Mong Hsu Flux Remnant — The Glassy Trace of Borax Treatment
Mong Hsu Flux Remnant — The Glassy Trace of Borax Treatment
Residual flux glass trapped in healed fractures of high-temperature treated Burmese ruby
Mong Hsu flux remnants are residues of borax flux trapped in healed fractures and surface-reaching cavities of heat-treated Mong Hsu rubies. The flux remnants are diagnostic features of the standard high-temperature treatment regimen applied to Mong Hsu material since the deposit's discovery in 1992: heating to 1,500-1,800 degrees Celsius in the presence of borax (sodium tetraborate) flux produces dramatic improvements in colour and apparent clarity, but leaves characteristic glassy residues in the healed fractures that gemmological examination can identify under magnification.
How flux remnants form
The standard Mong Hsu treatment heats rough or partly cut ruby to temperatures above the melting point of borax (approximately 743 degrees Celsius) and well below the melting point of corundum itself (2,054 degrees Celsius). At treatment temperatures, the borax flux melts and flows into the surface-reaching fissures and fractures present in the rough, with capillary action drawing the molten flux deep into narrow cracks. As the assembly cools, the borax solidifies as glass — amorphous, with the refractive index and other optical properties of borate glass — within the fissure structure of the host ruby.
The post-treatment effect on the fissures is what the trade calls flux healing: the appearance of the original cracks is greatly reduced, since the glassy fill provides a continuous optical path through the fracture rather than the disruptive air-filled void of the untreated stone. Apparent clarity improves dramatically, and stones that would have been commercially marginal in their untreated state become attractive and saleable.
Identifying flux remnants under magnification
Flux remnants in heated Mong Hsu ruby appear under microscope magnification as glassy, high-relief residues in the formerly fractured zones. The residues may be colourless to slightly cloudy, sometimes with visible flow lines, gas bubbles, or refractive-index differences relative to the host corundum that produce visible boundary effects. The flow patterns within the fissures often retain the original crack geometry but with the smooth, glassy character of solidified flux rather than the angular, planar character of unhealed fractures.
The Gubelin Photoatlas of Inclusions, the Lotus Gemology online inclusion library, and other major gemmological reference sources document Mong Hsu flux remnants with comprehensive photomicrographs covering the range of presentations. Together with the relict blue cores and the characteristic dissolution patterns of original silk, the flux remnants form a diagnostic suite that identifies Mong Hsu treatment with high confidence.
Treatment disclosure and value implications
Flux-treated Mong Hsu ruby must be disclosed under AGTA, CIBJO, and other coloured-stone trade-association codes of practice. Laboratory reports describe the treatment in standardised language, typically referring to heat treatment with residues or flux healing, with detailed observations of the specific flux remnants visible in the stone.
The treatment is widely accepted in the trade as a routine processing step for Mong Hsu material, and treated stones support the global commercial ruby trade at consumer-accessible price points. Untreated Mong Hsu rubies — rare in production and difficult to find in fine quality — command premiums for the absence of treatment, but the practical reality of the market is that almost all Mong Hsu rubies in circulation are heated. See also: Mong Hsu ruby; Mong Hsu blue colour core; flux healing.