Porcelain Warping

[colekeller]

Hello! I was hoping to have a discussion with anyone who may be more educated/experienced on the topic of porcelain warpage. 

Primarily what I am wondering is - is it possible to make porcelain wares without any warpage at all through glaze fire? I have tried a number of different drying and firing techniques but it seems impossible to achieve a perfect net form. I am slip casting, so my wares have very consistent wall widths. 

I have read time and time again that the design of the object dictates warpage (along with drying/firing techniques), but I can't really think of a design that doesn't have at least some varying wall thicknesses as a result, other than a perfectly spherical bowl or something. For my purposes, I will need to have at least some variation in thickness, and I hope to learn of a way to consistently achieve near net form through glaze fire. It would be great to hear from you all on the subject. 

Thanks for reading

Cole

[tinbucket]

Are your pieces warping in drying or firing? Evenly drying will take care of most drying warpage, I think. If your porcelain has a high amount of plasticizer this can require more attention. If your work is warping during the firing >>> Do you mix your own casting slip?  How important is translucency to your work?  Although changing the form will help I think the pyroplasticity of the porcelain is a more important factor. I think many porcelains are over fluxed to make them more translucent (a broad statement). For example you may be able to reduce the amount of feldspar in your casting slip, maintain 0% porosity, and reduce the pyroplastic deformation of your pots. Another option is adding a filler such as pyrophyllite or alumina. From what I have read potassium feldspars create a more viscous melt than sodium feldspars (custer will deform less than minspar in an equal level of vitrification). If you test/mix your own casting slip you will have much more control over the warpage of your pieces and have to compromise the design of the form less. I'm in over my head, calling on @glazenerd

[glazenerd]

Actually Tin, your responses were on target. 

Kaolin ( porcelain clay body) has a nearly net neutral particle charge. Which also means when you introduce a negative charge from plasticizers: ( or excess sodium silicate) as it dries and kaolin returns to a neutral charge: it also returns to its original particle orientation- which warps the piece.

1. If you are adding sodium silicate- how much?

2. Home made or purchased slip?  Particle size plays a role in warping as well.

3. Is the slip rated to your cone fire? Excess flux can also cause distortion if over fired. What you think is warping might actually be slumping.

Tom

[Rex Johnson]

I recently tried throwing Laguna porcelain. I'd stayed away from it since being a student where I found far it too challenging.

I found it surprisingly easy to work with though. But I also found that some simple bowls warped in the bisque.

I think partially it may have been from the lack of attention to making the warped bowls stressless in while throwing, rather, a consistent compressed and pulled wall.

That said, I think the stress was accentuated by the uneven firing of the kiln, that is, closer to the flame than the others. I'd say maybe 2 or 3 warped out of about 15.

I guess that's part of the porcelain challenge. Lesson learned...

You can see this one has a 'flat' in the roundness.

[hitchmss]

I use Bmix 10, fired to cone 11/12. While its not truly a porcelain, it behaves in a lot of ways like I porcelain. However, I used to work solely in porcelain at cone 10 redux/soda. With any of my work (porcelain/bmix) I find that even drying is a huge factor to not warping pots. Im constantly rotating work, flipping, changing locations in my studio, etc to maintain an even drying atmosphere. For example, if my freshly thrown pots are facing the two HVAC vents in my studio, and left to dry overnight, the sides of each pot facing the vents will shrink first, and will lean towards the drier/warmer air. I alleviate this by either drying under a piece of plastic loosely draped over my pots, or just every few hours I rotate the cart around. Simple things, but they are time consuming.

The other thing which used to plague my work in regards to warping was my kiln shelves. I used to fire on cordierite shelves, that after dozens of cone 10/12 firings had warped to banana like status. I would try wadding my feet so that I had "even" support on the foot of the pot, but it would still warp 9 times out of 10. I upgraded my kiln shelves to advancers about 2 years ago, and havent really had warped pots since then. Even old warped pots are refired, and come out perfectly flat. Now of course, advancers are not a cheap fix to warped pots (approx. $250 per 12x24" shelf), but it has solved the issue for me. I am a production potter, and for me the expense was justified, but for many it is not.

In regards to your porcelain clay body; like others have said, if you mix your own slip, you can alter your recipe to achieve the desired results, and minimize the warping. Remember, the more plastic your clay body, the more shrinkage, and consequently this also impacts the amount of warpage. If you can vary the particle size of your kaolin, flux, and silica this will help with the warpage. If all your particles are in the 250-325 mesh size, there arent any larger particles to "build" a solid wall. Think of it like a crushed stone foundation for a home; all the same size isnt nearly as strong as a mix of sizes which puzzle piece themselves into a much stronger wall. Porcelains, especially those desiring a translucent body, will contain 25%+/- of each flux and silica; when about half of your clay body is glass making materials, its going to move around. As tinbucket mentioned, there are different additives which can be introduced into your clay body to help better "bind" those particles together; small barbed needle like materials like kyanite can add a multitude of beneficial aspects to your clay body from thermal shock to warping, cracking, etc.

