The melting furnace is the crucible in which the glass is made from the mixture of vitrifying materials.
In a pot furnace, four successive operations are carried out: melting (passage of the materials to the liquid state), refining (elimination of gases produced by chemical reaction during melting), homogenization of the mass, thermal conditioning (reduction of temperature to according to use). The temperature is generally 1200 to 1600° C, and 1000 to 1300° C for the last phase.
The process dates back to antiquity. The Glasi Hergiswil in Switzerland still uses this type of oven. After 5 to 7 years of use, the furnace needs to be shut down, cooled down and demolished. It takes two months to rebuild it by hand.
A tank furnace allows all the stages of the process to be carried out continuously. The glass mass flows into a long refractory corridor where the manufacturing steps are carried out. Raw materials are introduced at one end and molten glass is removed at the other. This is the furnace used in industry; mechanization allows high production rates.
The first basin furnaces appeared in the 19th century. The economic and environmental criteria require a constant improvement of the equipment.
The life span – the so-called “furnace campaign” – of a basin furnace is a maximum of about 12 years. Thermo Fisher in Romont needs to rebuild its furnace after only 6 years.
In a tempering furnace, the glass cools slowly to prevent it from shattering.