A glory hole is a small refractory furnace kept above 1,100 °C, with an interior of about 300 × 300 × 300 mm and an opening just large enough for the work, used to reheat a piece between shaping operations. Without one a blower has roughly 30 seconds before the glass stiffens, and with one a reheat takes 15-30 seconds. It is fired by a gas burner or by charcoal with forced air, so the air supply and the chimney matter as much as the brickwork.
Part of Glass Blowing and Forming
The small reheating furnace beside the main furnace that keeps a piece at working temperature while it is shaped.
The Glory Hole Is A Separate, Smaller And Hotter-Than-Needed Chamber
The main furnace holds the crucible of molten glass, and every time its opening is used it loses heat. The glory hole takes the reheating job away from it. The carrier article specifies a small refractory chamber of 300 × 300 × 300 mm interior, with the opening facing the blower. Interior volume is 0.3 × 0.3 × 0.3 = 0.027 m³, or 27 litres.
It is built to exceed the working range. The carrier’s 1,100 °C+ clears the top of soda-lime’s range (1,050 °C) and the bottom of borosilicate’s (1,100 °C). The corpus’s Glass Melting article reports charcoal reaching up to 1,400 °C with forced air.
What You Need
- Refractory: firebrick or high-alumina clay; see Refractory Materials and Furnace Building, which describes secondary air ports for higher temperatures.
- Air: a double-action bellows or crank blower; see Bellows Building.
- Fuel: charcoal or gas. For a charcoal glory hole, lump charcoal of 3-8 cm works best and fines restrict airflow, per the glass-melting article.
- A refractory plug or door to close the opening when idle.
- Insulation packed around the outside; the glass-melting article suggests ash, sand or vermiculite.
Method: Building And Running It
Step 1: Build the chamber. 300 × 300 × 300 mm interior with one opening toward the blower, sized for the work. Dry it slowly, then fire it slowly; the glassmaking article gives a week of drying and 24 hours of firing for a glass furnace.
Step 2: Provide a chimney and an air inlet so the draught is steady and the exhaust leaves the workshop.
Step 3: Bring it to heat before the first gather. The pipe tip itself is preheated here.
Step 4: Reheat in short bouts. 15-30 seconds is the corpus figure for a cooled piece. Rotate the work continuously inside, as outside; glass sags if rotation stops.
Step 5: Close the opening when idle. The glass-melting article reports that every opening of a furnace drops its temperature by 50-100 °C; a plug limits the loss.
Worked Example
If a piece needs five reheats during a job and each takes 15-30 seconds, the time in the glory hole is 5 × 15 = 75 seconds to 5 × 30 = 150 seconds. The glassmaking article says 5-10 reheats may be needed in a job, so the range widens to 5 × 15 = 75 s up to 10 × 30 = 300 s (five minutes). The chamber must hold temperature through that, with the opening in use, and still recover between pieces.
Common Mistakes
- Too large an opening. It loses heat and exposes the blower to more radiation.
- Starving it of air. A glory hole that sits at 1,000 °C will not return a piece to working temperature quickly.
- Leaving the piece in too long. Glass above its working range runs like water, per the carrier.
Safety
Hot glass does not look hot
Once a piece leaves the glory hole it falls below the glow threshold of about 600 °C within a short time, and from then on it looks cold while still being far too hot to touch. The same is true of the glory hole’s own front face after it is shut down.
Carbon monoxide and radiant heat
A charcoal fire with a restricted exhaust produces carbon monoxide, a colourless gas that causes headache, confusion and death in an enclosed workshop (recalled general fact, not from the corpus). Vent the chimney outdoors and do not fire it in a closed room. The opening radiates enough heat and glare to burn skin and damage eyes at working distance; wear dark eye protection and cover bare arms.
What’s Next
- Learn next: Free Blowing Techniques — what the glory hole supports
- Combine with: Annealing Oven — the cool counterpart to this chamber