Glass furnaces

The glassmaking process takes place in the melting furnace, where the vitrifiable mix is obtained by intimately blending the pre-measured raw materials and adding a minimum of 10% crushed glass, known as cullet or groisil. This addition of crushed glass is essential to obtain correct glass quality, and can reach 90% with the increasing use of recycled glass. Glassmakers call this vitrifiable mixture "composition".

• The process takes place at temperatures between 1,200 and 1,600°C and essentially comprises four phases:

  • melting of the vitrifiable mixture, which changes from a powder to a liquid state;

  • refining, which eliminates the gaseous compounds resulting from the chemical reactions produced during melting;

  • chemical and physical homogenization of the glass mass ;

  • conditioning, which lowers the glassy mass to a temperature compatible with its subsequent use, generally around 1,000 to 1,300°C.

  In most cases, the conditioning process ends in channels made of refractory materials connecting the furnace to each of the forming machines. These channels, equipped with heating and cooling devices, are called "feeders" by glassmakers.

  • These different phases of the development process can be carried out :

    • or, simultaneously, in different parts of the furnace, in the case of continuous basin furnaces;

    • or, successively, in the case of crucible furnaces (pot furnaces) or "day tank" furnaces.

  • In all these furnaces, the distribution of heating is of great importance, as the glass currents generated by thermal gradients in the molten glass play a vital role in the quality of refining and homogenization.

• Depending on the heating mode, melting furnaces can be classified into two main categories:

  • flame furnaces using fossil fuels, in which thermal energy is transmitted to the glass by heat exchange between the flames and the surface of the glass bath;

  • electric furnaces, in which thermal energy is generated by the Joule effect in the molten glass.

  Electric auxiliary heating is often installed on flame furnaces, either to temporarily increase furnace output, or to improve glass quality.