Re-Firing ^10 Clay At ^6

[lincron]

 I have a number of bisque fired  ^10 clay pieces that I made during a class that used ^10 only.  At home, I work with ^5 - ^6 clay, and glazes.  Can I take my bisqued ^10 clay pieces, fire them in my kiln to ^ 9 or 10, (without glaze) and then glaze them with ^5 glaze and refire to ^5?  I know this is an odd question, and it seems to me it would work in theory, just wondering if anyone has any thoughts on this?  I no longer have access to ^10 glazes, and would rather not purchase some just for 6 things.

Thanks,

 

Linda

[bciskepottery]

Refiring your pieces to cone 9/10 will vitrify the clay, making it very difficult to apply glaze to as the bisque will no longer absorb water from the glaze. You may also have problems with glaze fit. Theoretically, yes it can be done; practically speaking, not a high certainty of success. Better off just glazing as they are and firing to cone 5/6 -- you may end up with some nice decorative wares but not functional as the clay will not be fully vitrified.

[lincron]

Hmmm, the pottery in question is functional ware, mostly mugs, so leakage would be an issue if the clay is not vitrified. BUT, if the mugs are glazed inside, would that keep them from leaking or seeping moisture, at least for an hour while I drink my coffee?  

[Chilly]

Hmmm, the pottery in question is functional ware, mostly mugs, so leakage would be an issue if the clay is not vitrified. BUT, if the mugs are glazed inside, would that keep them from leaking or seeping moisture, at least for an hour while I drink my coffee?  

 

Maybe, maybe not, the only way to know is to try.

[preeta]

lincron at a previous school we used ^10 clay and only fired to ^04. yup ^04 - both bisque and glaze. i made pinch pot and slab bowls and yes they should be good for an hour or so. ive eaten out of mine over a period of time and they were ok. the bowls were the right thickness of clay. not too thick or too thin.

 

why ^10 clay you might wonder. this was a class without a studio. just a kiln. proff himself fired ^10 gas so he knew the claybody very well. first semester students were not allowed to use any other clay. second semester kids could with permission. 

[lincron]

Thank you Preeta, I do believe I'll give it a try.  They certainly are not doing me any good sitting in a box in the garage.

[bciskepottery]

No harm in trying.

 

Just be aware that glazes eventually fail over time (craze, etc.) and you should never expect a glaze to compensate for non-vitrification. Your unglazed bottom will also absorb water from the dishwasher, etc. You may get some seepage if the beverage sits in the mug too long. You may also eventually get discoloration/mold from moisture in the bottom -- especially in high humidity areas.

 

Cone 6 to 10 is too wide a range when you fire at cone 6. Clay manufacturers should know better; so should studios teaching potters.

[glazenerd]

 

Bruce said --Cone 6 to 10 is too wide a range when you fire at cone 6.

 

Up to 20% less flux in cone 10 as compared to cone 6: not to mention the type of clay plays a role in porosity as well. The particle size of the clay can create even more of a porous body, when flux is lacking.The silica ratio in the body also adds to the problem: the more silica involved equates to that much more un-melted (free silica) due to the lack of flux: making for a weaker body. --which in turns creates COE issues during use.

 

When clay is fired to the prescribed cone: the flux in the clay body becomes fluid: which in turn creates the glassy matrix that results in vitrification. However, that same fluidity raises to the surface of the clay: creating an impermeable surface. This is the reason clay is fired in test bars unglazed: and then checked for absorption to determine vitrification.  A mature clay body will not absorb water regardless if glaze is applied or not. I can assure you from the extensive testing that I run on clay bodies: there are voids and micro-fissures in non vitrified bodies: rather you see them or not.

 

Part 2 : When the flux in the clay body raises to the surface during a correct cone firing: it bonds to the glassy matrix of a correct cone fired glaze. In essence, the fluidity in the clay interlocks with the fluidity of the glaze to form a completely vitrified piece. The lack of flux in the clay body, which also includes firing a cone 10 at cone 6; means that clay/glaze interface bond is also comprised. it will not appear compromised until it is subjected to thermal shock: then those beloved check cracks that all functional potters love begin to appear.

