Compressed air replaces the glassblower’s breath with a regulated, dry supply switched by a foot valve, which gives each bottle the same inflation and frees the blower’s hands, but it removes the natural limit of lung pressure, so the line needs a regulator, a relief valve and a vent. Mechanising the air does not change the glass: it still works at 900-1,050 °C for soda-lime glass and every piece must still be annealed.
Part of Pneumatic Systems
Mechanising glass bottle production with an air line connected to the blowpipe, so the air pressure, timing and cleanliness no longer depend on the blower’s lungs.
What the Air Line Changes and What It Does Not
In the manual method the blower inflates a gathered gob of glass through a blowpipe, either free or in a dip or split mould; see Glass Blowing and Forming for the glass, the furnace and the 30-60 bottles a day an experienced blower and helper make, and Glassblowing for the basics. The glass, the temperatures (soda-lime glass works at 900-1,050 °C), the mould and the annealing do not change. Only the air does.
What the air line gives:
- The same inflation each time, set by a regulator and a timed valve.
- Two free hands, because the blower no longer needs a breath at the right moment.
- Dry, clean air, if you condition it, without the moisture and contamination of breath.
What it takes away: the lungs are a natural pressure limiter and the air line has no limit but the one you fit.
What You Need
- A blowpipe with a bore of 8-10 mm (Blowpipe Making), with a swivel or a flexible hose coupling so the pipe can turn.
- A regulator to cut the line pressure to the blowing pressure, and a relief valve after it.
- A foot valve for the blower to switch air on and off, with a bleed hole so releasing it vents the hose.
- An air line with a water trap and drain; see Moisture Removal. Water in the air can flash to steam in hot glass and spatter.
- A heat-proof hose or a long section of metal tube near the pipe, so radiant heat does not melt the hose.
Method: Blow a Bottle With the Air Line
Step 1: Gather and shape the parison Gather glass and shape it as for hand blowing.
Step 2: Close the mould Lower the parison into the open mould and close it, per the corpus’s bottle production.
Step 3: Start low Open the regulator from a low setting and press the foot valve in short puffs. The glass should swell steadily and fill the mould.
Step 4: Release and vent Let the foot valve go. The hose vents through the bleed, the bottle holds its shape in the mould, and the blower opens the mould.
Step 5: Anneal Take the bottle to the lehr at once; unannealed glass can crack within hours, as the glass article states.
Common Mistakes
- Pressure set too high. The parison bursts, or the walls thin in one spot and rupture.
- Water in the line. It chills the glass and can spit.
- A hose with no vent. The pipe stays pressurised after the foot valve is released.
- Blowing into a plugged pipe. A blowpipe blocked with glass holds the full regulator pressure; treat a blocked pipe as a pressure vessel and vent the line first.
- A line that runs straight from the receiver. A regulator failing open sends full tank pressure to the pipe.
Safety
The line has no lungs to limit it
A blowpipe fed from a receiver can deliver far more pressure than a person can blow, and a hot glass bubble bursts violently, throwing molten glass and shards. Always run the line through a regulator and a relief valve set below what the glass and mould can take, keep the pipe vented when the blower is not pressing the pedal, wear eye and face protection and heat-proof gloves, and keep bystanders out of the line of the mould. Never let a hose rest on hot glass or the furnace.
What’s Next
- Learn next: Plastic Blow Molding — the same idea with thermoplastic
- Combine with: Air Storage and Treatment — where the clean air comes from