Melting vs. Dissolving: A Question of Flux

A recent article by sapphire expert and geologist Richard Hughes of Pala International, Fallbrook, Calif., notes that in the heating of sapphires and rubies, flux is used to melt the sides of the fissures, which then helps heal the two sides back together. The article, titled “All Fluxed Up,” appeared in Gemworld International’s Market News . However, like most gemologists—JCK‘s gemstone editor included—Hughes used the word “melt” incorrectly.

“I think ‘All Fluxed Up’ is a good piece and is a subject that needs to be discussed,” writes John Emmett, noted sapphire expert at Crystal Chemistry in Brush Prairie, Ore. But Emmett notes that the article makes no clear distinction between melting and dissolving.

“Melting is a definite property of a crystal and occurs at a definite temperature. If you heat an ice crystal, the temperature increases until it reaches 0°C. Even if you continue to add heat, the crystal stays at 0°C and begins melting. As more heat is added, the temperature remains at 0°C until all of the ice is melted to water, at which point the temperature will again increase with heat addition. For corundum this process occurs not at 0°C but at 2,045°C. Below this temperature corundum does not melt.

“Table salt melts at 804°C. It works just like the ice example above. Below 804°C there is no melting of salt. However, salt can be rendered into a liquid form by dissolving it in water at much below its melting point. Salt dissolves in water at almost any temperature where water is a liquid, and slightly below.”

Emmett makes the following points regarding melting vs. dissolving:

  1. Crystals have a definite melting point and are solid below that point and liquid above it. The liquid has the same composition as the crystal.

  2. Crystals can become liquefied by dissolving them in a solvent. This can, in principle, occur at any temperature. The liquid formed is a mixture of the crystal (solute) and the solvent liquid and thus does not have the same composition as the solid.

“When the flux fills a fracture,” writes Emmett, “it dissolves the wall of the fracture until that small amount of liquid in the crack becomes saturated with sapphire solute. At that point it dissolves wall material at the same rate that it deposits fresh material on the wall. But note that we now have an efficient mechanism of material transport in the crack. The system now tries to minimize the surface free energy of the crack by transporting material from the wall to the sharp tip of the crack, healing it.”