How to Cast with Jesmonite®: Step-by-Step Guide

Not sure how much to mix? Use our free Jesmonite® calculator — enter the water weight of your mould and it gives you the exact liquid and base amounts.

What you will need

You do not need a full workshop to begin. A small, tidy kit and a clear work surface will do. Here is the short list of essentials.

• Jesmonite® base powder and Jesmonite® liquid. For indoor decorative pieces such as coasters, trays and vases, reach for Jesmonite® AC100. It is the beginner favourite and comes as a mineral base powder plus a liquid acrylic. (Not sure which variant suits your project? Compare them in our AC100 vs AC730 vs AC84 guide.) If you would rather buy everything bundled together, an AC100 starter kit is the simplest way in (starter kits are available for AC100 only).
• A mould. Silicone moulds release cleanly and are reusable. A coaster or tray mould is an ideal first project because the shape is flat, forgiving and quick to fill.
• Mixing cups and a spatula or stick. A plastic or paper cup works fine; choose a silicone one to be more eco-friendly, as you can reuse it.
• Digital kitchen scales. Jesmonite® is measured by weight, not by eye, so accurate scales matter.
• An apron. Jesmonite® is water-based and gentle on skin, so you do not need gloves, but casting is still messy work.
• Optional: pigments. A little liquid pigment goes a long way and turns a plain cast into something special.

Getting the mixing ratio right

The ratio is the single most important part of learning how to cast with Jesmonite®. Get it right and your cast sets hard and even. Get it wrong and it may stay soft, crack or crumble.

For AC100 the standard mix is roughly 1 part Jesmonite® liquid to 2.5 parts base powder by weight. So for every 100 g of Jesmonite® liquid you would add around 250 g of powder. Because this is measured on scales rather than by volume, keep your cups on the digital scale as you pour each part.

Ratios vary between Jesmonite® variants, so use the one that matches what you are casting: AC100 mixes at 1:2.5 (Jesmonite® liquid to base), while the cement-based AC84 and AC730 mix at 1:5. Weigh both parts on a digital scale for a reliable result.

A useful habit: work out how much total mix your mould needs before you start, then split that into powder and liquid amounts. Mixing too little means a half-empty mould, and mixing too much wastes material, since Jesmonite® cannot be poured once it starts to set.

Not sure how much to make? Many of our own moulds list the amount you need right in the product description. For any other mould, use this simple trick for AC100: fill your empty mould with water and weigh it, add about 10% extra (some mix always stays in the cup), then divide that figure by 2 to get your Jesmonite® liquid amount and multiply the liquid by 2.5 to get your Jesmonite® base amount. You can also work out quantities with an online Jesmonite® calculator or the Jesmonite® app. For a full walkthrough, see our guide on how much Jesmonite® you need.

Step-by-step: your first Jesmonite® cast

Set everything out within reach before you begin, because once the Jesmonite® liquid and powder meet you have around 10 to 15 minutes of working time (a little less if your room is above 25 °C). Follow these steps in order.

1. Prepare your space and mould. Cover the table, check that your silicone mould is clean and dry, and place it on a level surface so the cast sets flat.
2. Weigh the Jesmonite® liquid first. Put your mixing cup on the scales, zero it, and pour in the measured amount of Jesmonite® liquid.
3. Add the powder gradually. Sprinkle the Jesmonite® base powder into the Jesmonite® liquid a little at a time rather than dumping it all at once. Adding powder to liquid (not the other way round) helps it wet through evenly with fewer lumps.
4. Mix thoroughly. Stir until the blend is completely smooth, like single cream, with no dry pockets at the bottom or sides of the cup. This usually takes a minute or two.
5. Add pigment if you want colour. Stir in a small amount of pigment now and mix until the colour is even. Start with less than you think, as a little goes a long way.
6. Pour into the mould. Pour in a slow, steady stream from one point and let the mix flow across the mould, rather than splashing it in from several places.
7. Settle the mix. Gently tap the mould on the table or wobble it side to side to help trapped air rise and the surface level out.
8. Leave it to set. Do not touch or move the mould while it firms up. Let the chemistry do the work.

That is the whole process. Your first cast will teach you more than any guide, so do not aim for perfection.

How to avoid air bubbles

Air bubbles are the most common cosmetic problem for beginners, showing up as little pits on the surface once you demould. The good news is they are easy to reduce with a few simple habits.

• Mix gently but fully. Whipping the mix fast folds air in. Stir with purpose, scraping the sides, but avoid beating it.
• Tap and vibrate the mould. After pouring, tap the mould firmly on the table several times, or wobble it, to bring bubbles to the surface.
• Pour slowly from a low height. A thin, steady stream traps far less air than a splashy pour from high up.
• Pour into the deepest point. Let the mix flow and rise through the mould rather than filling from the top down.

A few tiny bubbles are normal and can be sanded or filled later. Aim for progress, not a flawless first result. For more detail, see our full guide on how to prevent and fix Jesmonite® air bubbles.

Curing and demoulding times

Patience pays off here. Jesmonite® AC100 is typically firm enough to demould after about 30 minutes, though cooler rooms and thicker casts take longer. The piece will feel touch-dry and no longer cold or damp when it is ready.

To demould, gently peel the flexible silicone away from the edges and flex the mould to release the cast, rather than pulling on the piece itself. If it resists, give it a few more minutes.

Remember that demoulding is not the same as fully cured. A fresh Jesmonite® cast keeps getting stronger over the first 24 hours, so handle it carefully at first. Sand any rough edges with fine sandpaper and a sanding block soon after demoulding, while the cast is still relatively soft and easier to work. Leave sealing and everyday use until it is fully cured after 24 hours.

5 common beginner mistakes

Almost everyone hits at least one of these on their first few casts. Knowing them in advance saves a lot of frustration. For the full rundown, read our guide to the most common Jesmonite® mistakes and how to fix them.

1. Guessing the ratio instead of weighing. Measuring by eye or by volume leads to soft or crumbly casts. Always use scales and weigh both parts.
2. Panicking about the working time. Jesmonite® actually gives you a comfortable working window of around 10 to 15 minutes (a little less if your room is above 25 °C), so there is no need to rush or pour half-mixed. Have your mould ready, mix thoroughly, then pour before it starts to thicken.
3. Under-mixing. Dry pockets and lumps left in the cup cause spots and air bubbles in the finished piece. Scrape the sides and base until the blend is completely smooth.
4. Adding too much pigment. You can add up to about 2% of the total mix weight in pigment; go beyond that and it can affect how the cast sets. A little liquid pigment usually goes a long way.
5. Demoulding too early. AC100 is usually ready to demould after about 30 minutes; take it out before it firms up and you risk bending or breaking it. Once demoulded, still let it cure fully before use.

Ready to make your first cast?

Jesmonite® is genuinely beginner-friendly, and the best way to learn is to get your hands mixing. Browse the full Jesmonite® range to pick your materials, or grab a ready-to-go AC100 starter kit and a coaster mould and pour your very first piece this weekend.