It's easy to have a glass shop or hardware store cut replacement glass for you. But when you're replacing a pane in an old sash, the perfection of new glass will look out of place next to the wavy, bubble-filled originals. That's why Tom Silva makes a habit of saving old (pre-1950) glass from windows that are being tossed out. Then he just cuts his own as he needs it. Here's how:
Step 1: Lay the glass on the sash and line up one corner of the glass with a corner of a clean, putty-free opening. Leave the edge of the glass about 1/16 inch from the inside edges of the rabbets.
Step 2: Place a framing square on top of the glass and line it up with the inside edges of the rabbets in the opposite corner, which are visible beneath the glass.
Step 3: Lubricate the glass cutter's tiny wheel with a drop of oil.
Step 4: Holding the square firmly in place, set the side of the cutter against one side of it. Holding the cutter in a vertical position, pull it over the pane's surface with a single firm stroke. (You've only got one chance.) Listen for a continuous scratchy, crackling sound. If there's no sound, try again in a different part of the glass, bearing down a bit harder. Or buy a new cutter. Score only one side of the corner in this step.
Step 5: With gloved hands, line up the score line with the edge of a table.
Step 6: Grip the side hanging off in space and snap the pieces apart with a slight downward flick of the wrist.
Step 7: Take the glass back to the sash, line it up as before, and with the square in place, score the other side of the corner. Break as above.