Lately I have been seeing quite a few videos on YouTube about molten salt, specifically table salt (NaCl). One of the things that interests me about it is that regardless of what melting technique or type of crucible, the salt always ends up black afterwards.

Is the issue that the salt is highly reactive and is pulling chemicals out of the crucible regardless of what the crucible is made of? Wouldn't that mean that different crucibles would impart different colorations on the end result? That is the speculation I've seen on the comments of many videos (for example), but nothing definitive.

Or is it perhaps an optical change due to the change of the crystal structure? Or maybe something in the salt itself oxidizing or reacting with something in the air?

  • $\begingroup$ It may not be black. It may only look that way by reflecting/refracting light away from the camera. $\endgroup$ Jun 17 '18 at 12:05
  • $\begingroup$ @OscarLanzi That's part of what I was asking regarding possible optical changes. Large blocks of naturally-occurring NaCl are translucent white but it also generally doesn't form via melting in nature. $\endgroup$
    – fluffy
    Jun 17 '18 at 20:29
  • $\begingroup$ referencing a few of these videos would be helpful. $\endgroup$
    – A.K.
    Jun 19 '18 at 3:43
  • $\begingroup$ @A.K. I had linked to one of them; unfortunately most of them don't show a good view of the result or are built as clickbait or are using very dirty crucibles or are using "light salt" or the like. $\endgroup$
    – fluffy
    Jun 20 '18 at 4:21

Part of the color change might be due to creation of F-centers, crystal defects where there are electrons filling the place of an atom. A corresponding H-center (for halogen interstitial) may also be created for each F-center.

That said, such color changes tend to be rather pale, and as you state, $\ce{NaCl}$ heated to melting in an iron crucible may appear dark or even black. This is likely due to dissolution and oxidation of the crucible. The molten salt attacks the metal at grain boundaries and also dissolves protective oxide coatings, allowing oxygen from the air to attack the underlying metal. Even in a closed system, such as a molten (mixed) salt thermal energy storage system, corrosion is a problem.

The color change could also be due to trace impurities in the "table salt". For example, sodium or potassium ferrocyanide, $\ce{Na4[Fe(CN)6]·10H2O}$, is intentionally added to keep the salt free-flowing despite humidity changes. This might react at high temperatures to form a deeply-colored compound.

To find out what is causing the discoloration, you might try heating a small sample of high purity $\ce{NaCl}$ in a ceramic or platinum (a bit pricey) crucible, and you could analyze heated table salt for dissolved iron compounds.

BTW, F-centers and other crystal defects can be created intentionally with optical lasers and x-rays. If memory serves, there was an article in Scientific American's Amateur Scientist about creating F-centers using an electrostatic generator.

  • $\begingroup$ Quite logical , solid state in actikn $\endgroup$ Jun 18 '18 at 13:23

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