Basin furnaces have only recently been produced and used: it is only at the end of the 19th century, with the advent of moulding-machines replacing manual production, that the basin first appeared. This furnace is made for the continuous production of glass; the various phases which in the crucible furnace occur over time, here they occur over space.
The mixture is loaded continuously at the end of a large rectangular tank, whose surface varies from a few m2 to almost a hundred m2, in the case of furnaces for float sheets and the thickness of the melted glass bath varies from 80 to over 150 cm. Moving towards the opposite side, the mixture melts and forms a liquid which is refined and homogenized and thus goes out of the tank through a submerged hole (throat) to reach the production area.
These furnaces produce from 100 up to 1000 tonnes of glass a day. A furnace for bottles can produce a million pieces a day! And a float furnace produces enough sheets to cover a surface as big as ten soccer fields!
The whole process, from the weighing and mixing of the primary materials to the packaging of the final product, is completely automated.
The basin furnaces are constructed with blocks of refractory material of various types. They are mostly alumina and zirconium-oxide-based compounds. In the areas in contact with the molten material, arc welded blocks which are particularly resistant to corrosion are used, because they are compact and non-porous. The vault is in silica refractory materials (arc welded for the production of meltable glass at high temperatures) and the walls are generally in sodium-alumina material.
The energy required to reach fusion temperature is generally supplied by gas burners or combustible oil and the heating occurs by radiation. Mixed basin furnaces are increasingly being used more , in which part of the energy is supplied via electrodes immersed in the molten material (electrical boosting). Activation of these electrodes occurs only in particular moments to increase the quantity of glass produced.
In regards to the set-up of the burners, the basin furnaces can be divided as follows:
a) U-shaped flame or horse-shoe furnaces (small, with production less than 200 tonnes a day). In these furnaces, energy is recovered by preheating the air in metallic regenerators preheated by smoke going out.
b) Transversal flame furnaces (the biggest type, fed by 4-6 burners per side, which can be regulated independently so as to obtain, along the axis of the furnace, the temperature distribution required). The recovery of the heat occurs via regenerators, piling of refractory bricks placed at the sides of the furnace, heated alternatively by the smoke.
Depending on the production type, the furnaces can be a single tank (a so-called open furnace), rectangular shaped, lengthened in the direction of the glass flow, or two tanks. In the first, biggest one, the fusion and refining occur; from the second, called the production one, feeders come out which lead to the moulding machines (bottles, vases, glasses). The two tanks are connected by the throat.