Jesmonite® Air Bubbles: How to Prevent and Fix Them

Why air bubbles happen

Jesmonite® is a water-based two-part material: a mineral Jesmonite® base powder and a liquid acrylic. When you combine the two and stir, you unavoidably fold air into the mix. Some of that air gets trapped against the mould surface as the material sets, and those trapped pockets become the pinholes and craters you see on the finished face.

A few things make it worse. Stirring too fast whips in extra air, a mix that is too thick can’t release bubbles before it firms up, and intricate moulds with fine detail give air more places to hide. Understanding this is half the battle: your job is to fold in less air, and give what does get trapped a route to escape before the cast sets. AC100 demoulds after around 30 minutes and gives you a comfortable working window of about 10 to 15 minutes (a little less if the room is above 25 °C), so there’s time to work the bubbles out before the cast sets.

Mix to reduce bubbles

A good result starts at the mixing stage, and it starts with the ratio. AC100 uses a mix ratio of 1:2.5 (Jesmonite® liquid to base by weight), so weigh both parts accurately on a decent scale, because a mix that is too thick traps far more air and won’t let bubbles rise.

When you combine the parts, add the Jesmonite® base powder to the Jesmonite® liquid and fold it together to wet everything out. Then mix thoroughly to break up any lumps, keeping the blade below the surface so you aren’t dragging air down into the mix. For larger batches, a high-shear mixer on a drill gives you a smooth, lump-free blend quickly, which means less prolonged stirring and less folded-in air overall.

Once mixed, don’t rush straight to the pour. Let the mix stand for a few seconds and give the cup a firm tap on the workbench. This encourages the biggest bubbles to migrate up and pop before the material ever reaches your mould.

Pouring and tapping technique

How you pour matters just as much as how you mix. Pour slowly, in a thin, steady stream, from around 10 cm above the mould. A thin stream lets trapped air shear out of the mix as it falls, whereas dumping a thick glug from the cup drags a mass of air straight down into the pour.

Aim for the lowest point of the mould and let the material flow and rise to fill the shape. Keep the stream feeding into material that is already there so you aren’t splashing and folding in fresh air.

Tapping is the main way to beat air bubbles. Once the mould is full, get the bubbles moving:

• Tap and vibrate. Gently tap the sides of the mould and give it a light, sustained vibration on the workbench. This shakes trapped air loose so it rises to the surface. A few minutes of patient tapping does more than one hard bang. For larger pieces or very detailed moulds, a vibration plate makes releasing trapped air even easier.
• Pop the survivors. Watch for bubbles breaking the top surface and burst any that appear with the tip of a toothpick or pin.
• Work within the window. Do all of this promptly. As the mix thickens toward its demould point, bubbles can no longer travel, so your tapping needs to happen while the material still flows.
• Only for very detailed moulds. If a mould has a lot of fine structure or detail, you can first brush a thin layer into the cavity before backing up the pour, but for most pieces this isn’t needed, as tapping does the work.

Reslin®, a plant-based casting resin, benefits from the same slow-pour-and-tap approach, so these habits carry across whatever you cast.

Fixing bubbles after demoulding

Even with good technique, the occasional pinhole slips through, particularly on complex shapes. If you demould after around 30 minutes and find small surface craters, you can still save the piece. Let it cure fully first, which for Jesmonite® means keeping it for 24 hours as it continues to harden and strengthen.

Once cured, mix a very small, slightly looser batch of the same material and use it as a filler. Work it into each crater with a fingertip or a small tool, wiping away the excess so it sits flush with the surface. Let that repair cure, then smooth it back. A little at a time works better than trying to skim the whole face.

To finish, sand the repaired areas by hand. Start with a coarser sanding accessory to knock back the filler, then move to a finer variant such as P150 sandpaper on a sanding block to blend the repair into the surrounding surface. Because Jesmonite® is porous, remember to seal the piece before it meets water or goes outdoors, and any small filled spots will all but disappear under a sealer.

Put it into practice

Air bubbles come down to three habits: mix accurately, pour slowly in a thin stream, and tap the mould until the bubbles rise. Master those and your casts come out smooth almost every time. For the full casting routine, see our step-by-step casting guide. If you’re just getting started, an AC100 starter kit gives you the Jesmonite® base powder, Jesmonite® liquid and essentials in one go, or browse AC100 and the full range of accessories to build your setup for bubble-free results.