24 February, 2010

A tiny tale of tiny magnets - update with answers

The Slapdash Sewist asked how I know not to iron a magnet. Gold medal to those of you who guessed, correctly, that I learned not to precisely by doing it. It's a cute, or maybe merely salutory, story of the huge disconnect we sometimes experience between theory and real life. Way back when, I taught 2nd year electricity & magnetism while postdoc'ing, so I do know some theory, and later, in a grownup job, lectured on paleomagnetism and archaeomagnetism, including all the stuff on how rocks, ceramics, and sediments become magnetized, and all the lab tricks of measuring remanent magnetization, Curie temperatures, and so on. You'd think this large chunk of highfalutin' knowledge buried inside my head would give me pause when playing with magnets and heat. Alas: not so.

A couple of years ago I attended a workplace gala - a great and very formal black tie event, so of course made myself a silk gown for the do, and, equally of course, had a great time. Since a regular purse would've clashed, I made myself the simplest possible envelope clutch out of the dress fabric - for the bare essentials: a handkerchief, driver's licence, you know the drill. Nothing could be simpler. (click on the pic below to get an enlargement)

As it was to be a one-time item destined for a subdued-light environment, I didn't worry too much about making it fancy or any fancy closures, but ran over to Lee Valley, bought a 3/8" rare earth magnet (page 130 of their online hardware catalogue), sewed it and a flat iron washer onto opposing sides of the inside flap, and the job was done. Till I gave the finished item that "one last pressing". The magnet went dead on me, the theory-powered lightbulb snapped on in my head, and I had to run back for another 38 cent (sheesh! the high cost of sewing!) replacement.

The opened pic shows the large flat iron disk sewed onto the inside of the flap, and the small magnet sewed onto the envelope. The little magnet sitting on the clutch is 1/4", one of four I use to hold sheet music down onto a standard nickel-plated iron music stand, when the inclination to play outside in the wind strikes me. Yes, placed directly metal on metal these tiny babies are fiercely strong; but when separated by a couple of layers of cloth, their hold is surprisingly soft.

In the case of a jacket - daytime, possibly frequent wear - I'll mount the magnets differently, so they don't show on the outside or the facing. And I know better now than to - ever - iron them.

************************ ETA 27/2:

To answer your questions, Gwen and Kathi: magnets are affected by heat. Without getting unduly technical, the Curie temperature is like a magnet's melting point. Just like water molecules above water's melting point are free to move at will (turning ice into water), so above the Curie temperature magnetic spins are free to rotate at random.

This property is used to magnetize rare earth magnets in manufacturing. They're made by sintering rare earth alloy powders, sometimes in a plastic (non-magnetic) binder, in a strong artificial magnetic field. Then they're cooled from above their Curie temperature while they're still being held in this strong magnetic field. This aligns all the spins to one desired direction and freezes them that way permanently: turning the material into a permanent, very strong magnet.

But when we place a hot iron on a magnet we "melt" the magnetization again, the spins become free to move willy nilly, and, since the Earth's magnetic field is extremely weak, when the magnet cools back to RT, its spins "freeze" in random directions - no more magnet, just a chunk of rare earth material!

Cold water washing and dry cleaning won't affect a magnet. They're usually nickel plated for corrosion resistance, but can be physically brittle, so might get chipped if bounced around in a clothes dryer. So that's two good reasons not to machine dry them: the heat is likely to demagnetize them, while the bouncing may crack or break them.

Bottom line? cold water wash, but air dry. It should be OK to iron the item, but not too close to magnetic closures, and never directly on them.



  1. Goodness, that is too funny! Thanks for the story.

  2. So, it de- magnitized. Humm. Are magnets then affected by washing and dry-cleaning?

  3. Great story - love the way you tell it!

  4. I didn't know that. I have told people they can wash and dry purses with magnets, am I wrong? Goodness.

  5. That's a really fascinating explanation. I can't get my head around why you don't see the magnets "moving" though, as you can see water moving when it's not ice anymore. For some reason I never "connected" with atoms and science in school. Why that is, and why people are inclined to understand some things better than others, that would be interesting to know.