New kiln - cone 6 or cone 10???

darrellw

I plan to get a kiln rated to fire to cone 10, but everything I've read seems to indicate that firing to cone 10 is pretty tough on an electric kiln. The kiln maker offers two options that sound like a reasonable thing if I'm going to be doing much firing at cone 10, upgraded elements (APM, supposed to be more stable and last longer, $320) and an upgraded thermocouple ("type S, $200).

But right now I don't know how much I'll fire to cone 10. I've searched and read all I can, and it seems clear there isn't going to be a clear answer on if cone 6 stoneware is sufficient to be frost proof, or if you need cone 10. It does appear that cone 10 stonewear is likely to be a little more durable, but on the other hand, there doesn't seem to be much in the way of glazes for cone 10.

So I'm currently undecided on the extra expense (over 25% of the base cost of the kiln). If I end up doing a lot of cone 10, I feel it is probably a worthwhile investment, but I just can't tell right now which way I would be going. Anyone out there firing strictly electric, and if so, what cone do you find yourself using?

Thanks,
Darrell

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Joanie

Cone 5 - 6 glazes are fairly common now, and are indeed easier on the kiln. It isn't necessarily the cone temp that determines whether the clay body is frost proof or vitreous, it has as much to do with the clay itself. A good clay body at cone 5 should be vitreous, which means "like glass", non-absorbent.

I would suggest that you subscribe to the "clayart" list. It is a listgroup that discusses primarily stoneware handthrown work, and has many educators amongst its ranks. They freely post glaze recipes and tips, and they are excellent people to ask questions of. Like here, there are a lot of "characters" and personalities, but the clayart community is just as much a family as this forum is.

You can't really get the higher rated elements and fire to cone 10 without the higher rated thermocouple too. So it may be an all-or-nothing proposition. Personally, I wouldn't do the upgrade, because unless you plan on doing a lot of high range firing (or porcelain) it doesn't seem worth it.

Joanie

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darrellw

Thanks, Joanie!

I guess I could always upgrade the elements and thermocouple later anyway, since they sound like (somewhat) consumable parts, even at cone 6 (I downloaded the Skutt manual and have been busy reading....).

I'll check out the ClayArt list. I've been reading some of their archives from various web searchs, but just now found the URL to subscribe.

The local ceramics place unfortunately didn't have any absorption information on their mid-fire stonware bodies, but did point out a couple that are "tighter" than others (and they all happened to be nice tans, browns and reds, which would have been my choices color-wise). We get pretty good wet/freeze/thaw/refreeze cycles here in the Pacific NW, so I'll find out pretty quickly if they will stand up! Plain old terra cotta rarely lasts the winter here.

Hmm, what to spend the $500 I'm saving on.....

-Darrell

Joanie

Darrell, there are easy tests that you can perform on the absorption rate of the fired clay bodies. One involves weighing a test tile after firing (dry) then putting it in water for a day, then weighing it again. You can find out how much water it holds.

But it is my guess that any stoneware will withstand freeze/thaw/heat cycles as long as it has properly matured. The absorption rate probably only reflects the size of the spaces between the clay platelets. None of these tiny spaces (chambers) would connect with each other enough to bring water all the way through the clay wall. (which is why clay walls do NOT "breathe") They are only accessible to water when they are on the surface of the pot, not farther in.

One concern for the freeze/thaw would, however, be the glaze fit. If the glaze doesn't fit properly, then there is tension between the clay wall and the glaze. If the glaze is too tight, it cracks because the clay expands and contracts during repeated heating and freezing. (even steel expands and contracts during heating and freezing, right? Has nothing to do with absorbed water.) If the glaze is too big, then there is surface pressure on the glaze and it may pop off. It's really important to make sure that the glaze and the clay body expand and contract at the same rate during firing, so that the finished piece will be in harmony.

edit: what about the soil inside the pot freezing with water in it? Does the soil/water freeze enough to crack otherwise non-absorbant pots?

