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A glazing bar is the horizontal or vertical crosspiece that holds a pane of a window in place. Glazing bars can be made of wood, metal, or plastics. Before modern technology made it possible to make large uniform sheets of glass, glazing bars were necessary to make large windows. In modern architecture and design, they are used mostly for aesthetic purposes.
Since at least the Middle Ages, people have been using glass in windows. Glass is ideal for windows because it can be manufactured to be transparent or translucent, letting in light while still keeping out the elements. Glass is generally an isolator, so it keeps the heat in and the cold out, or vice versa.
Before modern technologies were developed, it was very difficult to make a large, flat piece of glass. Small panes of glass were much easier to make, but resulted in tiny windows. To solve the problem, pieces of wood were used to hold several panes of glass into a window, allowing for larger windows.
The glazing bar came with its own host of problems, however. The early glazing bar was made of wood, which warps and bends over time. This could result in broken panes at worst. At best it left cracks between the wood and the glass that let in the elements. Building owners had to have the windows resealed periodically for years after installation to repair the damage caused by warping.
As technology advanced, the glazing bar improved as well. Eventually it could be made out of materials that would not warp, such as plastic or metal. Even as this change occurred, glass-making technology was developing as well.
In modern times, the glazing bar is mostly unnecessary since it is possible to make huge panes of glass without too much trouble. Some people still prefer glazing bars for their aesthetic appeal. They break up the large plain expanse of glass, making a window more visually interesting and making it less likely that birds will fly into the windows thinking they are open spaces. Glazing bars may also be used in historic homes or homes that have a historic feel.
Modern glazing bars often add no structural integrity to the window. They are either inserted between two panes of glass or overlaid on top of a large piece of glass. Bow windows or curved skylights can be created using glazing bars that curve.