| Lead-Glass Filled 'Pao Mai' Rubies | | | | In the latter part of 2004, rubies began to appear on the market in quantity, after being treated by a process known as pao mai or new-bake* in English. The amount of filling used can vary from small traces to significant amounts. The rubies also display distinctive features and are in fact relatively easy to separate from other types of rubies available on the market. | |  | | A parcel of pao mai rubies. | | |  | | Residues which have seeped into surface reaching fissures. | | | | The treatment process involves heating the stones in the presence of lead and silicon oxides with fluxes at relatively low temperatures (~900°c-1400°c). During the treatment the oxides melt and seep into the surface reaching fissures and fill or 'repair' them with a residue. The difference in the refractive index of the residue and the surrounding corundum means the residue can sometimes be seen where it breaks the surface of the stone, occasionally even with the naked eye. The easiest way to observe the residue at the stone's surface, is to view the stone in reflected light or with fibre optic light. Under these conditions the duller lustre of the residue can be clearly visible (see image below). | | Examination with a loupe or microscope will reveal more. The filling resembles fillers I have seen in other types of rubies. Due to the difference in the R.I., the fillers or residues stand out in high relief. The filling can often show interference colours when examined in dark field illumination but clearer colours can be seen using a fibre optic light (see image below). Gas bubbles are also sometimes observed in these stones. Due to the relatively low temperatures used in the pao mai treatment, some inclusions normally affected by higher temperature treatments remain undisturbed. Sometimes it is possible to see un-dissolved rutile silk inclusions in these stones, which could lead the observer to miss-identify the stones as un-treated. | |  | | The surface of a pao mai ruby clearly showing filled fissures. | | | | |  | | Interference colours often seen in 'pao mai' rubies. | | | | During a buying trip in December 2004 I took the opportunity to examine as many of these stones as possible. Several things were immediately apparent including the fact that they were not being offered as anything other than pao mai. There was no cover-up or fraud as some people were suggesting. The treatment is being performed on relatively low grade Madagascan rubies which are lightly to heavily included. Without this process much of the material would only be suitable for industrial purposes - 'sand paper material'. | | | The rubies are selling at prices starting at tens of dollars per carat with large 5 carat plus stones selling at a few hundred dollars. Due to their relatively low cost, the best use for this material would seem to be in mass-produced silver or gold jewellery. There is a place for these stones in the market and as with all treated gems, they should be sold with full disclosure of treatment. | | | | * For the pedants amongst us; the Thai word 'pao' in pao mai is translated to 'bake' in English. Some refer to this treatment as 'new burn' but this would translate into 'mai mai'. The word mai has many different meanings in Thai depending on the way the word is pronounced. The words for new & burn are both mai & mai. Which then brings you to the Thai translation of 'Does new wood burn?' - Mai mai mai mai? If you were to reply 'New wood does not burn.', your reply would be Mai mai mai mai. All these mais being pronounced differently. | | | | |
