Using this method, it is necessary to observe some of the rules for heating the stones, otherwise they may break or change colour. For that reason, we recommend to avoid any drastic changes of temperature as they might cause a thermal shock. If cooling is necessary, it is advisable not to exceed the maximal cooling rate of 5 °C per minute. If the stones are not mechanically stressed during heating, even the temperature of 1100 °C does not present any risk. However, account should be taken of the stone’s colour and its heat resistance because some colour shades alter if exposed to increased temperatures. The Preciosa stone‘s heat resistance is tested at the temperature of 600 °C during 2 hours, and using a programme simulating the “lost wax” process‘s temperature regime (See Chart 1)
There are various methods of measuring the thermal-shock resistance. At Preciosa we use a method of sudden cooling by a given temperature difference resulting in an S-shaped damaged stones – thermal shock relation curve. One of the characteristic parameters of the curve is the thermal shock (temperature difference) causing damage to half of the stones. To illustrate the dependence on the stones size, the following table gives two typical size values.
|Size||∆T 50% [°C]|
Apart from thermal shocks, it is necessary to take account of possible mechanical damage owing to excessive pressure applied primarily to the edges of the stones, or resulting from impacts of hard objects.(See Chart 2)
While soldering metal parts together, you should prevent the stones from contacting the soldering agents (flux) used at an increased temperature, such as borax for instance (sodium tetraborate Na2B4O7.10H2O) as they etch the stone’s surface (cubic zirkonia melts in them).