Jesmonite® Mistakes: 10 Common Errors and How to Fix

1. Getting the mix ratio wrong

This is the number one cause of failed casts. Jesmonite® is a two-part material, and the balance between Jesmonite® base powder and Jesmonite® liquid is what makes it set hard. Too much Jesmonite® liquid gives you a soft, chalky result that never fully firms up; too little makes the mix thick and fast, so it sets before you’ve filled the mould. The fix is simple: measure by weight and stick to the correct ratio for your variant. AC100 is 1 part Jesmonite® liquid to 2.5 parts base powder by weight (1:2.5), while AC84 and AC730 are 1:5. Not sure how much to mix? Most of our own mould product descriptions list the amount you need, or you can use the AC100 water trick (fill the mould with water, weigh it, add about 10%, divide by 2 for the Jesmonite® liquid, then multiply that liquid by 2.5 for the Jesmonite® base), an online Jesmonite® calculator or the Jesmonite® app. Measure every batch the same way. For more on getting quantities right, see our guide on how much Jesmonite® you need.

2. Using inaccurate scales

Even with the right ratio in mind, a cheap or unreliable scale will quietly ruin your castings. Jesmonite® is weighed, not measured by volume, so a scale that drifts by a few grams throws the whole batch off, especially on small pieces where the margins are tight. Use a digital scale that reads in single grams, always zero (tare) your mixing cup first, and add the Jesmonite® base powder and Jesmonite® liquid one at a time so you can watch the numbers. If results are inconsistent from one pour to the next with no obvious cause, your scale is the first suspect.

3. Trapping air bubbles

Nothing spoils a smooth surface like a rash of tiny pinholes. Bubbles get whipped into the mix and then trapped against the mould wall. The main way to beat them is to gently tap and vibrate the mould after pouring, so trapped air rises to the top, then burst any stubborn surface bubbles with a toothpick. Pouring slowly in a thin stream from around 10 cm above the mould also helps, as bubbles pop on the way down. Only for moulds with a lot of fine detail, you can first brush a thin layer of mix (1 to 2 mm) into the cavity before you back it up with the rest, but for most pieces this isn’t needed. For larger pieces or very detailed moulds, a vibration plate makes releasing bubbles even easier. For a deeper dive, see our guide on preventing and fixing Jesmonite® air bubbles.

4. Mixing too slowly or leaving lumps

AC100 gives you a comfortable working window of about 10 to 15 minutes (a little less if the room is above 25 °C), so there is no need to panic, but a slow, half-hearted stir still leaves you with lumps of unmixed powder that show up as spots and trapped air bubbles in the finished cast. You want the powder fully wetted and the mix smooth before it starts to thicken. Add the Jesmonite® base powder to the Jesmonite® liquid (not the other way round), and mix thoroughly. A high-shear mixer attachment on a drill makes short work of larger batches and gives a much more consistent result than a hand stir, which matters more the bigger your pour gets.

5. Adding too much pigment

It’s tempting to keep adding colour until the mix looks rich in the cup, but pigment is powerful and a little goes a long way. As a guide, keep pigment to no more than about 2% of the total mix weight; go beyond that and it can affect how the cast sets. Start with a small amount, mix it in fully, and remember the wet colour always looks darker than the cured piece. Our liquid pigments are highly concentrated, so a little goes a long way across many projects. Because Jesmonite® is opaque, pigments give you solid, even colour. To test shades safely, cast a few small samples before committing to a full batch.

6. Leaving the cast in the mould too long

More time in the mould does not mean a better result. Left too long, a Jesmonite® cast can “sweat”, drawing moisture to the surface and leaving cloudy marks or a patchy finish where it sat against the silicone. The fix is to keep an eye on the clock rather than walking away for hours. AC100 is usually ready to come out at around 30 minutes, once it feels firm and cool to the touch. Demould at that point, then let the piece finish curing in the open air.

7. Demoulding too early

The opposite mistake is just as common: pulling the cast out before it has set enough to hold its shape. Rush it and you get bent edges, cracks, or fingerprints pressed into a soft surface, and thin or detailed pieces are the most fragile. If the cast still feels warm or flexes at all, give it a few more minutes. Press gently at the thickest point; it should feel firm and cool, not tacky or springy, before you ease it from the mould.

8. Not sealing before contact with water

Jesmonite® is porous, so a raw cast will happily soak up water, drink stains and moisture marks. People often discover this only after a coaster leaves a ring or a dish stains from the first use. The fix is to seal anything that will meet liquid. For indoor decorative pieces made with AC100, an acrylic sealer in satin or gloss protects the surface and makes it easy to wipe clean. Always let the piece cure fully first, then apply the sealer in thin, even coats rather than one thick layer. For the full method, see our guide to sealing Jesmonite®.

9. Using the wrong Jesmonite® for outdoors

AC100 is built for indoor decorative work, so using it for a garden planter or an outdoor ornament sets you up for disappointment when the weather gets to it. For anything that lives outside, reach for a hard-wearing formulation instead. AC730 is cement-based and suited to both indoor and outdoor use, while AC84 handles the outdoors too and is easy to pour. Whichever you choose, seal outdoor pieces with a penetrating or Flexiguard sealer for invisible water protection, and add drainage if live plants are involved.

10. Skipping the curing time

Demoulding is not the finish line. Jesmonite® keeps hardening and gaining strength over the first 24 hours after it comes out of the mould, and it needs that time before it’s ready for sealing or everyday use. Sanding is different: tackle it as soon as possible after demoulding, while the cast is still relatively soft and easier to work, tidying up any edges with a sanding block and P150 sandpaper. Save sealing for once the piece is fully cured, because sealing or stressing it too soon risks a weaker surface or trapped moisture under the sealer. Set your freshly demoulded casts somewhere dry and leave them to cure fully before you seal and finish them.

Start on the right foot

Most Jesmonite® mistakes come down to guessing about quantities, so the easiest way to avoid them is to begin with everything pre-matched in one box. An AC100 starter kit pairs the Jesmonite® base powder, Jesmonite® liquid and the essentials you need, which takes the guesswork out of your first few pours. Pick a kit, weigh carefully, and you’ll be past the beginner traps and casting clean pieces in no time. Order before 2 pm and it ships the same day.