Blast furnaces

Mario Moretti


We know nothing or very little about blast furnaces before the invention of the blow-rod in the 1st century B.C.. From that moment and until the 7th-8th century, glass production grew in an almost industrial sense. Big tank furnaces were used (the remains of some of them have been found in Palestine and Egypt) which could produce up to 10 tonnes of glass. Once the fusion was finished, which lasted several days if not weeks, the furnace was put out, demolished, and the big sheet of rough glass (its thickness was several tens of centimetres), was broken into blocks. These were transported to other places where the glass was remelted into crucibles situated in small furnaces to be moulded. In the middle ages and until the advent of the industrial era, the mixture was calcinated beforehand at about 800°C in reverbatory furnaces, transformed into frit which was then melted in crucible furnaces.

Crucible and Tank Furnaces

In crucible furnaces, there were one or more containers made of refractory material (fireclay or siliceous sand, but also earthenware in Roman times) in which, during the mixture fusion (or the frit or rough glass), there followed the production of the glass and then again the loading (discontinuous process).
In the production of artistic glass, the crucible furnace is still today made of heat-resistant bricks, using alumina-silicone material (for the walls) and silica (the vault). The shape of the crucibles is that of a cylinder opened at the top or elliptic for the smaller ones. Their capacity varies from a few dozen kg to over a tonne of glass. The crucible is placed in the centre of the furnace, supported by bricks leaned on the counter, so that the burner flame hearts it uniformly.
Once placed in the furnace, the crucible must be heated slowly until it reaches the operation temperature of over 1000°C over a few days, to prevent it from splitting due to the too fast expansion of crystal materials which make it up. Once it is running regularly, the crucible can continue to be used for several months before it needs to be replaced. The furnace is always kept on and the temperature ranges from 1400°C (fusion) to 1000°C (production).
In big artisan glass works, tanks are also used, furnaces without a crucible with the walls covered in perfectly square parallelepipeds (to prevent infiltration of glass through the joints) made of arc melted refractory material made of silica-alumina-zirconium, which is very resistant to corrosion. In the tanks, which can reach a size of even several tonnes, the flame heats the glass by radiation, passing over the melted tank. The flame is fed by methane (the least polluting fuel), and air combustible oil. To reduce the consumption of energy, this oil is first preheated in a heat regenerator (metallic tank heated by smoke coming out of the furnace).
The use of methane is not sufficient to reduce the pollution within the limits permitted by European regulations for the presence in the flame of nitrogen oxides in amounts which exceed the allowed amounts. For this reason, in the future, recourse must be made to oxy-combustion (oxygen substitutes the air in the flame) or electric furnaces. These electric furnaces are widespread today in countries in which electrical energy costs are low or are used in the fusion of the more polluting mixtures (like those containing lead or fluorine compounds).
There are two systems to electrically melt: radiate the crucible with resistors attached to the walls of the furnace or melt the mixture in tanks which is heated by molybdenum electrodes immersed directly in the glass.

Basin Furnaces

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.