Like metals, glass is a solidified liquid. It is produced by melting sand, soda ash, limestone, dolomite, alumina, feldspar, potash, borax, cullet (broken glass) and/or other ingredients, at about 1500° C, shaping it, and allowing it to cool slowly (annealing) to prevent cracking. Although the earliest forms of glass were produced a few thousand years ago, its large-scale production and use in buildings is less than two centuries old.
Glass is not an essential material for low-cost constructions in developing countries, but certain glass products or even waste glass can be quite useful in improving the quality of other materials, or indoor comfort in buildings.
• Flat glass, either as clear float glass (with undistorted vision and reflection), cast glass (usually translucent) or special variety (for solar control, thermal insulation, decoration, etc.) mainly for glazing of windows, sometimes doors, also for solar collectors, greenhouses, Trombe walls (thermal storage walls).
• Hollow glass blocks (made by fusing two trays of glass together) for non-loadbearing walls or screens to provide light and solar heat transmission.
• Glass fibre, in conjunction with other materials such as cement, polyester and epoxide resins, for lightweight roofing materials or infill wall panels, sun shades, cisterns and other items of any desirable shape.
• Glass wool, made of glass fibres sprayed with a binder and formed into boards or rolls, as thermal insulating material.
• Old bottles used as a substitute for hollow glass blocks.
• Waste glass, crushed to a fine powder and mixed with clay (7 parts powder: 3 parts clay), acts as a flux and reduces the temperature needed to fire the bricks by more than 50° C (saving nearly 50 % of the fuel). The bricks are tough and resistant to wind and rain. Very strong and resistant bricks are also made from 31 % crushed glass, 6 % clay, 7 % wafer end 56 % crushed old bricks.
• Durability, usually high in normal conditions, and good resistance to chemicals (with a few exceptions) and biological hazards.
• Sufficient strength and elasticity, so that an ordinary glazed pane will safely deflect up to 1/125th of its span.
• In regions with cold seasons, utilization of solar energy by trapping the heat within the building ("greenhouse effect"), providing indoor comfort and saving fuel for heating.
• Glass can be recycled.
• Glass is brittle and thus difficult to transport; incorrect installation, thermal stresses, sudden impact, etc. can lead to breakage.
• Broken glass can cause serious injuries.
• Most modern varieties of glass absorb most of the sun's ultra-violet rays, which is vitally important (especially for children) for the synthesis of vitamin D and to destroy harmful bacteria.
• Hydrofluoric and phosphoric acids, and strong alkalis (eg caustic soda, alkaline paint removers, cement products) attack glass; deterioration is also caused by prolonged action of water.
• Although glass is non-combustible, it breaks and later melts in fires.
• Small glass components are easier to transport end less likely to break. A good alternative to standard glazed windows are adjustable glass-louvred windows, especially in the humid tropics, where cross-ventilation is desirable.
• Cheaper, low quality glass, made primarily from quartz sand, does not permit undistorted vision, but allows the healthy ultra-violet rays to pass through.
• Water running off from fresh concrete or mortar must be properly removed from glass to prevent deterioration. In dry conditions, with regular cleaning, glass can be extremely durable.
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