|Corundum (sapphire and ruby) treatment||
I'd like to start off with this article written by Richard Hughes on the treatment of rubies. It was written in 1997 but was too shocking to some groups to print. This article was later revised in 2002 to remove the term glass filling because this treatment is mainly for the purpose of fracture healing, not fracture filling.
Another slightly updated article by Richard Hughes
Flux healing involves heating corundums with borax or other fluxes. These fluxes actually dissolve the surfaces, including the internal surfaces of cracks. The corundum within this molten material then re-deposits on the fracture surfaces, filling and healing the fractures shut. Undigested material cools into pockets of flux glass. Essentially this amounts to a microscopic deposition of synthetic ruby to heal the cracks closed.
In the broadest sense, this is akin to the oiling of emerald – both treatments involve reduction of reflections from included cracks/fissures. Similar to placing an ice cube in water, a filled fracture is much less visible because the filler replaces air (RI = 1.00) with a substance that has an RI that more closely matches the gem itself (1.76–1.77). However, the flux healing of Mong Hsu rubies differs in three important respects:
•The Mong Hsu ruby treatment is NOT a fracture filling, but a permanent healing of the fractures and fissures, with any filling merely a remnant of the process. In many respects, it is a welding of fractures, similar to the joining of two pieces of metal with heat and a flux to lower their melting point.
•The Mong Hsu ruby treatment is permanent and irreversible. Unlike the oil in an oiled emerald, flux remnants will not drain out in the future, nor can they be removed. There is no way to have a stone revert back to the untreated state.
•The Mong Hsu ruby treatment actually improves a stone’s durability, since the fractures are permanently healed shut.
A 2010 research paper submitted by GIA which applies to all rubies, not just from Mozambique
There are three types of flux treatment listed and it is important to understand their differences:
FAPFH – Flux Assisted Partial Fissure Healing
GFF – Glass Fissure Filled
LGFF – Lead Glass Fissure Filled
While FAPFH, LGFF and GFF stones all result in stones that have been “clarity enhanced”, the essential difference between the three types of treatment is that the fissures in FAPFH treated material are “healed” closed by synthesis while fissures in either LGFF or GFF are not, i.e., FAPFH fissures are stable (once any surface glass has been removed by HF) whereas LGFF and GFF treated fractures unstable (they will break down and become obviously visible). In severe cases of LGFF treated rubies the LMHC laboratories developed severely worded
descriptions; these are “ruby with glass” and “ruby‐glass composite”.
How is this done, you say?
The ruby is heated to temperatures that may reach as high as 1850°C in the presence of a molten flux. At these temperatures the surfaces of the ruby (alumina) in contact with the flux will slowly dissolve into the flux. During the gradual cool down period the alumina comes out of the flux and crystallizes on the nearest available surfaces. These surfaces may be those of the crucible in which the process is taking place or on the rubies being treated. If crystallization takes place on the rubies, this may be on the outer surfaces (facets) of the stones or on the inner surfaces of any fissures that may be present. If the crystallization is on the inside of fissures, the build‐up of synthetic material will cause the fissures to gradually close or “partially heal” (Figure 2). These partially healed fissures have the appearance of intricate networks of fine tubules contained in undulating planes within the stone. The material that can be observed within these planes is mostly composed of a glass; the transparent spaces within the planes are where the fissures have been partially healed with synthetic corundum.