Im not a slip caster guru, so others who have more experience here may be able to offer more insight, however I do know that if your slip isnt properly deflocculated, and the proper viscosity maintained, you could have an excess of water in your slip, which is going to lead to more shrinkage, and more warpage.

Without going through lots of testing and altering of your clay bodies I would suggest handling your green wares carefully, keeping your drying even, and firing your pots as "flat" as you can. If your firing in a gas kiln you can amend the bag walls to achieve a more consistent heat dispersion and hopefully eliminate those "hot spots" which can also lead to warpage. If your firing in electric there's not a ton you can do inside of your kiln's stack to control heat flow; creating "bag walls" around the perimeter of each shelf may even out the heat some, but will come at the cost of drastically reduced firing space. A zoned kiln would help with uneven temps from top to bottom, as would a downdraft vent system. Likewise, a slower firing will yield more consistent temps than a fast firing one, however, keeping your clay body in a state of melt for longer than needed may not be beneficiary.

[colekeller]

Thanks all for the detailed responses! This information is extremely helpful. My apologies for not including some pertinent information in my original post. I am using purchased slip, formulated for ^6, from my local pottery store in Seattle. They've been around for a long time and are very consistent. I hardly have to deflocculate at all and that's only because most of what I cast is quite small.  This porcelain slip is not intended to be translucent at all, and from what I've gathered it seems to be formulated to be the most forgiving porcelain slip that this store carries (if that makes any sense). They have another porcelain slip that is ^10 and semi-translucent, but as you all noted this would require much more precision. Translucency is not necessary for me at this point in time. 

The warping that I am experiencing becomes apparent during drying and seems to be slightly magnified in the glaze fire. I am using an electric kiln and the warping seems consistent no matter where the wares are located within the kiln, so I don't think hot spots are necessarily a huge problem. That being said, I have a tiny old Skutt kiln that I can't imagine is firing perfectly. 

I had never heard of Advancers, but that sounds promising. I'm planning on upgrading my kiln soon, and will definitely get those. 

Thanks again all for the info, it is greatly appreciated! 

Cole

 

[hitchmss]

3 hours ago, colekeller said:

Thanks all for the detailed responses! This information is extremely helpful. My apologies for not including some pertinent information in my original post. I am using purchased slip, formulated for ^6, from my local pottery store in Seattle. They've been around for a long time and are very consistent. I hardly have to deflocculate at all and that's only because most of what I cast is quite small.  This porcelain slip is not intended to be translucent at all, and from what I've gathered it seems to be formulated to be the most forgiving porcelain slip that this store carries (if that makes any sense). They have another porcelain slip that is ^10 and semi-translucent, but as you all noted this would require much more precision. Translucency is not necessary for me at this point in time. 

The warping that I am experiencing becomes apparent during drying and seems to be slightly magnified in the glaze fire. I am using an electric kiln and the warping seems consistent no matter where the wares are located within the kiln, so I don't think hot spots are necessarily a huge problem. That being said, I have a tiny old Skutt kiln that I can't imagine is firing perfectly. 

I had never heard of Advancers, but that sounds promising. I'm planning on upgrading my kiln soon, and will definitely get those. 

Thanks again all for the info, it is greatly appreciated! 

Cole

 

Again, not a slip casting guru, but your slip should come defloculated from the manufacturer. The size of your castings doesnt dictate whether or not the slip should be deflocculated or not. A defloculated slip basically allows the particles to build in an even layer on the surface of the mold. A non deflocculated slip will clump and build uneven walls. You may be referring to the viscosity of your slip; the manufacturer will also mix the slip to the "proper" viscosity. As the slip container is opened, and moisture evaporates, and as you cast and the slip loses moisture (the excess I assume is poured back into your container). Measuring the specific gravity of your slip regularly, and keeping the proper specific gravity will help with your casting. You can measure the SP GR using a number of methods, the most accurate involves using a 100 ml syringe and your digital scale, the least accurate is using a hydrometer.