These are just my opinions, and I don't work on stoneware, but have had some experience in glazing wherein the clay body and the glaze are incompatible.

Joanie

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darrellw

Good point, though at what temperature (during firing) do the clay body and glaze become "set"? I would assume something well above room temperature, and a bigger difference than say 25 and 100 F (the widest range I would expect the pot to see here). But I guess it could be that the fit is really good at say 500 F, and just gets worse as the temp drops. The good news (for me) is that I don't expect to glaze a lot of pots right now, I think I would be using oxide stains, raw clay and colored slips a lot more.



I would think that would be more of a function of the shape of the pot. For example, something with more of an open V shape would let the soil expand a bit upward, and as the sides get more vertical, or even narrower at the top than lower, I would expect more problems. Since most non-cascade bonsai pots are quite a bit wider than deep, there is still probably room for the soil to expand, but a very straigh-sided, deep pot might have problems (just my guess)



Thanks for any info, even your opinions. Ceramics seems to be a bit of a niche as it is, and a very subset seems focused on bonsai pottery (or similar applications that can be applied), so every little bit helps!

-Darrell

PS Happy Birrthday!

rockm

Joanie, the remarks about glazes popping off of pots is interesting. I've had this happen to some less expensive Chinese and Japanese pots. High fired pots don't 'breathe' as some people assume they do. Thanks for making some sense of why that is.

For what it's worth, expansion of soil in a pot can crack them. As Darrell says, this depends largely on the shape of the container, from I've seen in the last ten years or so. I've learned to stay away from pots with an overhanging incurve at the top. In other words, a pot that recurves in on itself at the lip over the soil. That incurve prevents the soil from expanding upwards when it freezes in winter storage. That will crack the pot pretty easily.

Joanie

The glaze popping off is called "shivering". And the opposite, when a glaze is too small for the pot, and gets cracks, that's called "crazing".

Another problem that I've seen in an older Japanese pot is the surface skin popping off. Let's see if I can explain it... it's a pot in the San Diego Bonsai Pavilion collection, and it was made immediately after the war I believe. The inside is very rough, full of grog (small, fired particles that are mixed into raw clay to buffer expansion and warping) But the outer skin is very smooth. The way is was formed is that the potter hand-built the pot in the cheaper, groggy clay, and then covered the outside with a thin skin of better clay. You could call this outer clay an "engobe". It made the pot look more expensive than it was.

Over the years, this thin outer skin has popped off in places. When I pointed it out to Steve, the curator, he showed me another pot in the collection that was doing the same thing. If you have "chips" in odd places... not on a rim or corner, say.... it may be this problem. Particularly if the clay inside the pot doesn't seem to match the clay on the outside of the pot.

Just thought that was interesting.

Joanie

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Joanie

My understanding is that the glaze and clay need to "fit" from the point at which the glaze begins to solidify, on the down cycle of the firing, all the way to room temperature. When the glost (glaze) firing is going on, remember that the glaze will be like a very thick liquid, sort of like honey. Some glazes are more liquid than others, and they run and drip. Some glazes are much drier, and they don't move at all. Anyway, there comes a point in the cooling cycle where the honey-like glaze begins to solidify. As the glaze solidifies, both it and the clay body are shrinking, because they are cooling. From the point at which both are becoming solid and cooling, they both need to "fit". When they reach room temperature, they should be in harmony, neither too big nor too small for each other. If you have this balance, then the pot will never be under stress, because they will never achieve such high temps again.

If you are mainly going to use oxides and such, you will miss out on one of the most exciting part of firing, Darrell! Glaze formulation and glaze firing is the pulse-racing part of making pots. You never really know what you are going to get! It's the wabi-sabi, it's the random factor, the unreproducible result, that leads potters to the edge of joy and madness. I bet that when you get the other parts of ceramics under control, you will be doing glazing, just because that's the yellow brick road for all potters.

Joanie

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darrellw

Oh, I'm sure I'll play around with glazes, just not so much on bonsai pots.

But maybe I just need more decidious trees....

-Darrell