Glazing (window)

Glazing, which derives from the Middle English for 'glass', is a part of a wall or window, made of glass. Glazing also describes the work done by a professional "glazier". Glazing is also less commonly used to describe the insertion of ophthalmic lenses into an eyeglass frame.

Common types of glazing that are used in architectural applications include clear and tinted float glass, tempered glass, and laminated glass as well as a variety of coated glasses, all of which can be glazed singly or as double, or even triple, glazing units. Ordinary clear glass has a slight green tinge, but special colorless glasses are offered by several manufacturers.

Glazing can be mounted on the surface of a window sash or door stile, usually made of wood, aluminium or PVC. The glass is fixed into a rabbet (rebate) in the frame in a number of ways including triangular glazing points, putty, etc. Toughened and laminated glass can be glazed by bolting panes directly to a metal framework by bolts passing through drilled holes.

Glazing is commonly used in low temperature solar thermal collectors because it helps retain the collected heat.

The first recorded use of glazing in windows was by the Romans in the first century AD. This glass was rudimentary, essentially a blown cylinder that had been flattened out, and was not very transparent. In the eleventh century, techniques were developed where the glass was spun into a disc, creating a thinner circular window, or a cylinder was again formed, but this time it was cut from edge to edge and unrolled to make a rectangle-shaped window. The newer cylinder method remained the dominant method until the 19th century, and individual panes of glass were therefore limited in size to the dimensions of those cylinders.

Continuous plate production was invented in 1848 by Henry Bessemer, who drew a ribbon of glass through rollers. This standardized the thickness of the glass, but its use in mass-production was limited by the need to polish both sides of the glass after manufacture, which was time-consuming and expensive. The process was slowly refined throughout the next century, with automated grinders and polishers being added to bring the cost down.

The breakthrough in large, mass-produced, continuous glass production happened in the 1950s with the development of the Float glass manufacturing process. Molten glass is poured over a surface of molten tin, where it flattens out and can be drawn off in a ribbon. The advantage of this process is that it is scalable to any size and produces high quality panes without any further polishing or grinding. Float glass has continued to be the most used type of glazing to the present day.

The most common glass used for glazing is Soda–lime glass, which has many advantages over other glass types. Silica (SiO₂) makes up the bulk of the composition of this material at 70–75% by weight. Pure silica has a melting point that would be prohibitively expensive to reach with large-scale manufacturing, so sodium oxide (soda, Na₂O) is added, which reduces the melting point. However, the sodium ions are water-soluble, which is not a desired property, so calcium oxide (lime, CaO) is added to reduce the solubility. The end result is a product which is high quality, clear, relatively cheap to produce, and recycles easily.