Glass is very fragile, and can be cracked or chipped with slight pressure, or a from a pebble or rock hitting the surface. This is most common with automobile windshields, but can happen to home windows as well, most commonly during a thunderstorm with high winds, or while performing lawn maintenance near windows. Depending on the type of window, or structure of the glass, replacing windows can be quite costly in todays market. Stopping a crack from spreading across a piece of glass is an alternative to replacing an entire window pane.
Purchase a glass repair kit. These are available at most home repair, hardware, or auto repair stores and usually cost less then £6. The kit will contain: Tube of resin or epoxy, bridge device (suction cup), and may contain a glass scraper as well. Read the instructions carefully on the repair kit.
Clean both sides of the glass with glass cleaner. Ensure that you clean away any dirt, or debris present in the crack.
Apply the resin or epoxy by squirting it in the centre of the crack, and work your way out to the end of the crack. If the window has been cracked from a rock chip there will be a central fracture point. Start at this point and work your way over the entire crack. If the crack has a starting point on the side of the window pane, start at the side and work along the crack.
Apply the suction cup or bridge to the centre of the crack, and tighten by turning the tightening bar on top of the bridge. This will help the resin to adhere to the glass properly.
Remove the bridge by turning the tightening bar and scrape the excess resin from the glass using a scraping tool. A plastic putty knife will work efficiently for this step if one is not provided in the repair kit. Clean the glass thoroughly again using glass cleaner.
As a rule of thumb, if the window crack is larger than the palm of your hand, then you will have to replace the entire window.
Avoid epoxy or resin contact with hands and eyes as this will cause extreme irritation.