Use soda-lime glass for bottles, jars and windows because it is the easiest to work, borosilicate for flasks and beakers that will be heated because it resists thermal shock, and lead glass for lenses and glass-to-metal sealing because it has a higher refractive index and a lower melting temperature. The price of borosilicate is a furnace that works at 1,100-1,250 °C instead of 900-1,050 °C. Lead glass carries a toxic-metal hazard that the other two do not.
Part of Glass Blowing and Forming
The three working glass compositions and the job each is chosen for.
Each Composition Trades Furnace Effort Against Performance
The carrier article defines three compositions. The recipe for the first is in Glassmaking, which gives 70-75% sand, 12-18% wood ash or soda ash and 10-15% crushed limestone by weight.
| Type | Recipe idea | Working range | Use it for |
|---|---|---|---|
| Soda-lime | Sand, soda ash, limestone | 900-1,050 °C | Bottles, jars, windows |
| Borosilicate | Some soda replaced with borax | 1,100-1,250 °C | Lab equipment, cookware |
| Lead glass | Some soda replaced with lead oxide | 850-950 °C | Lenses, decorative work, sealing |
Soda-lime is the baseline. Its weakness is thermal shock: a soda-lime beaker heated on a flame will shatter, as the carrier warns.
Borosilicate takes direct flame. The carrier gives 10-15% borax in the batch for laboratory ware. True low-expansion laboratory glass is harder to reach than adding borax; if you make it, test a sample piece in a flame before committing a batch to flasks.
Lead glass has a higher refractive index and a lower melting temperature, and it is the type recommended for optics and for sealing glass to metal. Its working range of 850-950 °C is the lowest of the three, so it is also the easiest on the furnace.
What You Need
- A furnace that can reach the upper end of the range you chose; see Glass Melting.
- Borax for borosilicate, or lead oxide for lead glass. Both are specialist inputs.
- A separate crucible for lead glass; do not mix lead melts into a crucible used for food or drinking vessels.
Method: Choose By The Job, Not By Preference
Step 1: Decide whether the item will be heated. If it will be heated on a flame or stove, borosilicate is the only one of the three that is suitable.
Step 2: Decide whether light is the point. A lens or prism wants lead glass; see Lenses and Optical Glass.
Step 3: Otherwise use soda-lime. Containers that are only filled and stored do not need the extra furnace effort.
Worked Example
Furnace effort is set by the top of the working range. Moving from soda-lime to borosilicate raises the top of the range from 1,050 to 1,250 °C, which is 1,250 − 1,050 = 200 °C. The bottom of the range rises from 900 to 1,100 °C, also 200 °C. Lead glass sits 50-100 °C below soda-lime at the bottom (900 − 850 = 50) and the top (1,050 − 950 = 100).
Common Mistakes
- Making flasks in soda-lime. They crack the first time they sit on a flame.
- Annealing all three on one schedule. Annealing points are 510 °C, 560 °C and 430 °C; see Annealing Oven.
Safety
Lead oxide is poisonous to breathe, swallow or handle carelessly
Lead oxide powder and fume from a lead-glass melt carry lead into the body; lead accumulates and its damage is cumulative. Weigh lead oxide wet or in a closed container, melt it outdoors or under a flue that draws away from the worker, and never use lead-glass vessels for food or drink. Wash hands before eating. Keep a separate crucible, tools and a labelled bin for lead work.
Hot glass does not look hot
Glass glows only above roughly 600 °C and the annealing points here are 430-560 °C, so a piece in or near its hold temperature looks cold and burns instantly. See Glass Properties for Blowing.
What’s Next
- Learn next: Tools and Equipment — the kit for any of the three
- Combine with: Laboratory Glassware — where borosilicate pays off