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Summary of Leadglass Filled Corundum Summary of Leadglass Filled Corundum - Last Update : 12/19/2013 2:59:43 PM - Views : 1007

Glass filled corundum has been around the market for a long time now. Cheap and unusable corundum with cracks are filled with lead glass to improve its color and clarity. It is basically a clarity enhancement treatment where a gemstone is treated to appear marketable and more beautiful. In a way it is a very good thing as it provides the market with affordable beautiful stones, which actually makes it possible for a common man to obtain a beautiful Ruby at a reasonable price. Treatment in itself is not deceit, as long as the costumer is provided with the correct information about the stone and the price is related to the real quality of the stone. So the matter here to be considered is its disclosure and the durability concern it pose.

The idea was to develop a glass compound suitable to repair fissures in rubies. In fact it is not a new treatment as similar treatment was applied with diamond. Yehuda diamonds which were clarity enhanced by filling it with high RI glass gave similar results. The efficiency of the treatment is such that a single large fracture in an otherwise clean ruby could be made to disappear to the unaided eye exactly as filled fractures in diamond and emeralds. However it is more stable than emerald treatment where only oil is impregnated. Glass filler may be unstable at elevated temperatures and to chemical agent, so special care should be taken when repairing jewelry items set with glass filled corundum. So, when such a stone is sold a proper disclosure should be made to avoid this problem.

The process how this treatment is done follow few steps :

First step: The stones are preformed to eliminate the matrix and other impurities that could hinder the treatment process.

Second step: The stone is then warmed to remove the impurities possibly present in the fissures that could obstruct when the glass is added. The warming could be conducted between 900-1400 degree Celsius depending upon the ruby type.

Third step: The stones are then mixed with oxide powder such as mixture of silica and lead, but other chemicals like sodium, potassium, calcium and metallic oxides like vanadium or bismuth can also enter in some glass composition. (The heating temperature also changes as composition of the glass changes). The mixed powder are added to the stones along with some oil so that it is covered fully and placed in the crucibles, which are then brought to the furnace. The heat fuses the powder turning it into liquid glass which then enters the fissure to cover and conceal it.

The stones can be heated several times to achieve the desired result. Sometimes the resulting stones are surface treated using several chemicals to improve their surface luster.

Identification Concern:
Although advance instruments like ED-XRF can positively conclude presence of lead or other chemicals, microscopic observation of an experienced gemologist will be able to identify the repaired ruby without much difficulty.

The fact that this treatment requires heat below 1400 degree Celsius makes the stone appear like natural unheated stone as many of the inclusions are unaffected by this temperature of heat, so it could pass off as natural unheated stone if further investigation is not carried out. But using dark field illumination, most Lead glass filled fissures will display blue/orange flashes owing to slight difference in the Refractive index of the stone and the filler material. In immersion method the flash effect is more easy to observe.

Careful observation might also reveal gas bubbles or platelets (remnants of former iron stains), that are formed during the filling process. Inclusions of flattened gas bubbles are also noticeable. Surface luster difference is prominent when seen with an overhead lighting. This difference in luster is caused by the difference in hardness the two material. Although adding lead to glass compound increases its Refractive Index, at the same time it makes it more soft or decreases its hardness, hence making it more prone to scratching and abrasion.


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