Glass Production

Float Glass Process

The term "float" glass derives from the production method, introduced in the UK by Sir Alastair Pilkington in 1959, by which process 90% of today's flat glass is manufactured. The raw materials (soda lime glass, 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 onto a bath of molten tin in a continuous ribbon.

he glass, which is highly viscous, and the tin, which is very fluid, do not mix so that the contact surface between these two materials is perfectly flat. When leaving the bath of molten tin the glass has cooled down sufficiently to pass to an annealing chamber called a lehr. Here it is cooled under controlled temperatures, until it is essentially at room temperature.

Rolled Glass Process

This describes glass made through a rolling process, whereby the semi-molten glass is squeezed between metal rollers to produce a ribbon with pre-defined thicknesses and patterned surfaces. This process is used for patterned figure and cast glass production.

Other Processes

There are some other processes, such as the Pittsburgh process, or the Libbey-Owens process, nowadays rarely used for the production of flat glass.

Glass Terminology

• Acid Etching The application of hydrofluoric acid to a glass surface to produce a matt frosted decorative appearance.
• Acoustic Insulation Using double or triple glazing or special acoustic laminates to limit the passage of sound.
• Alarm Glass A Laminated security product with wires in the interlayer connected to an alarm system.
• Annealed Glass Glass made under strict temperature control to reduce The stress within the finished product.
• Armour Plate Laminated glass, resistant to mechanical shock, composed of at least four panes of glass and usually at least 25 mm thick.
• Bending A process of curving glass in a kiln e.g. for the outer casement of revolving doors.
• Bevelling Producing a sloped edge as a decorative feature, often on the edge of mirrors in widths of 10mm to 60mm.
• Borosilicate glass Glass made from silica and boric oxide. Such glass is highly resistant to chemical corrosion and temperature change (thermal shock) and is particularly suitable for laboratory ware (test tubes, etc.), domestic cooking ware (oven dishes, etc.), high-power lamps and other technical glass ware. It is also used when glass has to be bonded to metal and low expansion is a key characteristic.
• Bullions Also known as 'Bulls Eyes' flat glass incorporating a bowl shaped piece of glass.
• Butyl Sealant used in the manufacture of double glazed units.
• Conductive coating A glass coating which is electrically conductive. Conductive coatings have been used to produce frost-free windscreens, and in a range of electro-optical applications. One way of producing a conductive coating is by depositing tin salts onto the glass.
• Cullet Scrap glass intended to be re-cycled.
• Embossing Carving or moulding in relief. The forming or application of figures or patterns to an object so that they stand out from the surface.
• Figured Glass This is the trade name for patterned glass.
• Fire Glass Glass that prevents the spread of fire and gasses for a set period of time and is classified accordingly.
• Flat Glass All types of glass (rolled, float, plate, etc.) produced in a flat form, regardless of the method of production.
• Grinding The removal of glass with abrasives or abrasive (grinding) wheels in order to shape, polish or otherwise finish both flat and hollow glass. Grinding processes include milling, sawing, edging and drilling.
• IG Abbreviation for Insulated Glass
• Laminated Glass Laminated (or compound) glass consists of two or more sheets of glass with one or more viscous plastic layers "sandwiched" between the glass panes. The solid joining of the glasses takes place in a pressurised vessel called an autoclave. In the autoclave, under simultaneous heating of the already processed layers of glass and special plastic, lamination occurs. When laminated safety glass breaks, the pieces remain attached to the internal plastic layer and the glass remains transparent.
• Low E Low Emissivity - often used in double and triple glazing units, this window glass has a special thin-film metallic or oxide coating which allows the passage of short-wave solar energy into a building but prevents long-wave energy produced by heating systems and lighting from escaping outside. Low-E glass thus allows light to enter while also providing thermal insulation.
• Opal Glass Translucent and white opal effect glazing.
• Polishing A finishing process to leave edges smooth and dark with all sharpness removed allowing butt jointing.
• PVB Polyvinyl Butyl used to bend sheets of glass together when making laminated glass, can be cut by knife or sawn.
• Pyrometer An instrument used to measure the temperature inside the furnace or kiln.
• Safety Glass Glass which does not disintegrate into sharp and potentially dangerous splinters when it is broken. Safety glass may be produced by laminating (see "laminated glass") or by tempering (see "tempering").
• Sand Blasting Giving an effect similar to acid etching but by the blasting of grit. The finished surface is rougher to the touch.
• Screen Printing A process for the decoration of glass whereby coloured ink is forced by a flexible "squeegee" through a fine-mesh screen, or "mask", (traditionally made of silk, now also made of nylon, polyester and stainless steel) onto the glass surface. A separate mask is used for the application of each colour.
  
  Considerable automation of the process has been developed, thus allowing extremely high printing speeds for even complex designs.
• Silvers Trade name for mirrors from the silvering process.
• Silica Silicon dioxide, a mixture that is the main ingredient of glass. The most common form of silica used in glassmaking has always been sand.
• Soda-lime glass The most common type of industrially produced glass. A typical soda-lime glass is composed of silica (71-75%), soda (12-16%) and lime (10-15%), plus small amounts of other materials to provide particular properties such as colour.
• Tempering (see toughening)
• Toughening Special process of solidification of a glass sheet in order to make it particularly resistant to breakages. The process may be physical (thermal) or chemical. In the former, the glass sheet is heated to a temperature just below its softening point and then immediately cooled by special jets of cold-air. These harden the surface of the glass, giving the inside more time to cool. This allows the external layer to crystallize while the inside solidifies with greater compression than in the crystal lattice. The result is a sheet of glass which is two or three times stronger than un-tempered glass and which, upon breakage, shatters into tiny pieces with blunt edges (the most common applications are for automotive glass). Glass sheets which have been chemically tempered are five to eight times stronger than those which have not undergone any tempering process.
• Wired glass Flat rolled glass reinforced with wire mesh and used especially for glass doors and roofing to prevent objects from smashing through the glass and also to hold pieces of broken glass together. By holding the glass together, it can also protect against break-in. Wired glass is produced by continuously feeding wire mesh from a roller into the molten glass ribbon just before it undergoes cooling.