Bubbles are not a fault in the material. They are air that you put into the mix and did not get out again before it stopped moving.
That is good news: every bubble has a cause upstream of it, and each cause has a fix.
Where air gets in
- Mixing too vigorously
- Fast, whipping strokes fold air into the mix in exactly the way you would aerate a batter. Mix firmly but deliberately, keeping the spatula below the surface.
- Pouring from height
- A stream falling a long way drags air down with it and traps it at the bottom of the mould — which becomes the top face of your piece.
- Pouring too fast
- A fast pour rolls over itself and folds air into the corners before the material has settled.
- Detail in the mould
- Fine texture and tight corners hold pockets of air. The material has to displace it, and it needs a moment to do so.
Getting them out
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Mix below the surface
Keep the spatula down in the mix. Scrape the sides and base rather than beating the top.
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Let the mix stand briefly
A short pause after mixing lets some air rise before the material ever reaches the mould — but only if you still have working time in hand.
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Pour low, thin and into one point
Bring the container close to the mould and pour a thin stream into the lowest point. Let the material travel outwards on its own instead of directing it around.
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Tap the mould
Firm, repeated taps against the worktop. Bubbles rise and break at the surface. Do this immediately after pouring, and give it more taps than feel necessary.
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Brush detail first, for textured moulds
For a mould with fine relief, work a small amount of mix into the detail with a soft brush before filling the rest. It displaces the air that would otherwise be trapped against the face you care most about.
Bubbles you find after demoulding
Small surface pinholes can usually be sanded back if they are shallow, or filled with a little mix and sanded flat once set.
Large voids inside the body of a piece cannot be repaired invisibly. That piece becomes a test — note the pour height and the mixing style you used, change one of them, and cast it again.
