Author: Duffy Hoffman
Contributors: Bob Yapp
Title of Treatment: Cutting Glass
Class of Treatment: [ ] Maintain, [ ] Stabilize, [x] Repair, [ ] Upgrade, [ ] Exception
Type of Treatment: [x] Traditional, [ ] Modern
Condition to be Treated:
Glass is too large to fit into the sash light.
Cutting old glass can be difficult because of it's irregular character. The thickness and hardness may vary a lot within one piece of glass. This treatment also applies to new glass, which is more consistent and easier to cut. Layout the size needed by marking the glass, score the glass with a special cutter and snap the glass into two pieces at the score. Careful work is required to size the glass pane correctly and safely.
Safety: Wear tight fitting glass handling gloves and safety glasses.
1. Work on a flat cutting table with a soft surface, such as tight-napped carpeting or rubber matting.
2. Most old glass has a slight curve or crown to it. Site down the edges to determine which side has the crown.
3. Lay the glass on the cutting table with the crown (convex) side up.
4. Mark the glass where it is to be cut with felt marker or grease pencil.
5. Cut with a sharp, hand held glass cutter. Dip the rolling cutting wheel in a solution of 50% boiled linseed oil and 50% gum turpentine. This lubricates the cutting wheel which helps make a better score on the glass and avoids chipping glass and dulling of the cutting wheel.
6. Lay a straight edge or T-square on the marks. Run the glass cutter slowly but firmly along the side of the straight edge. Press down firmly, but not so hard it breaks the glass. Run slowly to follow the up-and-down waviness of the glass. Run with a continuous stroke to form a single score. If the score is not continuous, flip the glass over and carefully score the other side in the exact same location.
7. Move the glass over the edge of table so the score mark is right above the table's edge. Hold the waste piece of glass to be snapped off and lightly tap the score line on the waste side with the ball end of the glass cutter. If the waste glass piece is over 6" wide it can usually broken off the glass at the score line by using two hands and a quick, even, downward motion. If the waste glass is less than 6" wide it is usually best to break the glass at the score line with a pair of glass breaking pliers.
If the glass does not break evenly and pieces are left behind, re-score on the other side and chip off. Then sand of edges with wet sanding block for a smooth edge.
• Boiled linseed oil
• Gum turpentine
• Sanding block with wet-dry sand paper
Quality of Results
Glass is scored properly and cleanly snapped on score line with no glass chips at the break. Glass fits into the sash's light with 1/16" play on all four sides.
Scores were not continuous and break edges have chips. Glass breaks in places other than the score line.
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