Glass has no single melting point; it passes through a working range, a softening point and an annealing point as it cools. For ordinary soda-lime glass the working range is 900-1,050 °C, the softening point is about 700 °C and the annealing point is about 510 °C, and borosilicate needs a furnace reaching 1,100-1,250 °C. The danger sits at the bottom of that scale: glass stops glowing below roughly 600 °C yet still burns skin instantly.
Part of Glass Blowing and Forming
The temperature bands at which glass can be shaped, softened and stress-relieved, and how composition moves them.
Glass Is Shaped Across a Band of Temperatures, Not at a Melting Point
Glass is not a crystal. It is an amorphous solid, and as the corpus’s glassmaking article puts it, the molecules freeze in a disordered arrangement instead of locking into a lattice. Because there is no lattice to break, there is no single temperature at which it turns from solid to liquid. It stiffens gradually over hundreds of degrees, and every glassblowing operation is a decision about where in that stiffening to work.
Three temperatures matter for a blower, and the carrier article’s table gives all three for each glass type.
| Glass type | Working range | Softening point | Annealing point |
|---|---|---|---|
| Soda-lime | 900-1,050 °C | 700 °C | 510 °C |
| Borosilicate | 1,100-1,250 °C | 820 °C | 560 °C |
| Lead glass | 850-950 °C | 630 °C | 430 °C |
What You Need
- A furnace that reaches the top of the working range. For soda-lime that is 1,050 °C at the pipe; the melting and fining stages in Glass Melting run hotter than that.
- A way to judge temperature. Colour is the only instrument most villages have: dull red 600 °C, cherry red 800 °C, bright orange 1,000 °C, yellow-white 1,200 °C and above, per the same article.
- An annealing oven. See Annealing Oven.
Method: Read The Three Temperatures As Three Different Jobs
Working range. Glass is fluid enough to shape yet viscous enough to hold its form. Below the range it cracks when forced; above it, it runs like water. At the bottom of the range you shape fine detail and finish; at the top you gather.
Softening point. At this temperature a finished piece starts to slump under its own weight. Never place a piece in a chamber hotter than this.
Annealing point. The hold temperature at which internal stress can relax. Every blown piece goes here before it cools. The stress mechanism is in Annealing.
Worked Example
A soda-lime piece leaving the glory hole at the bottom of its working range is at 900 °C. The annealing point is 510 °C, so the piece must lose 900 − 510 = 390 °C before the oven takes over. The carrier article gives about 30 seconds of working time before glass stiffens without reheating, which is why the glory hole sits at the blower’s elbow: the 390 °C drop has to be steered, not left to chance.
For borosilicate at the bottom of its working range the same sum is 1,100 − 560 = 540 °C, and lead glass is 850 − 430 = 420 °C.
Common Mistakes
- Treating the furnace thermometer as the glass temperature. The crucible surface and the glass at the pipe tip differ; judge the glass by its colour and how it hangs.
- Using one oven schedule for every glass. The three annealing points differ by 130 °C between lead glass and borosilicate; a soda-lime schedule will under-soak one and overheat the other.
Safety
Hot glass does not look hot
Glass glows only above roughly 600 °C. The annealing point of soda-lime is 510 °C, so a piece sitting at its annealing hold, or cooling through 500 °C, looks exactly like a cold piece of glass in daylight, and it will burn skin on contact. Treat every piece that has been in a furnace, lehr or flame as hot until the oven has reached room temperature, and never judge it by sight. Pick up work with tongs, and keep an obvious hot-glass tray so a finished piece is never left where someone will grab it.
Silica dust from batch mixing
Glass batch is mostly sand, and sand is crystalline silica. Fine silica dust that reaches the deep lung scars it, and the scarring is permanent, not an irritation that clears. Mix and sieve batch wet or outdoors, never sweep dry spill, and wear the best salvaged respirator available; see Dust and Respiratory Protection, which is written about coal dust but whose limits on cloth masks apply equally to silica.
What’s Next
- Learn next: Viscosity-Temperature Curve — why the three temperatures sit where they do
- Combine with: Glass Types for Different Uses — choosing a composition for the job