Today I asked this question on Facebook in pottery heads and on the Curly Creatures Page, and it has been getting a fair amount of discussion in both spots:
“Has anyone ever tried firing a piece with a magnet *inside* it? I’m wondering if I could put a strong magnet (rare earth or ceramic) inside a void in a piece and seal it in (maybe with an air hole, maybe not.) Allowing some space around it should keep shrinkage rates from being an issue. I believe most/all magnets have a melt point below the cone 6 glazing temp, but wondering if it could be remagnetized (or would need to be) afterwards.
Thoughts?”
Since there’s so much interest, I thought I’d keep the general gist here for posterity. A list of thoughts:
- It looks like clay firing temps are higher than a magnet can handle intact. There’s a table of curie temps here: http://en.wikipedia.org/wiki/Curie_temperature and the highest temp is only 1043F. Cone 6 can hit 2300F, but if you low-temp fire you might get away with it.
- Changing to low fire process isn’t a great option for me, because the studio is set up for cone 07 bisque, cone 6 glaze. Would also require retesting clay bodies, new glazes, etc… for some, this could be an option that would allow using normal magnets and not having to magnetize.
- Naturally, any magnets embedded should have enough space around them to allow shrinkage of the clay body.
- Possibly magnetize after the fact with something like this: http://www.amazon.com/Wiha-40010-Magnetizer-or-Demagnetizer/dp/B00018AONE (although I’m guessing a powered tool would be better)
- How about adding iron filings to the clay body and magnetizing it when the piece is finished? This clearly wouldn’t have the strength of a rare-earth magnet, but could work. (Can you remagnetize a rare-earth or ceramic magnet?) Also, attention would need to be paid to shrinkage – the clay body with all the ferrite would shrink less, probably glaze differently, etc…
- Another possibility: after a first glaze firing, leave a hole large enough to accept ferric material filings – pour in, glaze fire again. This is an option, but would also really define the form of the final piece, since any holes for adding the metal would need to be able to face up during firing.
- A sheet (or pieces) of metal that magnets will stick to could be embedded instead of a magnet. A few considerations:
- This decreases the thickness of clay that can be between the pieces needing to stick together. Two magnets obviously have higher attraction at further distance than one and metal, and gauss goes as the square of distance. With fairly heavy pieces, this doesn’t give a lot of leeway.
- If the metal melts below 2150F, in order to not break the piece it would need to either: not absorb/penetrate an open clay body, or expand/shrink at the same rate as the clay.
- Raku firing: lower temp (1470F-1830F) that’s still beyond the Curie point of every magnet I’ve found so far, although on the lower end just barely past cobalt’s 1400F…
- Add magnets after the fact, and disguise the spot. Considerations:
- With no part of the structure over the magnets, the glue holding them in needs to be strong.
- You can’t glaze over the spot (even cone 012 is 1623F – more than 200F over cobalt), so maybe a small glazed plug, and glossy epoxy. That’d be hard to get right, given warping of the plug and epoxy’s trickiness…
- Epoxy or glue to level with the glazed clay around it, cleaned up so the seam is smooth, could get a layer of gold leaf and you wouldn’t know there was anything under it. However, I’m not sure how gold leaf would handle repeated impacts/abrasions from the opposite piece, so it would need to be protected more than usual. (Another layer of epoxy?)
I know I could just make the piece with voids and glue the magnets in, but I was hoping to hide the “how” for this work. So far I haven’t had a chance to try this, but I’ll come back with the results of any experiments, so Watch This Space.
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