Thermal enhancement of some blue sapphires from madagascar

Abstract

Heat treatment is the most conventional method for gem enhancement. Color and clarity improved from this process are accepted by the international trading organizations as a general disclosure as long as there has been no color chromoproric causing element diffusing from an external source. Significant factors for the heat treatment are the maximum temperature reach, soaking time, heating rate, cooling rate and environmental condition. However, some intrinsic characteristics of those material stones (for example its characteristic trace element content which may appear as milky zone) are the most crucial concern for all burners. Some blue sapphires from llakaka-Sakaraha deposits, southwestern Madagascar were collected for this study; they were subsequently categorized into 7 color groups (e.g. dark blue, medium, very light blue, milky very light blue, dark violet, medium violet and light violet). Determination of physical and optical properties of raw sapphire samples were carried out using basic equipments and advanced instruments. Step-heating treatments under reducing condition using an electric furnace were carried out of different highest temperatures (1000℃, 1200℃, 1400 ℃ and 1650 ℃). Three hours soaking time were used during experiment. Characteristics of all samples were then examined again after each heating step. Results of this study clearly indicate that most sapphires can be intensified their blue colors at 1,400 ℃ ̶ 1,650 ℃; however, some sapphires turn colorless at high temperature. These appearances can be conclusively explained by Fe : Ti : Mg ratio contained in sapphire samples. In addition, sapphires with color-change effect are not recommended for heat treatment, because they appear to have lost this valuable phenomena. In conclusion, llakaka-Sakaraha sapphires have a potential for thermal enhancement; however, great care must be taken for particular cases.