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I recently bought a couple of 61-mix toroids, and as usual I wrote the mix number on them before they become mystery parts. However, the ink soaked right into the ferrite.

I've left ferrites hanging out in the weather, getting wet and frozen without issue, but never mix 61. Given their porosity, I'm concerned wet, freezing weather may fracture them. Anyone have experience to share on that point?

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  • $\begingroup$ I've used 31 and 73 material outdoors with no issues. But if I ever use a different mix, l'll freeze test it. Probably soak it in water for a day or so, then throw it in the freezer. Also, it might be interesting to measure the loss, etc. of a ferrite core transformer, both dry and soaked. $\endgroup$ – Mike Waters Feb 12 '17 at 15:53
  • $\begingroup$ My experience from breaking ferrites is that they're hard ceramics. The surface is unglazed but the centre is like glass. They might still absorb some water though. If the hole is full of water and they freeze, that could certainly break them. $\endgroup$ – tomnexus Feb 14 '17 at 2:24
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I've had a couple turn up cracked when left outside in an enclosure that wasn't waterproof, but I can't say if that was freezing that did it, or just thermal cycles on a compromised toroid causing stress.

These days I hit pretty much all of them with a light coat of decent spray paint, or even a light coat of flex-seal type sealant before putting them outside. I've never had a painted toroid break.

A quick test would be to weigh a dry toroid on a digital kitchen scale with 1 gram resolution or better, let it sit in a dish of water for a minute, dry it with a towel then weigh it again. If it's gained a measurable amount of weight, it probably pulled up enough water to exert some real force.

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    $\begingroup$ Were they 61 mix? $\endgroup$ – Mike Waters Feb 12 '17 at 15:56

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