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Copper is a very good heat conductor.

In the Mishkan the outside Mizbeach was made of copper, and had a fire burning on it continuously. (Actually it's a dispute if it had 3 or 4 fires burning on it daily.)

Certain types of Avoda (Temple services) required the Cohanim to step onto the actual Mizbeach.

For example:

  • Every morning they had to do Trumat Hadeshen - take a spoonful of ashes off the Mizbeach.

  • They frequently removed all the ashes.

  • They had to prepare new bonfires daily and then light them.

As we know, the Cohanim had to serve barefooted.

How did the Cohanim walk on the copper Mizbeach without getting their feet burnt? We don't find this mentioned as one of the miracles that happened in the Mikdash.

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  • If the Mizbeach was covered with eonugh copper to conduct heat shouldn't it also melt (or at least transform in some way which probably can be considered as Pgam)?
    – Zeev
    Mar 23 '15 at 9:59
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    Correct, copper is a good conductor of heat. That also means that it's a good dissipator of heat. And while we don't see them mentioning the feet of the kohanim as a nes, we DO see them talk about the miracle that the thin film of copper (ovei marde'a) and the wooden substrate it was sitting on didn't burn or melt at the extreme temperatures (IMHO, a much greater miracle). The large surface area of the mizbeach means that while yes, it would be rather warm on the top where the kohanim walked, the thickness of the conducting surface probably channeled enough of the heat to make it safe.
    – Isaac Kotlicky
    Mar 23 '15 at 10:06
  • Wasn't that regarding the gold?
    – Zeev
    Mar 23 '15 at 12:11
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    @IsaacKotlicky can you source the "ovei marde'a thickness"? All I have is מה מזבח הזהב שאין עליו אלא כעובי דינר זהב from the last Daf in Chagiga - where the source of the miracle of the wood not burning is mentioned (כמה שנים אין האור שולטת )
    – Danny Schoemann
    Mar 23 '15 at 14:08
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    Was the copper on top as well, or only around the wooden retaining wall? IIRC the mizbeach was filled with earth at each stop.
    – J. C. Salomon
    Mar 24 '15 at 3:06
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With thanks to Danny Schoemann who pointed me at a reference to what I’d remembered without a source.

Here is how the Torah describes the tabernacle altar (Shemot 27:8):

נְבוּב לֻחֹת תַּעֲשֶׂה אֹתוֹ; כַּאֲשֶׁר הֶרְאָה אֹתְךָ בָּהָר, כֵּן יַעֲשׂוּ׃
Hollow with planks shalt thou make it; as it hath been shown thee in the mount, so shall they make it.

As Seforno explains (ad loc.),

נבוב לוחות.‏ כמו תיבה בלי שולים ובלי מכסה׃ כאשר הראה אותך בהר.‏ שימלאו חללו באדמה בשעת חנייתם, ועל האדמה אש תמיד תוקד׃
“Hollow with planks,” like a box without bottom or top.
“As was shown to you on the mountain,” that they filled its hollow space with earth when they built it, and on the earth an eternal fire burned.

So the kohanim were not, after all, walking on a copper sheet that had a fire on it.

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