Coldworking is the part of glass that has no thermal drama. A fully annealed piece can be ground, flat-lapped and polished at room temperature, and the only thing that limits the result is the grit sequence.
Why grit sequence matters
Each abrasive stage removes the scratches left by the previous one. If you jump from a coarse grit straight to polishing, the coarse scratches are still there and the polish simply makes them shiny and more visible. There is no shortcut: step through the grades in order.
| Stage | Typical grit | Purpose |
|---|---|---|
| Shaping | 80 - 220 | Remove material, establish geometry |
| Fine grinding | 220 - 400 | Remove shaping scratches, true the surface |
| Pre-polish | 400 - 600 | Remove fine scratches, prepare for polish |
| Polish | Cerium or tin oxide | Optical clarity, no material removal |
Mark the surface with a pencil before a grinding stage. When the pencil marks are gone, the stage is complete. It is the simplest way to avoid under-grinding.
Wet, always wet
Grinding glass dry produces airborne silica dust, which causes silicosis. Every grinding and polishing operation on this site assumes a water flood. Water also cools the work and carries away swarf so the abrasive keeps cutting instead of burnishing.
- Never grind dry. Full stop.
- Keep the slurry wet and covered so dried material does not re-aerosolise.
- Wear a face shield, not just glasses. Slurry spray carries grit.
- Clean the floor around the machine. Cerium slurry is extremely slippery.
The machines and the hand tools
A grinding wheel handles geometry: flattening a base, taking a rim down to length, shaping an edge. Flat laps do large flat surfaces. Hand tools such as diamond files and pads handle detail and small corrections where a machine cannot reach.
A glazed wheel generates heat instead of removing glass. Dress it whenever it stops cutting freely, and the cut stays predictable.
Polishing
Polishing removes almost no material. A felt pad charged with cerium oxide, run wet at moderate speed, smooths the microscopic scratches left by 600 grit until the surface transmits light again. It is slow, and it only works if every earlier stage was done properly.
Confirm the piece is fully annealed
Coldworking an unannealed piece can release locked stress and crack it on the wheel.
Clamp or jig the work
Small pieces held by hand are how fingers get cut. Use a jig wherever possible.
Step through the grits
80, 220, 400, 600, then polish. Check with a pencil line at each stage.
Keep the pad damp and the speed moderate
Heat from a dry pad can crack even cold glass.
Clean between stages
A single coarse particle carried onto a fine pad puts a deep scratch back into a finished surface.
Coldworking is patient work with a very high ceiling. A well-polished flat on a simple form can be more impressive than a complicated shape left dull.
This article is educational reference material. Hot glass work involves severe burn, fume and fire hazards. Wear eye protection, heat-resistant gloves and natural-fibre clothing, ventilate every heat source to open air, fit a carbon monoxide alarm, and take hands-on instruction before operating any hot glass equipment.
No guarantee of results. Techniques, temperatures and schedules described here are reference guidance based on common studio practice. Glass behaviour depends on your materials, equipment and technique. Test everything on your own setup and follow the manufacturer's instructions.