Treatments of Rubies
The treatments of rubies to improve their look and feel has been a common practice down the ages. The level and type of treatment however has differed considerably and this has impacted upon the price of the gemstone accordingly. Some treatments occur in almost all cases and are (therefore) considered “acceptable” practices. As with most gemstones, the most accepted and most occurring treatment is heat treatment. Although heat treatment affects the price of a top quality ruby, most if not all rubies at the lower end of the market are heat treated. Heat treatment is performed on the rough stones to improve color, remove purple tingle, blue patches and silk. These heat treatments typically occur around temperatures of 1800°C (3272ºF). Some rubies undergo a process of low tube heat, when the stone is heated over charcoal of a temperature of about 1300ºC (2372ºF) for 20 to 30 minutes. The silk is only partially broken as the color is improved.
A less acceptable treatment, but one which has gained notoriety in recent years, is “Lead Glass Filling” of Rubies. By filling the fractures inside the ruby with so-called “lead glass” the transparency of the stone is dramatically improved making previously unsuited rubies now fit for applications in jewelry. The process is typically split into 4 steps:
• 1) Rough stones are pre-polished to eradicate all surface impurities that may affect the process
• 2) Rough is cleaned with Hydrogen Fluoride
• 3) First heating process whereby no fillers are added. The heating process eradicates impurities inside the fractures. Although this can be done at temperatures up to 1400°C (2552ºF) it most likely occurs at a temperature of around 900°C (1636ºF) since the Rutile silk is still intact
• 4) Second heating process in an electrical oven with different chemical additives. Different solutions and mixes have shown to be successful, however mostly lead-containing glass-powder is used at present. The Ruby is dipped into oil, then covered with powder, embedded on a tile and placed in the over where it is heated at around 900°C (1636ºF) for one hour in an oxidizing atmosphere. The Orange colored powder transforms upon heating into a transparent to yellow-colored paste, which fills all fractures. After cooling the color of the paste is fully transparent, that dramatically improves the overall transparency of the Ruby.
In case a color needs to be added, the glass powder can be “enhanced” with Copper or other Metal Oxides as well as elements such as Sodium, Calcium, Kalium etc.
The Second heating process can be repeated three to four times consecutively, even applying different mixtures.
You do need to know then if the ruby you are buying has been treated and by what method. As some are quite unsuitable for jewelry.