Jesmonite® Not Setting? Causes, Fixes and Cure Times

How long should Jesmonite® take to set?

First, make sure you are not just being impatient. Jesmonite® sets in stages, and the piece is fragile long before it is finished.

For AC100, the go-to variant for indoor decorative casting, you can usually demould after roughly 30 minutes. At that point the piece may still feel warm from the chemical reaction that has taken place, but it is firm enough to demould. It should hold its shape and no longer take a fingerprint when you press it gently; if it still dents, give it a little longer. It will never be sticky, though it can still feel slightly damp at this stage. It is not fully cured yet, though. AC100 keeps hardening over the first 24 hours, so it feels a little soft and chalky when you first pop it out and steadily gets tougher over the course of that first day.

The cement-based variants take longer to reach that stage. AC84 is ready to demould after about 2 hours and AC730 after around 4 hours, and both then reach full strength over the first 24 hours, exactly like AC100. So always check which variant you are working with before you decide the cast has failed: a piece that looks like it is “not setting” at 30 minutes is often simply an AC84 or AC730 cast that just needs its longer demould time.

So if your cast has only been sitting for ten minutes, that is normal. Give it the full demould window for your variant first. If it is still genuinely soft, sticky or crumbling well past that time, one of the reasons below is the culprit.

1The mix ratio is off

This is by far the most common cause. Jesmonite® is a two-part material: a mineral Jesmonite® base powder and a liquid, combined at a specific ratio by weight. AC100 uses a standard ratio of 1 part Jesmonite® liquid to 2.5 parts base powder, but always use the exact ratio for your own variant, as it varies between variants (AC84 and AC730, for example, use 1 part liquid to 5 parts base).

Get that balance wrong and the chemistry cannot complete properly:

• Too much liquid gives you a soft, weak, chalky cast that stays tacky and never firms up the way it should.
• Too little liquid makes the mix thick and fast, so it starts curing before you have poured, trapping bubbles and leaving a rough or crumbly result.

If your last batch was soft and sticky, you almost certainly added too much liquid or not enough powder. Adjust and try again with a fresh, accurately weighed batch. Our guide on how much Jesmonite® you need helps you get quantities right.

2Inaccurate scales

Even with the right ratio in mind, you can only hit it if your scales are honest. A lot of “why won’t it set” problems trace back to measuring, not mixing.

Small, cheap kitchen scales often round to the nearest 5 grams or drift as the battery fades. On a small casting that is a big proportional error, easily enough to push you into the soft-and-sticky zone. A few tips:

• Use digital scales that read to 1 gram (0.1 g is even better for tiny jewellery pours).
• Always weigh both parts, Jesmonite® base powder and Jesmonite® liquid, rather than measuring the liquid by eye or by volume.
• Tare (zero) the scale with your empty mixing cup on it before you start, and check the battery if numbers jump around.

Weigh accurately and you remove the single biggest source of a failed set.

3It was not mixed thoroughly

Right ratio, accurate scales, and it still came out patchy? Then the two parts may not have been fully combined. If pockets of dry powder or unmixed liquid are left behind, those areas cannot cure and will stay soft, lumpy or chalky, often with spots and trapped air bubbles, while the rest of the piece sets fine.

To mix properly, stir until the blend is completely smooth and lump-free, scraping the sides and bottom of the cup where dry powder loves to hide. For anything more than a tiny pour, a high-shear mixer on a drill is far more reliable than a hand stir, and it doubles as your best defence against air bubbles. AC100 gives you a comfortable working window of about 10 to 15 minutes (a little less if the room is above 25 °C), so work steadily and thoroughly before the mix begins to thicken.

4Not enough cure time

Sometimes nothing is actually wrong. The cast simply has not finished. As noted above, a demoulded AC100 piece feels soft and chalky at first and only reaches full strength after 24 hours. If you seal or start using it too early, it can feel weak and disappointing even though it would have been perfectly solid the next day.

A related trap is leaving the cast in the mould too long. Jesmonite® can “sweat” if it sits well past its demould time, leaving surface marks or a slightly tacky skin. Aim to demould around 30 minutes, sanding is easiest now, while the cast is still relatively soft, then let the piece cure in the open air for a full day before sealing or use.

Cool, damp conditions can also slow curing down, so give your pieces a warm, dry, well-ventilated spot to harden.

5A wet or dirty mould

Your silicone mould needs to be completely clean and dry before you pour. Water still sitting in the mould, or leftover residue from Jesmo Clean or another cleaner, interferes with the surface as the cast sets, leaving marks and patchy spots or soft, weak areas where the mix met that moisture. Wash moulds with warm water and mild soap, rinse well, and let them dry fully before the next pour: a proper drying time is all it takes to avoid this.

How to rescue or prevent it next time

Can you save a bad cast? Honestly, a piece that has properly under-set is best treated as a learning batch rather than rescued. You cannot re-cure a cast that was mixed wrong. What you can do:

• If it is merely young, wait. A soft, chalky piece that was mixed correctly usually just needs the rest of its 24 hour cure. Leave it somewhere warm and dry before you judge it.
• If it is crumbly or permanently tacky, start fresh. Note what likely went wrong (usually too much liquid or a measuring slip) and mix a new, accurately weighed batch.
• Sand early, seal once fully cured. Tidy edges with P150 sandpaper on a sanding block as soon as you demould, while the cast is still relatively soft, then protect the porous surface with a suitable sealer once the piece is fully cured, before any contact with water.

Prevention is simpler than any rescue. To get a reliable set every single time:

1. Start with a clean, completely dry mould, with no water or cleaner residue left behind.
2. Use the exact mix ratio for your variant (1 part Jesmonite® liquid to 2.5 parts base by weight for AC100).
3. Weigh both parts on accurate digital scales that read to at least 1 gram.
4. Mix thoroughly until smooth, scraping the sides and bottom.
5. Demould at about 30 minutes, sand while the cast is still soft, then cure 24 hours before sealing or use.

If measuring is where things keep slipping, one of our AC100 starter kits takes the guesswork out by pairing the Jesmonite® base powder and Jesmonite® liquid in the right proportions, so you can focus on the fun part.

Ready to try again with confidence? Browse our full range of Jesmonite® variants, pigments and moulds at gigacrea, all in stock and dispatched fast, and get your next cast setting perfectly. For more troubleshooting, see our guide to the most common Jesmonite® mistakes.