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Viking Potter

Why not underfire clay

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When my family took a class we were given a block of clay and each of us had some left over that we took home.  Since we were used to it, we bought more of the same and have continued to use it as we have progressed.  Now, as I learn about clay bodies, I find that this is a mid fire clay that is to be fired to Cone 6.  We have been using it as a low fire clay, with bisque to 04, and our low fire glazes at 06.  So my question is, why not continue to use this clay (or even others) at low fire temperatures?  We are having plenty of success, and satisfaction with our low fire efforts, and frankly, I doubt my old Scutt 231 would ramp up to Cone 6 without a whole bunch of new elements (they are ordered and on their way).  I have some High Fire clay that is reputed to be excellent for beginners and students, but I could never get my kiln to High Fire temps.  What happens if you don't heat the clay up to the recommended cone temp?

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At low fire temps, it doesn't matter if it's under fired. Low fire bodies don't vitrify either, so if your glazes are working fine on a cone 6 body, then go for it. The big issue is when you're under firing at higher temps. Mid range and above, glazes are more likely to craze than low fire glazes, and when that happens the pot will leak if the body isn't fired to maturity/vitrification.

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By firing ^6 clay only to bisque temperatures it is going to be open and very porous. Bisque temps are meant to leave the clay in this state so the glaze will adhere to the pot. If your glaze has any crazing or micro fissures whatsoever or the pot is unglazed on the bottom the pot will take water into the clay body itself. Doing the washing up or using the dishwasher will introduce water into the clay, now put that pot in the microwave and you’ll feel the pot itself getting very hot from the absorbed water in the clay. This might take a little while to happen but it will happen. Absorbed water in a pot will in turn cause crazing of the glaze in and can grow mold.  Added to this is ^6 bisque isn’t going to be as strong as ^6 clay fired to maturity. Yes, low fire clay is porous, with the exception of fritware, but the clay is fired to maturity, it’s not vitreous but it’s matured.

edit: if you have a piece of your current clay that is bisqued but not glazed try filling it with water and leaving it on a piece of newsprint for a couple days. This is a simple test for clay that is to be used for functional pots, the pot shouldn't leak even without glaze. Guessing the paper will be quite damp and wrinkled with this test with your ^6 clay fired to 04

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Cone 5-10 bodies are primarily fluxed with potassium/ sodium spars; which at cone 04 only fuse materials together. These body types are actually expanding, becoming more porous until they hit 2050F, when they begin the early process of vitrification. These body types have absorption rates in the double digits if only fired to 04-06. So they will fail over a period of time if used for functional purposes.  Plenty of threads on fired pieces absorbing water causing mold, foul tastes, and odors. 

Low fire bodies use  different fluxes; such as talc ( magnesium) or boron. These have lower melt temps and do increase density, although still not for functional use. The more applicable effect is changing the COE of the clay to be  more in line with low fire glazes.

Nerd

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Case in point: I did a series of lovely soap dishes I meant to sell at christmas, they were in a cone ^6 brown clay which I'd found to be fairly foolproof at the old shop I was taking it to and where I bought it. They fired everything to cone 6.

When I moved last fall, noticed odd things were happening to my ^6 glazes at the new shop. Because of this I decided to glaze and 'test' one of my soap dishes all the way through with my most dependable low fire underglaze (which always did ok up to ^6) but put a ^6 clear glaze over it. What I got back was -for about 20 min- a beautiful luscious red soap dish, but making suspicious pinging and popping noises as it proceeded to craze all over. Took it home still trying to figure out what was wrong, put it on the sink and put a wet bar of soap in it: as soon as the water hit it, the dish started fizzling like dry terra cotta, and glaze pieces started falling off. The dish was so messed up I tossed it rather than even save it for mosaic shards. Really glad I didn't do the rest of the soap dishes, I'm saving them for my new kiln. 

Later found out the shop is actually only firing to ^.06 (I've complained about this a lot already, I apologize). This may happen to you, beware!

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Most places that do Ceramics for public firing will fill their kilns with poured pottery. At least in the day when pouring/casting was big. Most of this was done with a ^06 slip that was fine. However, when someone brought something in to fire, they would just throw it in with the other pieces assuming that it was 06. 

One of the reasons when I taught HS that I did not allow anything in the shop that was not our clay. As we fired to ^6, not ^06,  I was not about to risk damage to kiln or shelves just to please someone. This policy usually caused some negative feed back, and name calling of me, but the equipment did not get damaged.

 

best,

Pres

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Ok, makes perfect sense.  We had a family meeting and have decided to step up our game and work at ^6.  Which resulted in a planned road trip to get new glaze from our favorite spot, Archie Bray.  We are doing both 06 ceramics and will now do ^6 pottery so I am setting up a second Kiln to keep things separated.  Thanks to all for the valuable education.

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Just a note or consider it a tip.I have seen it myself in many others situations .

When working with a low fire body and another higher temp body its a matter of time when the low temp body end up in the high temp kiln and it can be a big costly mess.Glazes running onto shelves and elements not to mention the deformed body slumping .

So take precautions as you can NEVER be TO careful in this situation.

I just want you to know you are playing with fire-no pun intended .

 

 

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Not sure If I can add a picture to a post, but I did post a picture in the gallery, we did our first run of cone 6 material and generally all was well.   only issue, we had some stilts of unknown origin, and they melted into the bottom of a couple of pots.  Had used them a few times for 06 work but never thought that they would not stand up to higher temp.  The wire went right into the base and the base became part of the pot.  New stilts were purchased the next day.  I can now claim to be a potter because I made pots and they hold water and do not leak.  Neil, I am listening and learning buddy.  : )

pottery success2.jpg

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@Viking Potter, I can't tell if you have clear glaze on the foot of your pot but if you don't have glaze there then you don't need to use stilts. It's uncommon to stilt pots for ^6 since the feet/bases are typically not glazed. Lowfire needs to be as sealed as possible with glaze therefore it's common to glaze the entire pot, with higher fire the clay should hold water even without glaze therefore no need to glaze the feet/base. (If your clay leaks at ^6 then it would be preferable to find a new clay rather than glaze the feet/base and stilt)

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