Annealing Furnaces

Shaped glass (blown, moulded etc.) retains high temperature (500-600°C) and do not resist rapid spontaneous cooling. As it is a bad thermal conductor, the surface cools rapidly compared to the mass, and the increase of internal tensions would lead to the object breaking. Slow cooling is thus adopted, called, inappropriately, distemper or annealing with special furnaces which are of two types.

They are seldom used and only for small production: they are rectangular chambers constructed with blocks of refractory material. Glass objects which have just been moulded are placed in these furnaces heated to about 550°C. Once the production has ended, the furnace is closed, the heating is turned off and it is left to cool spontaneously until the next day when it is opened, emptied and turned on again for the next production.

This is a type of degrading temperature tunnel from 550°C up to room temperature; the objects which have just been moulded are placed in the hot end, on a metallic belt which moves slowly through all the tunnel, with a rate that varies depending on the size of the objects. In the sheet furnaces, the belt goes forward on rollers at a variable rate depending on the thickness and, at the exit, is cut into sheets of about 4 metres in length. At the end of the annealing tunnel, there are several automatic systems to control the product quality.