How to use Jesmonite

What is it?

Jesmonite is the brand name for a mineral powder and resin mixture. We buy it from Polysil. There are different versions of Jesmonite so check the details to make sure it is for the purpose you require.

Health & Safety

Mixing Jesmonite gives off a vapour, which you may or may not be able to smell. It is temporary but can also cause headaches so work in a well ventilated area.

Always use a dust mask because the fine powder will be in the air. Also wear a set of clothes that you don’t wear regularly as dust will settle on them.

What equipment should I use?

Plastic mixing bowl.

Whisk and mixing sticks. Silicone based tools are ideal because the Jesmonite does not stick to them permanently and can be washed off. Having said that, you still need to do the washing up pretty promptly to avoid thick crusts of dried mixture.

A table that you don’t mind getting dirty (or cover it with a plastic table cloth).

A weighing scale.

Silicone moulds.

Sandpaper of various grades.

Acrylic sealant.

How do you use it?

Jesmonite arrives as a bag of powder and a bottle of liquid resin. Once these two substances are mixed together in a plastic bowl, a chemical reaction will occur. The mixture will be thick like pancake batter, so you need to use a whisk or electric mixing blade to get out all the lumps. It is ready when it is nice and smooth. You have around 15 minutes to play around with it, add colours and pour it into a mould.

You need to gently vibrate or tap the sides of the mould to get rid of hidden air bubbles. You can also use a cocktail stick to get into any hard to reach areas.

Over the next 45-60 minutes, the mixture will heat up and then cool again. Depending on the size of your mould, the mixture should have set and then tip out your item!

How can I colour it?

Jesmonite produce their own range of coloured pigments but there are many other options including acrylic paint, mica powders, and alcohol inks. You need to be mindful of not adding too much colour, as it can affect the setting time and strength.

How do I clean up?

Scrape off excess mixture into a bin because you don’t want it clogging up your drains. Just use tap water and a sponge to wipe equipment into a bucket. Fragments of solid mix will end up at the bottom of the bucket, then it can go in the bin.

How do I finish off?

Most items will need a bit of sanding afterwards. Dampen the back of your item with water, or do the sanding in a basin of water, to reduce dust in the air.

Jesmonite is a porous material which needs sealing. Use several coats of an acrylic sealer (follow instructions of the manufacturer) applied with a brush or sponge.