To blow bottles in volume, gather a parison slightly smaller than a two-part hinged mould, lower it in, close the mould and blow firmly, then open the mould, move the bottle to the pontil, finish the neck and lip, crack it off and anneal it. An experienced blower with a helper makes 30-60 bottles a day. At roughly 270 g of glass per half-litre bottle that is 8-16 kg of glass a day, so the furnace and fuel supply, not the blower, set the real limit.
Part of Glass Blowing and Forming
Repeat production of standard bottles and jars in a hinged mould, and what a day’s output demands of the furnace.
A Bottle Run Is Limited By Glass And Fuel Before It Is Limited By Hands
The carrier’s seven steps are a single-bottle method. The production figure is 30-60 bottles per day for an experienced blower with a helper. The helper opens and closes the mould and tends the glory hole and the annealing oven, and this is a two-person job at that rate.
What You Need
- A split mould (see Mold Types and Mold Construction).
- A furnace and crucible sized for the run; see Glass Melting.
- An annealing oven big enough to take the day’s output; see Annealing Oven.
- Corks or stoppers, if the bottles are to be sealed; the glassblowing article finishes the lip with a string rim to reinforce the mouth for corking.
Method: A Bottle In Seven Steps
Step 1: Gather and form a parison slightly smaller than the mould.
Step 2: Lower it into the open mould.
Step 3: Close the mould halves.
Step 4: Blow firmly so the glass expands to fill the mould.
Step 5: Open the mould and remove the bottle on the pipe.
Step 6: Transfer to the pontil and finish the neck and lip.
Step 7: Crack off and anneal.
Worked Example
Take an illustrative half-litre bottle, assumed 7 cm across and 13 cm tall with a 3 mm wall. Surface area is about π × 7 × 13 + 2 × π × 3.5² = 286 + 77 = 363 cm². Glass volume is 363 × 0.3 cm = 109 cm³, and at a density of about 2.5 g/cm³ (recalled, not from the corpus) that is about 270 g. These dimensions are an assumption for illustration, not a corpus figure.
Daily glass at 30-60 bottles: 30 × 270 g = 8.1 kg to 60 × 270 g = 16.2 kg, call it 8-16 kg. The glass-melting article describes a 5-10 kg melt, so the run needs between 8 ÷ 10 = 0.8 and 16 ÷ 5 = 3.2 such melts, in practice one to three. The same article says a 5-10 kg melt burns 50-100 kg of dry hardwood over 6-10 hours, so one melt is 50 kg and three melts are up to 3 × 100 = 300 kg. A day’s bottle run is therefore a wood supply of roughly 50-300 kg, plus air from bellows or a blower, which a helper or a water- or hand-driven blower has to deliver for the full melt time.
Common Mistakes
- Planning bottles per day without planning melts and wood. The numbers above show the furnace limits the output.
- A parison as large as the mould. The carrier says to make it slightly smaller.
- One wooden mould for a long run. A wet wooden mould lasts 50-100 uses per the carrier, so at 30-60 bottles a day it is spent in between 50 ÷ 60 ≈ 0.8 and 100 ÷ 30 ≈ 3.3 days. Keep spares.
Safety
Hot glass does not look hot
A bottle lifted out of the mould is below about 600 °C within a short time and shows no glow while still burning skin. A line of fresh bottles on a bench looks cold and is not. Put them on a marked tray or straight into the oven.
Steam and pinch points at the mould
A wet wooden mould bursts into steam on contact with hot glass, and a hinged mould pinches fingers. Wear gloves and eye protection and keep the helper’s hands clear of the joint while the blower is working.
What’s Next
- Learn next: Annealing Oven (Lehr) — the second bottleneck after the furnace
- Combine with: Mold Construction — making the mould that makes the run possible