hand-dug clay works out.  That's one of my favourite materials to use.  I have a feeling that if it is a grey clay, it might turn red on firing, or maybe a buff colour that could benefit from slightly higher than cone 04 firing.  This colour change happens a lot with Northern Ontario clays where the iron content is present in the form of black magnetite rather than hematite.  But sometimes it just comes out brown, or speckled, or some variation.  You can get a hint that it's gong to go red if you let the clay age in plastic.  It will develop an outer layer with a slight orange hue where the iron digesting bacteria naturally found in the clay have converted the FeO to Fe2O3.

 

Thanks for the compliment about my work, it means a great deal coming from someone so talented.

[High Bridge Pottery]

I love the forms you do, they have so much weight. I can feel how they are holding themselves. Are they hollow?

[hanee]

Thank you, Highbridge. The older laying figures that are fired and from 2009 are all solid... the waiting-to-be-fired work at the top of the page are all hollow in the torso (newspaper)... they're all in the 1/4th scale range (18 inch figure), but I have some that aren't ont he website that I'll be putting up soon that are 1/3 (24) and 1/2 (36) as well which I had to hollow in the legs as well (very tricky!!). I also have been in the habit of making hollow heads lately just by shoving my thumb in the top of the head early on and then lightly building around that cavity. The hollowing is probably not necessary for firing so much as weight. If the head or torso is heavy it tends to slump during modeling and you lose the gesture after a while without even realizing it...

 

I support the standing ones while modeling initially with a pronged armature which comes in around the sacrum. Once the figure is firm enough to almost support itself I add a little buttress if needed (like on cold-shower-no4 and pulling-no-4) and then gently pull the pronged armature away.

 

The buttress wouldn't be needed if I was willing to use a wooden base instead of a ceramic one, though. You can also just do the figure down to the feet with a very small base and then dry and fire it horizontal and afterwards mount it on a wooden base. But mounting on a wood base seems like a hassle when I could just make it all one peice terracotta. Keeps my after-work down which means I can spend more time sculpting.

 

Bruno Lucchesi's "Terracotta: The Technique of Fired Clay Sculpture" is a good resource on the basic method.

[hanee]

Tyler,

 

I'll definitely post in the spring when I try to use this local clay. I think I'll throw a small sample (maybe a small relief) of it in the next batch just to see how it fires, either way. Regardless of the color I'm sure I'll be happy knowing its origin. Plus, it's free and sustainable which goes with my whole ethos since I'm trying to give my work away for free as well and want to keep my costs down...

 

I think I'll try 05 for the next batch and 04 for the following if that works...

[hanee]

Benzine,

 

Good to know that raku is a possibility, but I don't see the need to subject anything to such stress if you're not looking for the effects -- on the other hand, I would like to do wood-fired work if I ever have the facilities to do so (as a fuel/firing system, not for the wood-ash, etc)... And of course, I would absolutely love raku and unpredictable wood-ashy stuff if I were going to make a simple form that was purely decorative or functional...

 

Your figures have a lot of varying thickness but overall are quite small, so I'm not sure how different things would be with, say, a 36 inch figure (if I wood-fired then I'd probably be working quite large since I wouldn't be limited by affordable electric kiln-size as much anymore)... also people coming from a ceramics background who then come to figural work tend to work very different than people coming from a sculpture background, so I wouldn't consider the success of their firings to indicate that mine might be so... For example, I would never find myself carving clay and I work lots of small pellets, strictly *additively* whenever possible, putting nice little air pockets all throughout the piece probably... if my work was sliced in half it would not look very well made by ceramic standards...

[timbo_heff]

Hi,

A couple ideas: try the Sheffield MGB : it is based on the native clay but with a good bit of grog for sculptural applications: it does NOT have lots of Red Art in it so it is not as red and staining as regular terra cotta clays. Without all the Red Art is is a much more tame color. Still orange /red though to some extent: if you take it up to 01 or 1 even, it will get browner.

 

Another idea is to go with a low fire white with grog and add a bit of Mason Body stain to get your desired color.

Or do a tinted slip over either clay to get your color would be another approach.

Just some thoughts

[hanee]

timbo_heff,

 

I'm right on your line of thinking: I just called sheffield this morning and am swinging by on Wednesday on the way to visit family in the Hudson Valley. I was thinking I'd get a few boxes of MGB, and also try the same formula with less("Mass Red") or no("Sheffield") grog... I'm okay with orange/red depending on intensity, I think I will definitely try firing a tiny bit higher to toast it a bit darker.

 

I thought about that with the white body, exactly that in fact because I just bought some mason stain the other day to try adding to slip for a patina (ended up not doing it though). I was thinking, though, that if I was going to take white moist clay and dye it brownish, I'd be better off just mixing the clay from scratch since I was guessing it'd be quite a bit of work to get it evenly dispersed...

 

Thanks!

[Callie Beller Diesel]

If you want to try adding stain to your clay, mixing it from dry is easiest, but adding stain to very wet reclaim slurry with a drill and a paint mixing bit is very doable.

Another tack might be to do a surface application of some sort of flux, such as Gerstley Borate, borax or soda ash. Maybe even baking soda, in the name of using something inexpensive and sustainable. Spraying with a spray bottle *or brushing on a solution of any of these will bring out the iron content by fluxing it gently on the surface, and your structural integrity isn't altered in any way.

 

 

*(Don't put a solution of soda ash in a spray bottle: it hardens badly. )

[Benzine]

hanee, I think your work looks good by any standard.

 

My figures are small.  They were partially limited by my Raku kiln size/ the size of my reduction bins.  Also, it was for my home, and I really had no idea where to put it, so I didn't want to go too large.  I just had the idea to make it, where it went after that...

 

You mention using local clay.  I will note something, from my experience.  I was given some blue green found clay, by a coworker.  It fired to a golden yellow color.  However, it was extremely weak.  I could snap a bisqued half inch thick test tile, I made, in half with my bare hands.  It definitely needed some additives.  A classmate of mine in college, brought in a similar clay.  She had it fired, in a scrap bowl, to see what would happen, at the glaze temp (I think Cone 6-8?).  It melted pretty well.  It was quite a puddle, but it was getting there.

[High Bridge Pottery]

Chris Campbell has some tutorials on using mason stains.

 

http://www.ccpottery.com/murrinis.html