Production Techniques

Mario Moretti

The disc technique

The disc technique entailed blowing a sphere which was then attached to a prop at the opposite end of the blowing tube. Once the tube was detached, the prop was progressively opened from the hole left by the blowing tube. When heated intensely and spun rapidly, the hemisphere opened into a disc by the centrifugal force; this technique is still used today in the production of artisan dishes, open bowls and shield polychrome discs connected to lead. Once detached from the prop, the disc, if small in dimension (up to about 20 cm in diameter) was used as it was. The bigger discs, which could be larger that a metre in diameter, were cut into squares. The thickness is irregular (thicker in the centre, less at the edges), but the surfaces are clearer, because they were formed in the air without contact with other surfaces.
The disc technique, like the cylinder one, started in the first centuries AD and their production continued up to the 19th century. There is still some limited production today (cathedral glass) of artistic stained-glass windows.

The cylinder technique

The cylinder technique used rotation blowing to give a lengthened form to the prop until it became a cylinder, which was closed at one end by a hemisphere and, on the other, by the blowing tube. Once cooled, the cylinder was cracked-off and grinded (cutting the two closed ends), engraved and cut length-wise.
The cylinder was then placed in a heating furnace at a temperature of about 700°C.
Under its own weight and with the aid of appropriate tools, the cylinder would open and flatten out in a big sheet which was then annealed. The two surfaces were different: the top one was shiny, the one resting on the furnace floor was rough with small defects.

Casting, after the Roman era, began again in France in the 17th century to form big sheets for use in the production of mirrors which were to substitute the inimitable - but very expensive - Venetian mirrors. The melt was poured onto an iron plate and a roller attached to rails was run over the glass mass; this guaranteed a quick and uniform distribution of the glass in the rectangular mould. Using this technique, sheets of up to 2x4 metres could be produced, but with rather high thicknesses (over 6-8 mm).

The lamination technique

Imperfections on the surface - the top few millimetres - were eliminated, after annealing, with a laborious system of abrasion, flattening and shining. In the 19th century, this inconvenience was overcome with the lamination technique of the molten mass, which was cast directly from the crucible, between two cooled cylinders (Bicheroux procedure). The length of the cylinders was about 4 metres. There is an iron sheet under the cylinders of the same width, which moves at a regulated rate and on which the rolled sheet is laid which is cut by a specific device and then placed in the annealing furnace. Sheets 4x 6.6 m in size can be produced, with abrough thickness of 5 mm. The irregularities in the thickness do not exceed 0.6 mm, so there is a notable reduction in grinding and shining.
This technique continued to be used even in the industrial era and underwent great developments with the advent of the automobile industry in the first half of the 1900s.

Wired and moulded glass

Based on the same casting technique, we can, once taken out of the furnace, insert a framework of inoxidizable metallic wires in the glass sheet, which is still in the molten state, before putting it through the rolling cylinders: at the end of the process, the framework is completely immersed in the glass. The aim is to strengthen resistance to breaking and to avoid dangerous pieces of glass if it were to break.
With the same technique, the glass taken out of the furnace is passed through the two rolling cylinders which, either one or both of them, then print bas-relief designs on the surface of the glass.

Continuous sheet printing

Printing the glass directly from the incandescent mass was the dream of many researchers, especially for producing sheets. Attempts were made to prime the glass, via iron bars, to lift it in the form of a large strip from which the sheets would be obtained. But the strip shrank, it became thinner towards the bottom once it was lifted. The consequence was that of an 'ironing' effect.
The industrial solution is due to two different systems which are named after their respective inventors, the Belgian Fourcauld and the American Colburn.

Fourcault procedure

The fundamental concept of this procedure is an original solution to avoid the shrinking of the strip during the lifting after the priming by means of the rod grip. The main point is this: pulling is avoided. And in fact Fourcault 'pushes' the strip out of the melt. Pressure is obtained by the effect of difference in height formed in the tub of the molten glass. This is the famous effect of communicating vases!
Indeed, imagine a 2 m-long, 0.5 m-wide beam of porous refractory material (chamotte), with raised edges, so that it looks like a box with a very thick bottom in which, lengthwise, a crack is made. If you put the beam on the surface of the molten glass, it will float; but, if you force it to sink until the inside surface of the box is some centimetres below the surface of the melt, it will, due to the hydrostatic law, penetrate the crack in the form of a glass blade which is primed with an iron bar and lifted slowly between two lateral series of rollers. The sheet is thus produced. Once the sheet goes out of the crack, it is artificially cooled so that its state of rigidity begins straight away. The lifting occurs in a double-walled chamber which acts as an annealing furnace; the heat of the melting furnace is exploited, regulating it with valves along the chamber, on top of which the sheet is cut to the required length. The width of the sheet is determined by the length of the crack in the float (Fourcault's débiteuse); the thickness is given by the relationship between the rising rate and the temperature and the viscosity of the glass within the crack of the float.
Furnace production was 800 - 1200 m2 in 24 hours and the process was economical due to the limited expertise required and the low fuel consumption.
The advantages of the product are: completely flat surface, good fire polishing, absence of internal tensions.

Libbey Owens (Colburn) procedure

Unlike the Fourcault procedure, in Colburn's method - better known with the name of the company which owns the patents, Libbey-Owens - the glass is, in the real sense of the word, pulled vertically from the melt. To prevent shrinking of the width of the sheet, as soon as the pulling was begun, the edges were rolled between small rotating rollers, placed horizontally, 3-5 cm above the tub level. The peripheral rate of the rollers is lower that that which the strip is lifted: the rollers thus do not act by drawing, but by restraining and they form a step of molten mass available at the edges, which, once they are made, do not shrink anymore. The glass strip, which is pulled vertically for about 1.5 m, is folded square on an appropriately heated cylinder before horizontally entering the annealing furnace on rollers.
These pulling methods of the glass sheets were progressively replaced by the float procedure which produced glass of a higher quality and at a lower cost.

Float procedure

The glass, once the gas bubbles have been removed and has been homogenized in the refinement zone at about 1350oC, is progressively cooled and then cast in another furnace, containing very pure molten tin. The atmosphere in this furnace must be a reducing one (hydrogen atmosphere) to avoid the oxidisation of the metal.
The glass, which is less dense than the tin, floats and forms a strip of natural thickness of about 6mm. Special devices allow the slowing down or acceleration of the expansion of the strip, to regulate the thickness and the parallelism of the faces.
The two faces are perfectly shiny and flat, due to the action of the fire (the top one), and due to contact with the molten tin (the bottom one). When taken out of the molten tin, the temperature of the glass is about 600°C and stiff enough to be continuously carried by rollers into an annealing tunnel, in which the temperature is lowered progressively until it reaches room temperature, to prevent the occurrence of internal tensions and to prevent its breakage.
When the glass strip has been slowly cooled down in the open air, it is automatically cut into 6x3 metre sheets. These are moved using suction caps, they are taken to a store area and sent with special forms of transport.
This procedure was devised by Pilkington (England) in 1964 and is currently the most popular way in the world to produce sheets.
The advantages of this procedure are simplicity, high yield (a furnace produces over 600 tonnes a day with small amounts of scrap), perfect flatness and parallelism of the surfaces of the sheet and the possibility of obtaining any thickness between 1.3 and 24 mm.