No More Casting Bubbles: How to Avoid Air Bubbles When Casting

We all hate those pesky bubbles when casting with plaster, Jesmonite, concrete and similar materials. They can spoil crisp details, leave little holes on the surface and make an otherwise great cast feel a bit disappointing.

Luckily, there are a few simple and interesting ways to reduce them significantly. Here are my tips for getting cleaner casts and keeping those bubbles where they belong: out of your mould.

Mix Slowly and Properly

A lot of bubbles begin in the mixing cup. Fast, vigorous stirring whips air into the material ,a bit like making cake batter and thicker mixes can hold onto those bubbles all the way through curing.

Jesmonite AC100 & AC730

Both materials benefit from a steady, controlled mix, but AC100 and AC730 do not feel the same in the cup.

AC100

Jesmonite AC100 is a finer acrylic-modified gypsum composite. It flows more easily and is brilliant at picking up crisp surface detail, but that also means tiny bubbles can show up clearly on smooth casts.

For a standard mix, use the stated 2.5 parts base to 1 part liquid by weight. Add any pigment to the pre-weighed liquid first, then slowly add the base while mixing.

Use a silicone spatula for small batches, scraping the sides and bottom of the cup as you go. Do not beat it like cake batter. Keep the mixing motion low in the cup and steady, folding the material together until it is smooth and free from dry lumps. Once it is smooth, stop. Continuing to stir just adds more air.

AC730

Jesmonite AC730 is a cementitious composite with decorative aggregate, so it is naturally thicker and has more of a stone or concrete feel. It needs a little more care because air can sit around the aggregate and in the thicker mix.

Work to the manufacturer’s 5 parts powder to 1 part liquid ratio by weight and again add the powder into the liquid rather than the other way around.

For a small AC730 batch, use a sturdy flat spatula and press the mix against the side of the cup as you fold it together. This helps break up dry pockets without whipping in air.

For larger batches, a suitable mixing blade on a drill makes life easier and gives a more even mix, but keep it controlled and avoid running it fast enough to create a whirlpool.

Super Plasticiser

For a more fluid pour, Jesmonite Super Plasticiser can be really useful with AC730. It lowers the mix’s viscosity without adding extra water, helping it flow into fine detail, narrow corners and lettering more easily. It is particularly handy when you are casting a detailed silicone mould and need the material to settle before it starts to thicken.

Use the recommended amount for your total mix weight. Jesmonite lists 8–12 g per kilogram and add it during mixing. It is not a replacement for good technique, but it gives you a more workable mix and a little more help getting air out of the awkward areas. Always do a small test first, especially when using a new mould or colour of AC730, as the different bases can feel slightly different in the cup.

Calculation

Jesmonite batch weight (total liquid and powder) x8 /1000 or x12 (max) /1000

Example: 250g (Jesmonite batch) x8 = 2000 / 1000 = 2g (Plasticiser)

With both materials, make sure your mixing pot and tools are clean and dry before starting. Leftover cured material, water or dust can affect the mix, while an under mixed batch can leave dry pockets that tempt you to keep stirring for too long.

Once mixed, give the cup a few firm taps on the table. You will often see small bubbles rise to the surface straight away. This is a useful little pause before you pour, especially with AC730.

Do not be tempted to add extra water or liquid just to make either material runnier. Stick to the correct ratio and use the right pouring, brushing and vibration techniques instead. Changing the mix ratio can affect the final strength, cure and finish.

Plaster

Casting plaster needs a slightly different approach to Jesmonite. Start with clean, cool water in a clean mixing cup, then sprinkle the plaster powder evenly across the surface rather than tipping it into one pile.

Let the powder absorb the water for a short moment before you begin mixing. This is often called slaking. It gives the plaster time to wet out properly, which means you do not have to stir as hard later to break up dry lumps and that means fewer bubbles.

Once the plaster has absorbed the water, mix slowly with a flat spatula. Scrape around the sides and bottom of the cup, pressing any small lumps against the side rather than whisking the mix quickly. You are looking for an even, creamy consistency with no dry powder left.

Do not keep mixing once it is smooth. Plaster has a limited working time, and over mixing can introduce air and make it begin to thicken faster. Give the cup a few taps on the table before pouring, then pour into the mould in a thin, steady stream.

It is important to use the water-to-powder ratio stated for your particular casting plaster. Adding extra water might make it seem easier to pour, but it can leave the final cast weaker, more porous and more likely to chip. If you need more working time or a more fluid mix, choose a plaster designed for that purpose rather than guessing with the ratio.

For detailed moulds, use the same first-layer method: pour in a small amount, brush it into the fine detail, gently tap or flex the mould, then fill the rest. It is a simple extra step that can make a huge difference to the surface of the finished cast.

Concrete

Concrete is usually the trickiest of the three because it contains cement, sand and often larger aggregate. It is naturally heavier and less fluid than AC100, so air can get caught around the aggregate and against the surface of the mould. Those little holes are often called ''bug holes''.

Start by following the mix ratio for the concrete or casting mix you are using. Do not add extra water just to make it pour more easily. It may feel helpful at the time, but too much water can weaken the cured piece, make it more porous and encourage the heavier ingredients to separate.

