Glass moulds are made from wet fruitwood, cast iron or graphite. Wet fruitwood such as cherry or apple lasts 50-100 uses, because the steam it makes keeps glass from sticking. Cast iron lasts longer but the glass sticks unless the cavity is coated with lampblack soot, and graphite is the best material because glass does not stick, it conducts heat evenly and it lasts very long. Clay fired to stoneware temperature is a middle choice that is easy to carve.
Part of Glass Blowing and Forming
Choosing and making mould materials for blown glass, and how each wears out.
The Material Decides How The Glass Releases And How Long The Mould Lasts
| Material | Release | Life | Source |
|---|---|---|---|
| Wet fruitwood | Steam barrier stops sticking | 50-100 uses (carrier); 50-200 blows for hardwood (glassblowing article) | Cherry, apple |
| Cast iron | Sticks unless coated with lampblack | Longer than wood | Needs casting |
| Graphite | Glass does not stick | Very long | Hard to source |
| Stoneware clay | Carved before firing | Durable | Fired in a kiln |
The two life figures for wood overlap: the carrier says 50-100 uses for wet fruitwood and the Glassblowing article says 50-200 blows for hardwood moulds including cherry and maple. Plan on the lower figure.
What You Need
- Wood: a dense, close-grained fruitwood block, and a bucket of water.
- Iron: a casting mould and the metal. The corpus’s Mold Making article is about brick moulds and does not cover casting iron.
- Lampblack: soot scraped from an oil lamp or a smoky flame.
- Graphite or stoneware clay, if available.
Method: Building A Two-Part Wooden Mould
Step 1: Split the block or take two blocks, and join them with pins or hinges so they close exactly.
Step 2: Cut the cavity, with the joint line through the centre so the finished piece can lift out.
Step 3: Smooth it. The glass copies every detail of the interior, so rough surfaces will appear on the bottle. Any sharp edge can crack the glass.
Step 4: Soak in water before first use and keep it wet.
Step 5: For an iron mould, coat the cavity with lampblack before each blow.
Worked Example
If a wooden mould lasts 50 uses and a day’s run is 30-60 bottles from one mould (see Bottle and Jar Production), one mould gives 50 ÷ 60 ≈ 0.8 to 50 ÷ 30 ≈ 1.7 days of work. A run of weeks needs a spare mould for every day or two.
Common Mistakes
- Letting the mould dry out. It chars and sticks.
- Rough cavity. Marks and cracks.
- No lampblack on iron. The glass sticks.
Safety
Hot glass does not look hot
A graphite or iron mould stays hot for a long time after it stops glowing, and a cavity that was in contact with glass may be near 600 °C with no visible sign. Treat every mould as hot until it has been left to cool.
Graphite and soot dust
Sawing or carving graphite, and scraping lampblack, makes fine dust; do it outdoors or wet, and keep it out of the lungs.
What’s Next
- Learn next: Bottle and Jar Production — the moulds in use
- Combine with: Mold Types — the cavity shapes to cut