Mould blowing inflates a parison inside a mould so the glass copies the mould interior, producing consistent shapes with less skill than free blowing. The parison is first blown to about 60% of the mould volume, lowered in while still rotating, then blown firmly to fill the mould. A dip mould shapes only the body with a free top, a split mould gives the whole shape with a seam, and a wet wooden mould lets the glass slide on steam.
Part of Glass Blowing and Forming
Using moulds to repeat a shape in blown glass instead of freehand forming.
A Mould Replaces Hand Shaping With A Fixed Surface
Mould blowing trades the blower’s judgment for the mould’s geometry. It is faster, repeatable and needs less skill, which is why bottles, jars and patterned ware are made this way. The corpus’s Glassblowing article presents it as the technique that “requires less skill than free-blowing and produces more uniform results”. The skills of gathering and starting a parison from Free Blowing Techniques still apply.
| Mould kind | What it gives | Article |
|---|---|---|
| Dip (open-top) | Body shape, free-formed top | Mold Types |
| Split (two or more parts) | Whole shape including top, with a seam | Mold Types |
| Pattern | Ribs or diamonds pressed into the surface | Mold Types |
What You Need
- A mould of the right kind and material; see Mold Construction.
- A parison on the pipe, at working temperature.
- Water for wooden moulds, and an assistant to open and close a split mould.
Method: Blowing Into A Mould
Step 1: Make the parison slightly smaller than the mould, by free blowing to about 60% of the mould volume.
Step 2: Wet the mould, if it is wood. The steam keeps the glass from sticking.
Step 3: Lower the parison in, still rotating the pipe.
Step 4: Close the mould, if split.
Step 5: Blow firmly. The glass expands to fill the mould and copies every detail of the interior.
Step 6: Open the mould and lift the piece out.
Step 7: Transfer to the pontil, finish the lip and anneal.
Worked Example
For a mould of 500 cm³ interior, 60% is 0.60 × 500 = 300 cm³ of enclosed air in the parison before it goes in. The final blow expands that to 500 cm³. The walls thin as the bubble grows, so a gather large enough for the finished wall must be taken at the start; the bottle-blowing article works this through.
Common Mistakes
- A parison too large. It will not fit, or it chills against the mould before filling.
- A dry wooden mould. It chars and sticks.
- Blowing gently once the mould is closed. The glass needs firm pressure to reach every corner.
Safety
Steam from a wet mould
A wooden mould soaked in water produces a burst of steam when hot glass touches it. Keep the face to one side and wear gloves and eye protection.
Hot glass does not look hot
A just-released piece and the mould itself fall below about 600 °C and lose their glow within moments, yet burn skin. Do not touch a mould to test it.
What’s Next
- Learn next: Mold Types — which kind to choose
- Combine with: Bottle and Jar Production — a production run