Red hot

Okay, here’s a question my father sent in to Popular Science a while back but never got an answer on.

It’s probably a stupid question, but it’s the only one I have. :slight_smile:

A metal heated long enough becomes “red hot.” Is “red hot” the same temperature for different metals/alloys? I mean, I can’t picture mercury ever getting red hot, but steel and iron often do–would steel and iron glow red at the same temperature or different ones?

Just curious. He never told me what, if any, response he got from those hoighty toighty bastards at PS. :smiley:

short answer: Nope
Longer answer: red is not the “hottest” color for heated metals, but let’s just say it is. The glow comes, basically, from electrons going from high energy levels to lower energy levels. Since energy is conserved, the energy must be radiated from the atom. Sometimes, it is radiated as light. The energy associated with red light is the same, regardless of the metal being heated, but the amount fo energy needed to excite electrons to the point where they will spontanneously emit light is not the same for different metals, due to different numbers of electrons and energy levels.

Yes, the temperatures are pretty much the same no matter what you’re looking at. The color of a hot object (i.e., its radiation spectrum) is given by the celebrated Planck radiation formula. (It’s celebrated because in its derivation (Oct. 1900), Planck made the bold decision to quantize energy. Some years later, he realized he had spawned quantum mechanics.)

The spectrum of emitted radiation depends only on the object’s temperature. (This is only strictly true is you’re dealing with an ideal black body. Chunks of metal are ideal enough, though, that you won’t know the difference without a little effort.)

The seeming contradiction between these two answers–HR says no, and Pasta says “yes”–may be resolved once you realize that there are degrees of “red hotness”. Blacksmiths learn by experience how red to let the piece of iron get before they start hammering on it. Going from cooler to hotter, it can be a dull glowing red, it can be a bright cherry red, it can be a bright orange-red. These all count as “red hot”, in blacksmithing terms.

And yes, different metals, like copper and tin, have different temperatures at which they become “red hot”, but again, there’s a range of “red hotness”.

Now, if you were scientifically inclined, like Pasta, you could go out to the forge with a special thermometer and measure the heat of various metals for “dull glowing red”, “bright cherry red”, and “bright orange-red”. And I’m sure that somewhere on the Web there’s a chart of these temperatures. Why don’t you go and have a look, because although it’s killing me to admit it, I can’t find it? :slight_smile:

Just to add some thought. I routinely heat molybdemum to about 800C. I measure the temperature using an “optical pyrometer” which I believe determines the wavelength(s) of the emitted light (orange/red in my case). Any metal however has a “emissivity” factor that is related to how reflective it is (silver is more reflective than zinc for example). I have to turn the emissivity dial to the correct number depending on what I’m heating. So, if the dial is too low my moly heater may read 870C but if it is too high the temp will read 790C. Basically, it does matter, but not a whole lot.

Just to clear up any further confussion Duck Duck Goose may have caused. The first answer was nope, they do not turn red at different temps. The second answer was yes, they do turn red at the same temp.

So they were both saying the same thing.

And now I feel like a fucking idiot because I misread the first four posts in this thread. Haven’t woken up yet completely, haven’t had my coffee yet. Ignore me.

Here ya go, Duck Duck Goose:
Metal Temperature by Color

Search for “incipient red heat” on google to find it. Don’t ask me how I knew that :wink:


Thanks guys.

I think I’ll be scratching my head over this one for a while. :slight_smile:

Just to clear up any further confusion, Pasta and Holden MacRoyne are both right, but answered different questions.

Holden MacRoyne’s point was that the amount of heat energy you must dump into a metal to cause it to glow to a certain wavelength depends on the metal- different metals rise to different temperatures with a constant amount of heat energy added.

Pasta’s point was that it doesn’t matter what the metal is- the same wavelength glow = same temperature.

So, the glow<->temperature relationship doesn’t depend on the material. The amount of heat energy needed to raise x grams of material to a certain temperature/glow starting from room temperature depends on the material.