Why your Jesmonite® casts have air bubbles (and how to fix it)

Why do air bubbles keep showing up? 🫧

Air bubbles are one of the most common frustrations when working with Jesmonite® materials.

You mix carefully, pour slowly, do everything you think you should… and they still appear. Tiny ones across the surface, or larger ones sitting right where you don’t want them.

The good news is, it’s rarely one big mistake causing them. More often, it comes down to a few small parts of the process.

What’s actually causing them?

Most air bubbles come from how the material is mixed, poured, and settled into the mould.

The most common causes are:

  • Mixing too aggressively, which can whip air into the material without you realising
  • Mixing for too long, allowing more air to build up in the mix
  • Adding powder first, which creates lumps that need extra mixing to break down
  • Pouring too quickly or moving around the mould, trapping air in different areas
  • Not working material into edges and fine details early on

Once that air is in the mix, it needs a way to escape. If it doesn’t, it will stay trapped and show up on your surface after curing.

Simple changes that make a big difference ✨

The good news is that a few small adjustments can noticeably improve your finish.

When mixing:

  • Add liquid first, then powder
    → This helps prevent dry clumps forming at the start, meaning you don’t need to overmix later to break them down
  • Mix steadily, not aggressively
    → A controlled, consistent mix helps combine everything properly without whipping unnecessary air into the material
  • Use the right tools for your batch size
    → For larger mixes, our Jesmonite® mixing blades are designed specifically for Jesmonite®. They help create a smooth, even mix while minimising trapped air and actively helping knock bubbles out during mixing
    → For smaller batches, simple silicone spatulas work really well. They’re great for folding the mix, smoothing out lumps, and gently working out trapped air without overmixing
  • Scrape the sides and bottom as you go
    → This ensures everything is fully combined and avoids dry pockets that can affect consistency

When pouring:

  • Pour in a thin stream into one spot
    → Letting the material flow naturally from one point reduces turbulence and helps avoid trapping air as it spreads
  • Start with a small amount in the mould first
    → Pressing the material into edges and detailed areas early helps push out trapped air before it becomes an issue

After pouring:

  • Tap or gently vibrate the mould
    → This helps bring any remaining bubbles to the surface so they can escape before the material begins to set

Use a vibration plate for more consistent results
→ Our Artmore® vibration plate is designed to gently agitate the mould, helping air bubbles rise and release more effectively. It’s especially useful for detailed pieces or when you want a really clean, professional finish

A smoother finish starts with small tweaks 💡

If air bubbles have been affecting your casts, it doesn’t mean you need to completely change your process.

In most cases, it’s about refining a few simple steps and making small adjustments to how you mix, pour, and settle the material.

Often, it’s those small changes, like switching your mixing order or using the right tools, that make the biggest difference.

Once those tweaks become part of your process, you’ll start to see a much cleaner finish, and air bubbles will become far easier to manage.

Save this for your next pour 💛

Download our Jesmonite® air bubbles cheat sheet – a checklist for next time you’re making!