INFORMATION ABOUT GLASS
We have listed below some comprehensive information on glass to help you make an informed decision in purchasing your glass fence from Absolute Glass Products Pty Ltd.
The Building Code of Australia requires glass fencing to comply with the requirements of two Australian Standards:
- AS 1288:1994, Glass buildings-Selection and installation
- AS/NZS 2208:1996, Safety glazing materials in buildings.
AS 1288 includes information on the selection and installation requirements for glass in buildings,
including toughened glass, as well as human impact safety requirements. The Standard specifies that glass used in pool fencing is Grade A (toughened and laminated) safety glass.
How Glass Is Made
Glass is made naturally from a fusion of silica (sand), soda and lime. This fusion can be achieved merely by lightning striking in a place where the right ingredients happen to be adjacent to each other. When glass is made by man, other ingredients are added, such as potash, lead oxide and boric oxide. Some of these ingredients are used to make glass clear, some to colour it, and others to give it a frosted effect. Glass was made by potters in Egypt for glazing stone beads as early as 12,000 B.C.
As Egyptian culture progressed, craftsmen used glass for the manufacture of personal ornaments and bottles. A tremendous step forward in the use of glass was made by the Phoenicians in about 300 to 200 B.C. by the invention of the blowpipe. The blowpipe is a hollow iron tube with a mouthpiece at one end and a knob shape at the other. The knob-shaped end is dipped into hot, viscous glass. A “gather” of molten glass remains on the end when the pipe is withdrawn. This hot glass can be blown by the worker into a hollow ball. The harder he blows, the larger the ball. During the Roman civilization the art of glass-making reached near-perfection. In the 3rd Century, the Romans cast glass on flat stones and produced the first window panes. The break-up of the Roman Empire and the ensuing Dark Ages brought an end to such cultural developments. The glazing of windows did not become widespread over the whole of Europe until the 15th and 16th Centuries.
Glass is a made by melting a mixture of sand and other minerals in a furnace at temperatures of 1700 degrees Celsius.
What is Float Glass?
Float glass is a term for perfectly flat, clear glass (basic product). The term “float” glass derives from the production method, introduced in the UK by Sir Alastair Pilkington in the late 1950’s, by which 90% of today’s flat glass is manufactured.
The raw materials (silica sand, calcium, oxide, soda and magnesium) are properly weighted and mixed and then introduced into a furnace where they are melted at 1500° C. The molten glass then flows from the glass furnace into a bath of molten tin in a continuous ribbon. The glass, which is highly viscous, and the tin, which is very fluid, do not mix and the contact surface between these two materials is perfectly flat. When it leaves the bath of molten tin, the glass has cooled down sufficiently to pass to an annealing chamber. Here it is cooled at controlled temperatures until it is essentially at room temperature.
Step by Step Manufacturing of Float Glass
Information From Pilkington Glass
Sir Alastair Pilkington – who was honoured with a knighthood in 1970 – invented the “float” method of glass making which revolutionised the industry in the 1960s. He had the idea in the early 1950s, but it took seven years of hard work to prove that he was right, and the cost of developing the process was high, particularly for a company that was at the time family owned. Although the process was announced to the public in 1959, it was not until 1962-3 that it became consistently reliable and profitable.
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 1,500oC 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 1,100oC and leaving the float bath as a solid ribbon at 600oC.
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.
All toughened glass should have a permanent stamp in the corner identifying Australian Standard compliance.
Toughened glass starts life as float glass. Float glass, when shattered breaks into dagger like pieces and can be harmful, it is therefore unsuitable for some applications. Before undergoing the toughening process the glass parts must be cut to size. Any additional machining must be completed before the glass is toughened as it would shatter if it was cut in its toughened state.
In the toughening process, the surfaces of the glass are heated in a furnace. Recommended temperatures vary but the glass reaches temperatures of over 600°C. The hot glass is then cooled rapidly by a blast of air over a period of between 3 and 10 seconds. As a result, the surfaces shrink, and (at first) tensile stresses develop on the surfaces. As the bulk of the glass begins to cool, contracts. The already solidified surfaces of the glass are then forced to contract, and consequently, they develop residual compressive surface stresses, while the interior zone develops compensating tensile stresses. The tension zone in the core of the glass takes up about 60% of the cross-sectional area of the glass. Compressive surface stresses improve the strength of the glass in the same way that they do in other materials. The nature of the stresses is depicted in Fig. 1.
After the process the toughened glass has a greater resistance to thermal stresses and thermal shock and has improved flexural and tensile strength. However, following characteristics remain unchanged; • Colour, • Clarity, • Chemical composition, • Light transmission, • Hardness, • Specific gravity, • Expansion coefficient, • Softening point, • Thermal conductivity, • Solar transmittance, • Stiffness.
The higher the coefficient of thermal expansion of the glass and the lower its thermal conductivity, the higher the level of residual stresses developed, and the stronger the glass becomes. Thermal toughening takes a relatively short time (minutes) and can be applied to most glasses. Because of the high amount of energy stored in residual stresses, tempered glass shatters into a large number of pieces when broken. The broken pieces are not as sharp and hazardous as those from ordinary glass.
Above is a representation of the main elements of a glass toughening facility. The size of the machinery required is dependent on the size of toughened glass parts one wishes to produce.
Reference: Manufacturing Engineering and Technology Serope Kalpakjian.
Spontaneous Breakage of Toughened Glass
Spontaneous glass breakage is a phenomenon by which toughened glass (or tempered) may spontaneously break without any apparent reason. The most common causes are:
- Minor damage during installation such as nicked or chipped edges which later develop into larger breaks
- Binding of the glass in the frame causing stresses to develop as the glass expands and contracts due to thermal changes or deflects due to wind
- Internal defects within the glass such as nickel sulphide inclusion.
- Thermal stresses in the glass
- Inadequate glass thickness to resist wind load
The latter is not an issue with our glass as it is engineered and fit for purpose.
If toughened glass does implode it is designed to break into small uniform chunks and is relatively harmless providing the person handling the glass is wearing appropriate PPE.
Heat Soaking Glass
Heat Soaking is a destructive process in which a pane of tempered glass is subjected to temperatures up to 280° C for several hours over a specific temperature gradient to induce fracture. This test insures that if there is probability of breakage then the infected panes break inside the furnace at the factory itself. Up to 95 % NiS infested panes are usually destroyed inside the heat soak chamber at the factory premises and hence reduce the chances of onsite breakages.