Glassware

Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and various low melting point alloys were used in the past. This method gives the sheet uniform thickness and very flat surfaces. Modern windows are made fr om float glass. Most float glass is soda-lime glass, but relatively minor quantities of specialty borosilicate and flat panel display glass are also produced using the float glass process. The float glass process is also known as the Pilkington process, named after the British glass manufacturer Pilkington, which pioneered the technique (invented by Sir Alastair Pilkington) in the 1950s.

Float glass uses common glass-making raw materials, typically consisting of sand, soda ash (sodium carbonate), dolomite, limestone, and salt cake (sodium sulfate) etc. Other materials may be used as colourants, refining agents or to adjust the physical and chemical properties of the glass. The raw materials are mixed in a batch process, then fed together with suitable cullet (waste glass), in a controlled ratio, into a furnace where it is heated to approximately 1500 °C. Common flat glass furnaces are 9 m wide, 45 m long, and contain more than 1200 tons of glass. Once molten, the temperature of the glass is stabilised to approximately 1200 °C to ensure a homogeneous specific gravity.

The molten glass is fed into a "tin bath", a bath of molten tin (about 3–4 m wide, 50 m long, 6 cm deep), from a delivery canal and is poured into the tin bath by a ceramic lip known as the spout lip. The amount of glass allowed to pour onto the molten tin is controlled by a gate called a tweel.

Tin is suitable for the float glass process because it has a high specific gravity, is cohesive, and is immiscible with molten glass. Tin, however, oxidises in a natural atmosphere to form tin dioxide (SnO₂). Known in the production process as dross, the tin dioxide adheres to the glass. To prevent oxidation, the tin bath is provided with a positive pressure protective atmosphere of nitrogen and hydrogen.

The glass flows onto the tin surface forming a floating ribbon with perfectly smooth surfaces on both sides and of even thickness. As the glass flows along the tin bath, the temperature is gradually reduced from 1100 °C until at approximately 600 °C the sheet can be lifted from the tin onto rollers. The glass ribbon is pulled off the bath by rollers at a controlled speed. Variation in the flow speed and roller speed enables glass sheets of varying thickness to be formed. Top rollers positioned above the molten tin may be used to control both the thickness and the width of the glass ribbon.

Once off the bath, the glass sheet passes through a lehr kiln for approximately 100 m, wh ere it is cooled gradually so that it anneals without strain and does not crack from the temperature change. On exiting the "cold end" of the kiln, the glass is cut by machines.

Manufacturing of Float Glass

Watch the magic of this science-based process begins to unfold, in a series of stages on a float line that may be nearly half a kilometre long. Raw materials enter at one end. From the other, plates of glass emerge, cut precisely to specification, at rates as high as 6,000 tonnes a week. In between lie six high integrated stages...

Stage 1: Melting and refining

Fine-grained ingredients, closely controlled for quality, are mixed to make batch, which flows as a blanket on to molten glass at 1,500 ^oC in the melter.

Float makes glass of near optical quality. Several processes – melting, refining, homogenising – take place simultaneously in the 2,000 tonnes of molten glass in the furnace. They occur in separate zones in a complex glass flow driven by high temperatures. It adds up to a continuous melting process, lasting as long as 50 hours, that delivers glass at 1,100oC, free from inclusions and bubbles, smoothly and continuously to the float bath. The melting process is key to glass quality; and compositions can be modified to change the properties of the finished product.

Stage 2: Float bath

Glass from the melter flows gently over a refractory spout on to the mirror-like surface of molten tin, starting at 1,100^oC and leaving the float bath as a solid ribbon at 600^oC.

The principle of float glass is unchanged from the 1950s. But the product has changed dramatically: from a single equilibrium thickness of 6.8mm to a range from sub-millimetre to 25mm; from a ribbon frequently marred by inclusions, bubbles and striations to almost optical perfection. Float delivers what is known as fire finish, the lustre of new chinaware.

Stage 3: Coating

Coatings that make profound changes in optical properties can be applied by advanced high temperature technology to the cooling ribbon of glass.

On-line chemical vapour deposition (CVD) of coatings is the most significant advance in the float process since it was invented. CVD can be used to lay down a variety of coatings, less than a micron thick, to reflect visible and infrared wavelengths, for instance. Multiple coatings can be deposited in the few seconds available as the glass ribbon flows beneath the coaters. Further development of the CVD process may well replace changes in composition as the principal way of varying the optical properties of float glass.

Stage 4: Annealing

Despite the tranquillity with which float glass is formed, considerable stresses are developed in the ribbon as it cools.

Too much stress and the glass will break beneath the cutter. To relieve these stresses, the ribbon undergoes heat-treatment in a long furnace known as a lehr. Temperatures are closely controlled both along and across the ribbon. Pilkington has developed technology which automatically feeds back stress levels in the glass to control the temperatures in the lehr.

Stage 5: Inspection

The float process is renowned for making perfectly flat, flaw-free glass. But to ensure the highest quality, inspection takes place at every stage.

Occasionally a bubble is not removed during refining, a sand grain refuses to melt, a tremor in the tin puts ripples into the glass ribbon. Automated on-line inspection does two things. It reveals process faults upstream that can be corrected. And it enables computers downstream to steer cutters round flaws. Flaws imply wastage; while customers press constantly for greater perfection. Inspection technology now allows more than 100 million measurements a second to be made across the ribbon, locating flaws the unaided eye would be unable to see. The data drives ‘intelligent’ cutters, further improving product quality to the customer.

Stage 6: Cutting to order

Diamond wheels trim off selvedge - stressed edges - and cut the ribbon to size dictated by computer.

Float glass is sold by the square metre. Computers translate customers’ requirements into patterns of cuts designed to minimise wastage. Increasingly, electronic systems integrate the operation of manufacturing plants with the order book.

Float glass

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Glass bottle

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Glass jar

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