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Name: Bob
Status: student
Grade: 9-12
Location: MS
Country: USA
Date: Fall 2010


Question:
What color will any metal be (zinc, for example) radiate when it is heated to its melting point?



Replies:
Hi Bob -

Molten metals usually seem to have simple thermal-glow colors. Oxide crystals, glass, metal salts, and organic chemicals show much more variety and color-saturation.

Most liquid (melted) metals are silvery, which is high reflectivity and low emissivity, so they try not to radiate much of any color. But, usually the reflectivity is less than 90%, so the emissivity is >10%, so some glowing will happen. Impurities on the surface, such as oxidation or slag or carbon, by being dark instead of silvery, could enable additional radiation. And if the impurity added a color of its own, the glowing could pick up some color bias.

The metal can only glow in the visible range if it is hot enough for a black body to glow red, orange, yellow, white, or hotter. Look up black-body radiation and color temperature. Molten Aluminum at 660degC, for example, is just barely hot enough to glow deep dull red in a dark room. Low-melting metals such as tin, lead, indium, gallium, zinc, and others, melt at temperatures too low to glow in the visible spectrum. They will be glowing at some distance into the infrared, though. Even a warm human body glows at a far-infrared wavelength of 20 microns. Red light is 0.6 to 0.7 microns. So everything at temperatures between room temperature and 660degC will be glowing at some band of wavelengths between 1 and 20 microns.

A little hotter than aluminum are silver (960C), gold (1060C), and copper (1080C). While these are molten in a vacuum-evaporation chamber, you can look at these through rather dark sunglasses and appreciate the warm orange or yellow colors. And as they cool down, the colors get redder and dimmer as they should, but also seem slightly odd because the light that leaks out through their extremely silvery surface is a bit color-filtered in some way.

Then there is a good set of metals near iron on the periodic table, which seem to melt at about 1500degC. Iron, nickel, cobalt, and palladium all glow pretty brightly and almost whitely. In principle, for a color temperature only 1800K, their glow should be rather yellow, more yellow than an inefficient long-life tungsten-filament bulb. But it seems whiter than that when I saw it. Maybe their surface emissivity is lower in the red than in the green or blue, partially color-correcting their glow towards white. On the other hand, I have seen video-clips of foundries pouring molten iron, and they sometimes seem to glow yellowish if I remember correctly.

I have melted high-melting metals such as molybdenum, iridium, and rhenium. A little quarter-inch spot of one of these lights up the interior of the two-foot dull gray vacuum chamber so brightly that you hardly dare to look in through the window, even with extra dark glass filters. But the color seems simply whitish. It is a little like deciding what color the sun is. Most of the time it is just too bright to think of it as anything other than white.

Maybe someday someone will measure and publish the color temperature of various molten metals, as if they were lamps for illumination.

Jim Swenson


Your question does not have a simple answer because there is no single answer. Here is a (short) list of factors involved: 1. The temperature. Any object, regardless of and independent of its composition will have a "color" from dull red to blue-white that depends only its temperature. If you want to track this source down search the internet for "black body radiation" and look at the X-axis scaled in wavelength between 400 (blue) to 700 (red) nanometers, which is the wavelength range of visible light. Also you can see the "red" end by looking at the burners on an electric stove. 2. The color also depends upon reactions with atmospheric gases, especially oxygen and nitrogen, which are the major components of air. The oxides and / or nitrides that form are usually less dense than the metal and tend to float to the surface. Oxides and nitrides tend to be white to gray, but this is also piled on top of the black body radiation, and some oxides--iron for example, may be red to black, depending upon which oxide is present. These colors apply at room temperature. What happens at the melting point of iron is difficult to say. 3. Sodium is an impurity that is difficult to exclude, since it is a component in many materials, e.g. glass to mention just one. Sodium has intense yellow emissions at 569 & 590 nanometers (called the sodium D-lines). These can obscure any other visible emissions by other metals. Of course the presence of carbon can give the characteristic orange of a carbon flame.

As I said this is a short list. There are no doubt a number of others depending upon the metal, its purity, and the wide range of melting temperatures of metals.

Some conspiracy addicts have tried to invoke the "thermite reaction" in the 9/11 tragedy, and part of their claims has to do with the color of flames from the Twin Towers. However, those claims are not credible if the chemistry is examined carefully and impartially. They know the answer they want and will bend the data to fit their presumptions.

Vince Calder



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