Things You'll Need
Needle nose pliers
Window glaze tool
Arched windows, while beautiful in design and high in character, can be difficult to repair when broken. With the arches at the top of the window, all work must be done at the top of a tall ladder. Additionally, the arch of the window itself makes replacement glass more tricky to come by. Use a sturdy free-standing ladder to reach the top of the arch. Ask a friend to help hold the ladder sturdy, and to hand you your tools as needed, but have your friend stand back for the first part of the project, when you're cleaning out the old glass from the window.
Remove all broken glass from the sash. If the glass is simply cracked but still in one piece, you may be able to do this by removing the old glaze and glaze points. Use a utility knife and 5-in-1 tool to cut away the old glaze at the sides of the window, then remove the glaze points with needle nose pliers. If the glass has been shattered, wear thick work gloves and safety glasses. Remove old glaze with the utility knife, then reach in and start pulling out shards. To catch falling pieces of glass, lay a towel beneath the window, and spread a disposable tarp on top of it.
Remove all debris from the mullions around the window. Use the sharp edge of a 5-in-1 painter's tool to scrape out the old glaze, and use needle nose pliers to pull out old glaze points.
Hold a sheet of blank sketch paper or newspaper up to the empty arch in the window. Cut down the paper to fit inside the mullions as the old pane of glass once did. Leave a 1/16-inch gap between the edge of the paper and the window mullions. The paper should fit against the back of the lip of the mullion, but inside the sides of the mullion, just as the glass will. This paper is what you will take to the hardware store or glass shop to show the desired shape of your window pane when you order the new pane of glass. Cut down the sides of the paper with care, so that the paper is as close to the actual pane of replacement glass as you can get.
Take your paper and a shard of the old glass to your local hardware store. Order the piece of replacement glass. Show the old glass to the clerk at the store so they can try to order a similar glass for your window.
Slip the new piece of glass in the window to be sure that it fits. Do not attempt to fit a piece of glass that is too tight into mullions.
Insert the glaze points into the wood around the glass. Use the edge of the 5-in-1 tool, or the edge of the window glaze tool, to push the pointed tips into the wood so that the wider edges hold in the glass. There should be at least 3 glaze points per window side: one at each corner and one at the middle. If you wish to insert more glaze points for extra security, you may do so.
Roll out the window glaze into a snake shape and press the window glaze into the edge of the window pane so that it forms a seal over the edge of the window pane and the mullion.
Use the edge of the window glaze tool to make the edges of the window glaze straight and even. Carefully scrape away any extra glaze on the window.
Wait for the window glaze to dry according to the manufacturer's instructions.
Once the window pane is replaced, you will likely wish to repaint the mullion around the new window pane, if not the entire window frame.