How-To  

 

...Cast solid with Jesmonite AC100 - AND get rid of air bubbles

This How-to will tell you how to cast an item solid using Jesmonite AC100. This article assumes you already have a mould to make the item from, if not check out the articles on 'moulding it and casting it solid' from the main How-to index.

Jesmonite AC100 is supplied as a liquid and a powder which are mixed together at a ratio of 1 part liquid to 2.5 parts powder. A Jesmonite thixotrope additive is also available to make the mixed resin into a gel which won't be as runny - this is useful for the first coat inside the mould when we are making hollow items.

Jesmonite is non-toxic and non-flammable as it is water based. Although it has no dangerous fumes like solvent based resins it should still be used in a ventilated area as it has some odour. In this example the work has been carried out indoors with a nearby window open.

This article shows two methods, one that is quick and easy, and another way to do it properly eliminating bubbles and other imperfections from the cast.

For this example we are using a latex mould which has also had layers of cloth added to make help make it rigid and prevent it from shrinking as it dries. The mould has no rigid support jacket as it is quite small and keeps it's shape by itself, therefore we can fill it with resin without it stretching.

Method One - quick and easy, but not as good as Method Two

Jesmonite AC100 is mixed up by hand. However, as we are going to pour it into the mould and we want it to take the detail it has been mixed up a little thinner - around 1 part liquid to 2 parts powder instead of 1:2.5 as directed. The mixed resin is then poured into the mould to the brim:

As we mixed the Jesmonite thinner than directed it takes a little longer to solidify and finally set. After around an hour the mould it pulled off, but in the process some of the base snapped off and it appears that it has not properly set as some is stuck to the mould. However, the rest of the item is in fairly good shape:

Although the base can be cleaned up, on closer inspection we can see that there are also some air bubbles on surfaces that were facing upwards when the item was being cast. This is because air rises out of the resin which is introduced during mixing, and can't easily escape:

So altogether not very satisfactory. However, with a little more effort we can reduce reduce the effects of both issues without any specialist equipment.

Method Two - doing it properly

This time, we have mixed up a smaller amount of Jesmonite AC100 and added the thixotrope additive. This is added at around 2-6g per Kg of mixed resin so you need hardly any. The thixotrope makes the resin less runny so it will stick to vertical surfaces, so it is very useful for making hollow items as per the Jesmonite laminating How-To. For this example the thixotrope will help us eliminate the air bubbles while also allowing us to mix the resin at a ratio of 1:2.5 as directed so the piece will be much stronger.

An initial coat of thickened Jesmonite AC100 resin is applied to the inside surface of the mould. This is brushed on and therefore bubbles on the surface are eliminated. Two coats of thickened resin are applied followed by whatever was left in the pot:

The rest of the space is topped up with Jesmonite that is mixed directed at a ratio of 1:2.5. After around 40 mins it has set, and although to be safe it should be left for longer (1-2 hours) we carefully demoulded it. As you can see, the entire base of the item is intact this time as the resin was mixed as thick as it should have been to start with:

The item looks pretty good all over - mainly as resin was brushed into all the details.

This time there are no air bubbles in the piece:

Jesmonite AC100 can painted with a wide range of paints from cellulose car paints and lacquer to water based acrylics or even house hold gloss and emulsion paints.

Check out the Jesmonite AC100 Strength Testing page with video of destruction testing.