 

Just from my own testing: a cone 10 clay fired to cone 6 will have well over 10% absorption.

 

I would strongly advise you heed Bruce's warnings..  Use them to practice your glaze technique or testing colorant additions or even new glazes.

 

Nerd

 

You also run the risk of glaze shivering while enjoying your favorite green tea.

[lincron]

Thank you all for taking the time to reply to my post.  After reading all this, it seems that I should bite the bullet and pay for studio time and use ^10 glaze with a subsequent ^10 firing.

[porcelainbyAntoinette]

lincron:

 

Bone china, a type of porcelain clay is fired in a similar way you want to fire your work. Because bone china becomes very pyro plastic ( that is very soft because of the fluxes or melting agent in the clay), it slump easily at the highest firing temperature. For that reason the cups, plates etc is buried in silica ( or alumina) during the first firing which is the high fire ( we would do a lower firing bisque). The highly refractory silica prevents it from slumping. 

To glaze these pieces; that is the Noritake and other china ware that you can buy at high end stores, it must go back into the kiln with glaze that will melt and stay on at a much lower temperature -  maybe a bisque temperature.

The requirements for such a glaze is that it must 1. stick to the already vitrified ( watertight) clay  2. It must fit the clay body and not craze 3. It must be strong enough that it endure food acids, knife scratching and dishwasher abrasives. 

 

It is possible to do what you suggest, but it will take some testing on your part. 

 

Firstly you will have to fire a few pieces in the way you suggest- that is fire the clay body to its vitrification stage. Test and make sure the clay is as watertight as it can be ( I am not sure what is acceptable for stoneware - for porcelain it is 0 -.5%) 
 

Secondly you must test your glaze on the test pieces to see how the fit is. To do that you can change  a cup of your glaze into a brush- on glaze. - you can mix a brush- on additive for your glaze yourself, but the short path is to buy an already mixed up brush- on additive from your ceramic supplier and add that to your normal glaze. 

Now your clay becomes a paint-on glaze ( which in larger volumes, can also be used for dipping - avoid that option for now)

 

Once you have your glaze painted on the test pieces, you can fire to the temperature your glaze requires to melt and the result can be satisfying. Note I say CAN be satisfying. If you did the freezer test and the glaze did not craze, you can continue on this path.  If not; you have more tests to do. 

 

If this is a once off deal, I would say it will take your experience with clay and glaze chemistry to a next level, but if you are still in school and this is an ongoing situation that you have to deal with, I will advice you to keep the two separately from each other. Use the stuff at school in the way it is set up and set your studio at home up the way you would like to. 

 

All the best with your pottery. 

[preeta]

off topic...

 so should studios teaching potters.

 

i feel i need to explain things here. 

 

sadly using ^10 clay is a reflection of the state of ceramics in academia at a community college in a rural setting. the proff. is hanging on to dear life to not get his program cut off. he is hanging on to dear life to this one electric kiln he has there. if the kiln goes the whole ceramics program in this little outlet center goes. he had to go through a wringer to get his kiln 'serviced' once a year. he tells us and warns us about the vitrification of ^10 clay - so the information and knowledge is there. he is saving his kiln and his job (you'd be horrified the glazes we were allowed to use in the beginning class).

 

Linda : it all depends on how much you value your pieces. how much work did it take. i am not really attached to my pieces. i am a beginner and at the end of semester i really understand how bad my work it. not judgement, but reality. right now i am just starting to get consistent. now i no longer treat glazing as painting to cover my work and make it functional. my glazing nowadays is testing and deep exploration. if you now live in the world of ^5 why bother with ^10. 

 

this summer i threw with quite a few ^10 clay pieces. i wasnt deeply connected to the piece. it was more about throwing and how the clay felt. some visiting students were using ^10 clays (what they had) to return back to throwing and they generously shared some of their clay with me. i bisqued them 04 and put ^5 glazes on them (that we had in the studio) and they came out really nice. mind you my intent was not the firing and glazing aspect. 

 

you just have a few pieces. i would just buy a bottle or two of high fire glaze and ^10 fire them. seems cheaper than trying to go the studio time route (unless you can just rent a few hours as opposed to the month).