A concrete plasticiser or superplasticiser is the better option when you need a more workable mix. It improves flow without simply increasing the water content, making it easier for the concrete to settle into fine detail and allowing trapped air to rise out. Use one that is compatible with your chosen mix, at the stated dosage, and test it before using it on a final piece.

Mix steadily until everything is evenly combined. With a decorative concrete mix, use a flat spatula or paddle and scrape the sides and base of the bucket. Avoid whipping it too quickly but make sure it is mixed long enough for the water to coat the cement and move around the sand and aggregate properly.

A note on plasticiser

A plasticiser is one of the most useful additions for concrete casting. It makes the mix more fluid and easier to pour into detailed moulds without needing extra water.

This helps the material settle into corners and gives trapped air a better chance to rise out, leading to a cleaner cast face. Use a plasticiser made for your concrete mix, follow its dosage instructions and test it first a little can make a big difference.

Prepare Your Mould and Pour in Stages

Even a perfectly mixed batch can still trap bubbles once it reaches the mould. Fine text, sharp corners, deep grooves and textured details are the usual trouble spots, so this is where a little extra care pays off.

Before pouring, make sure your mould is clean and completely dry. Any leftover casting material, dust or moisture can affect the surface of the finished piece. Have it sitting on a level, sturdy surface before you mix so you are not trying to move a full mould around later.

High Detial Moulds

Rather than filling the mould straight away, pour in a small amount of material first. Tilt and rotate the mould gently so it reaches the corners, edges and fine details.

For moulds with lettering, texture or sculpted detail, use a small clean, dry brush to push this first layer into the surface.

This helps remove air from the areas where bubbles are most likely to get trapped.

You do not need a thick layer. Just enough to coat the mould face and make sure the material has reached every detail.

Pour slowly and from one point

Once the detail layer is in place, or your mould is ready to be filled . Pour the rest of your mix in a thin, steady stream. Aim for one area of the mould and let the material flow across naturally, rather than moving the pour around quickly.

Pouring from too high up or too fast can create more turbulence and trap air as the material lands. A slow, controlled pour gives the bubbles a better chance to rise to the top before the material begins to set.

Fill in stages

For deeper moulds or larger casts, it can help to fill gradually. Add a little material, tap or flex the mould, then add more.

This gives trapped air several chances to escape rather than sealing it underneath a full mould of material. It is especially useful with thicker mixes such as AC730 and concrete.

Gently squeeze, flex and massage silicone moulds

Silicone moulds are flexible, which is very helpful when you use that flexibility properly. Once there is a small amount of material inside, gently squeeze the sides, flex the corners and lightly massage around detailed areas.

This encourages the material to move into tight spaces and helps trapped air work its way upwards. Do this gently enough to move the mix around, not enough to distort the shape or force material out of the mould.

Once filled, give the mould one final check around any detailed areas, then move on to tapping and vibration.

Tap, Shake and Vibrate

Once your mould is filled, vibration helps trapped air rise to the surface and escape. You do not need expensive equipment to start, but the right method can make a big difference on detailed casts.

Vibration tables

A small vibration table is useful when you are making larger pieces, deeper moulds or several casts at once. Place the filled mould on the table and use short bursts of vibration, keeping an eye on the surface as you go.

You should see small bubbles rise and pop, while the material settles into the mould. A few seconds is often enough. Stop once the bubbles have slowed down over vibrating can cause heavier aggregate, terrazzo chips or pigment effects to settle and separate.

Always keep the mould level while it is vibrating. It is also a good idea to place it on a flat board or tray first, especially with soft silicone moulds, so the base stays supported.

Small vibrating tools for detailed areas

For smaller moulds, or one stubborn corner that keeps catching bubbles, a little handheld vibrating tool can be really useful. A small electric toothbrush, mini massager or similar vibration tool can be held against the outside of the silicone mould.

Move it slowly around the edges, corners and detailed sections rather than keeping it in one place. This gives you much more control than shaking the whole mould and is particularly helpful for fine lettering, small sculpted details and narrow channels.

Keep the tool on the outside of the mould. There is no need to put anything into the wet material. You are simply transferring a gentle vibration through the silicone.

Tapping still works

Do not underestimate a good tap. Lift the mould slightly and tap it down onto the work surface a few times, then tap around the sides with your fingers or a mixing stick. For small, simple moulds, this may be all you need.

The best results usually come from combining the methods: a careful first layer, a gentle squeeze of the mould, then a few taps or short bursts of vibration once it is filled.

Final Thoughts

Bubbles are one of those frustrating parts of casting that everybody deals with at some point. The good news is that you do not need to change everything at once to get better results.

Start with slower mixing, use a small first layer to work material into the details, then tap, flex or vibrate your mould before the mix begins to set. For thicker materials such as AC730 and concrete, a suitable plasticiser can also give you a much more workable pour.

Every material, mould and design behaves a little differently, so keep experimenting. A quick test cast can save a lot of sanding, filling and disappointment later and once you find the combination that works for your mould, stick with it.

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