The surface diffusion treatment was first developed at Union Carbide's Linde division in the US. Linde, the first company to synthesize star corundum, had problems obtaining both uniform color and uniform silk in the same stone. Here's the recipe for cooking sapphires the Linde way:
Pack stones in a crucible filled with the kind of chemicals which produce both rutile silk (Ti) and color (Ti, Fe, Cr, etc.).
Heat to near the melting point (1800–1900° C) for several days or even weeks. Periodically recharge the crucible with chemicals.
After cooling, lightly repolish, and voila – the gems now have color and/or stars.
How does it work? Like this: when a stone is heated to near the melting point, the crystal lattice is expanded to the maximum. Heat it too much and the bonds break completely. That's melting (and that's tough titty if it's your stone). But if you heat it to just below the melting point, where the bonds stretch but do not break, the gem will absorb the chemicals, creating color, asterism or both. However, atoms of Fe, Ti and Cr are fairly large and so cannot move easily into the stone. Thus the color and asterism are confined to a thin layer (0.10 to 0.50 mm) near the surface. This process was patented by Union Carbide in 1975–1977 and is termed surface diffusion.
Surface diffusion works best on stones which have little or no color of their own, as it makes little sense to add color to something that already has plenty. The starting material is generally near-colorless sapphire from Sri Lanka. Due to the shallow penetration of surface-diffused color, stones must be cut (but facets left unpolished). After treatment the surfaces will be pockmarked from the high temperatures. And so the stones are lightly repolished (with the emphasis on lightly), to leave intact as much of the color layer as possible. Too heavy a hand on the polishing wheel results in both loss of color and $$$$.
Both faceted and star sapphires of blue and orange colors treated in this way first appeared in the gem trade in the late 1970s and early 1980s. Initially the stones were produced by the Swiss company Golay Buchel, which bought the process from Union Carbide. Later, stones treated in Thailand and elsewhere appeared. But they soon disappeared when gemologists became familiar with their characteristics. And by the mid 1980s they were rare indeed.
Pack stones in a crucible filled with the kind of chemicals which produce both rutile silk (Ti) and color (Ti, Fe, Cr, etc.).
Heat to near the melting point (1800–1900° C) for several days or even weeks. Periodically recharge the crucible with chemicals.
After cooling, lightly repolish, and voila – the gems now have color and/or stars.
How does it work? Like this: when a stone is heated to near the melting point, the crystal lattice is expanded to the maximum. Heat it too much and the bonds break completely. That's melting (and that's tough titty if it's your stone). But if you heat it to just below the melting point, where the bonds stretch but do not break, the gem will absorb the chemicals, creating color, asterism or both. However, atoms of Fe, Ti and Cr are fairly large and so cannot move easily into the stone. Thus the color and asterism are confined to a thin layer (0.10 to 0.50 mm) near the surface. This process was patented by Union Carbide in 1975–1977 and is termed surface diffusion.
Surface diffusion works best on stones which have little or no color of their own, as it makes little sense to add color to something that already has plenty. The starting material is generally near-colorless sapphire from Sri Lanka. Due to the shallow penetration of surface-diffused color, stones must be cut (but facets left unpolished). After treatment the surfaces will be pockmarked from the high temperatures. And so the stones are lightly repolished (with the emphasis on lightly), to leave intact as much of the color layer as possible. Too heavy a hand on the polishing wheel results in both loss of color and $$$$.
Both faceted and star sapphires of blue and orange colors treated in this way first appeared in the gem trade in the late 1970s and early 1980s. Initially the stones were produced by the Swiss company Golay Buchel, which bought the process from Union Carbide. Later, stones treated in Thailand and elsewhere appeared. But they soon disappeared when gemologists became familiar with their characteristics. And by the mid 1980s they were rare indeed.
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