 A thought came to mind as I was typing this; when you turn your molds over to empty them, you may be building up a slightly thicker wall on the side of the mold which is "lowest". You should be using some kind of a stick to put under one edge of the mold as you empty your slip out; this will prevent drips from building up in the bottom of your casting. If there is another 1/16-1/8" of thickness on one side of your casting because of this tilted effect, that could easily explain your warpages. Try marking the tilted "lower" side of your casting, fire it, and see if your warpage matches with your mark. If it does, then you may have to turn your mold over to empty your slip and rotate your mold to try and even out that last bit of slip. If you had a roto-mold machine then you could just slap it in there and go on.

Advancer kiln shelves are an "advanced nitrite bonded silicon carbide" kiln shelf. 1/4" thick, extremely hard, very light, perfectly flat,  and has a glass layer on the surface of the shelf which basically eliminates the need to grind your shelves if glazes run as the shelf has a near 0% porosity. They are made in numerous shapes, and are pricey. the crystons (almost the same composition and design as the advancers) are another model they make . The crystons are slightly more pororus, as Im led to believe they do not go through the second, very long, specialized firing which distinguishes an advancer from a cryston. The crystons will allow glazes to stick a little more, but they are cheaper. Still maintain perfect flatness even at high temps. Marshall, the tech at KilnShelf told me their testing for deflection under load and temperature is like 800 pounds at 2400* with less than 1/8" deflection....beasts! If you make your living from ceramics, it is in my opinion these shelves are a no brainer. They basically pay for themselves (not a paid promotion, I promise, I just LOVE these shelves).  You get more stacking space, less thermal mass to heat, and better finished products.

[Mark C.]

You could try to add a small amount of EPK to your slip-that will make it warp less in fire -You should run some tests-like add 2% and 4% and see if they helps warpage in fire(test small amounts not the whole batch.EPK as alumina in it and is a cheap source for that. We did this isa slip business I had for about 15 years. Mine was cone 10 aroma therapy lamps.The EPK stopped all warpage-I have them add it to our custom 1 ton mixes from Laguna Call co.We ordered it dry by the ton and mixed it ourselves.We added about 10-15% you will need less in cone 6 bodies.

[Russ]

Some Japanese fire their porcelain on a disk made of the same clay called a "hamma". It shrinks at the same rate as the piece relieving stress at the foot. For larger pieces this might be practical.

Hamma' s are a one time one use throw away deal... but if you spend multiple hours on a piece I'd sure make one.

[glazenerd]

Point of interest: how warm/cold was it when you poured? Was the slip warm or cold?

T

[colekeller]

@Russ That's an interesting method. I agree that it would definitely be worth it for bigger pieces.

@glazenerd I pour in my basement, which is by far the coldest place in the house. I'm in Seattle and there isn't a heater in the basement, so I would say on average I'm pouring in 50° F ambient air with slip around that same temp. I'd never thought about the role that temperature plays, what are you thinking?

[neilestrick]

I haven't done a lot of casting, but when I did I found I never liked porcelain bodies that don't have ball clay in them. I hate ball clay for throwing porcelain bodies, but in casting it seemed to be more stable in drying and firing, and a lot more durable in raw form.

Evenness is everything with porcelain, especially in wide forms like bowls. If the bottom is even a little thicker than the walls, it will warp.

[glazenerd]

@colekeller Johnson, then later Lawrence did some extensive research on slip/ deflocculation, and clay water systems decades ago.  They found that cooler/ cold temperatures had a direct effect on the particle charges on a deflocculated slip. There were changes in particle distribution and packing density. Being applied: the effects included warping. So the real test would be: did the pieces you cast in the summer react differently than those during the winter? Most commercial slips have a long chain polymer (Darvan) used to deflocculate: these tend to be more reactive to temperature than sodium silicate.

@neilestrick Pointed out wall thickness  can contribute to warping in cast pieces- also applicable.

actually a properly deflocculated slip creates a denser body than a clay body. Deflocculation does more than just "thin" the slip; it suspends particles that directly effect packing density/ green- fired strength. 

T

[Mark C.]

Even wall thickness should be a goal in slip work. If its even they can be really strong depending on the form.They also can be fired fast if they are thin and dry. We used to stack the entire electric with no shelves with our lamps for bisqueing. Did many many thousands of 5 inch lamps this way. stacked on top of on another.They where strong like eggs.

[tinbucket]

Another thing to consider is the clay memory. Porcelain remembers. Say you remove a leatherhard pot from the mold and put too much weight on one side of it (imagine holding a cup sideways). Even if the pot does not go oval in that moment it may be enough for it to cause warping later on. However I suspect this is a problem with uneven drying, even a little bit can cause significant distortion

[colekeller]

@glazenerd You were absolutely right about the temperature during casting. After reading what you posted I've been casting with a space heater keeping the room around 65F. I've had drastically less warping. Thanks a lot for mentioning those studies.