mold question

[slats]

I bought a ceramic dish I really liked because of the unique shape. Can I sandblast the costing, kiln wash and slump?

[Brad Walker]

Yes.

[slats]

right to the point thanks

[Gail Jensen]

Everytime that I have done this, the mold cracks on the first firing. Perhaps I'm doing something wrong, but I would try something inexpensive first.

[rosanna gusler]

quartz inversion again. rosanna

[Tom White]

Gail, If you sandblast the glaze from only the top surface of a item to use it as a slump mold and leave it on the bottom surface you set up an unbalanced condition. In a properly finished glazed ceramic item the glaze on the surface is in tension with relation to the clay body of the item because the glass in the glaze shrinks more than the clay body when they both cool after firing. The thin layer of glass melted over the clay surface is nearly matched to the coe of the clay but is still in tension. The still glazed bottom surface of the slump mold has the tension of the glaze pulling on it and as the item heats it is strong enough to pull the clay apart causing the crack that can split the mold completely. Also, when slumping we tend to ramp up our kilns somewhat faster than the suggested rate for firing clay items. This puts additional stresses on the mold and the combination of the fast rate of heating and unbalanced tension from the glazed side of the mold make it very subject to cracking or outright splitting. If you find a glazed item you simply 'must' have as a slump mold I would suggest you heat it up much slower than you would bisque or ss molds, especially through the quartz inversion temp.

Best wishes,
Tom in Texas

[Greg G]

what is the quartz inversion temperature and when does it happen? Are there two temperature points/zones?
thanks, Greg

[rosanna gusler]

1064f. google quartz inversion. it is really interesting. i learned alot about clay. explained some possible reasons why be molds behave well in the kiln as compared to just plain bisque ware. rosanna

[Leslie Ihde]

Please, please, please nobody yell at me for raising this issue, but you say the piece is a "unique shape." Some potter might have spent a lot of time developing that shape and may have an issue with a glass artist using the shape. You might just want to ask the artist if he or she minds, or change the piece enough that it is no longer the same. They might take it as a compliment and be delighted to see their form continued, like a musical variation on a theme, but as a potter for whom form is the thing, I could see having some feelings about it. I know as a slumper, one tends to look around and see everything as a potential mold, but why not grab a piece a of clay and develop some of your own? You don't have to be a pottery to do it if you use slabs. I'm not trying to suggest there are any copy right issues here, I doubt there are, but it's something to consider.

Leslie (not trying to stir up trouble, just trying to explore an issue)

[slats]

YOU'RE KIDDING RIGHT LES! Maybe I should travel to China and look for the "artist"......PLZZZZZZZZZZZZZZZ for 1 $5.00 plate from Marshall's...Why not offer some more important input like what to do during this Quarts inversion.............slow down temps? or go faster?

[Brad Walker]

YOU'RE KIDDING RIGHT LES! Maybe I should travel to China and look for the "artist"......PLZZZZZZZZZZZZZZZ for 1 $5.00 plate from Marshall's...Why not offer some more important input like what to do during this Quarts inversion.............slow down temps? or go faster?

Actually, I think Leslie makes a very good point. Not all "found items" are free for the using. It's entirely possible that the piece could be copyright and resulting from someone else's hard work. Even a $5.00 plate from Marshall's (maybe especially a $5 plate from Marshall's) can be protected by law. You should always check out found pieces for copyright or trademark symbols before just assuming they're free to copy. If this topic interests you, then you might want to download and read this pdf file on design copyrights: http://www.copyright.gov/title17/chapter13.pdf

As for quartz inversion, the rule of thumb is that you slow down or soak during this period. During a slump, I usually make a long initial soak at this point; often that accomplishes the slump as well as protects the mold.