...Mould it and cast it in something solid

This How-to will tell you how to make a flexible mould for a piece with a rigid support jacket and then cast it in something solid so it turns out the same as the original sculpt.

Firstly you will need to sculpt your piece, the picture below shows the custom item sculpted in an non-air drying, sulphur-free oil based clay. This clay is 'chemical free' so it will not affect the moulding substance and cause any issues.

Important: Oil Based Clay is reusable - you can use it time and time again to make different sculpts once you've finished with the moulding stage of the process.

The piece is placed onto a plastic board - an old chopping board in this case:

We will use latex to make the mould, although you could investigate other moulding rubbers such as Silicone RTV or Urethane moulding compounds. Latex is the cheapest but you generally get what you pay for, so the downsides are that it will shrink as it dries and can also only be built up in thin coats which can take a while. However, latex is good for low cost one off pieces and suitable for the purpose of this How-to.

Although latex is not solvent based, it is always best to use it in a well ventilated area as it has a bit of a smell to it - rather like ammonia.

Latex is air drying, so any brush you use to apply it will also get dried latex in which is very hard to get out. To avoid ruining too many brushes you should keep the brush submerged in latex and covered with a plastic bag in between coats. This will allow the brush to at least last as long as it takes to make the whole mould:

Although latex is brushed on in this example, you can also dip the item in latex to built up thin coats, however this is only really practical for items that can easily be suspended from the base such as chess pieces etc.

You'll need to apply thin coats of latex to start with, and try to brush out any bubbles, allow around 30 mins to one hour drying time in between coats. You can tell when the latex is dry enough to apply the next coat as it will become almost clear to start with, and as you build up layers it will become brown:

Here's the first coat just on and still wet:

Here's the piece after about ten coats have been applied, you can see the latex has gone a very brown colour:

You could continue to apply coats to build up the mould thickness further, but this becomes time consuming. Depending on the piece, you may prefer to add layers of cloth to the mould - this will help build up the thickness quicker and will also stop the mould shrinking so much as it dries. However, it will also prevent the latex mould from being as stretchy, so you must consider that any details may get trapped inside the mould as you'll never be able to get the piece out.

In this case the cloth has been applied like a 'tent' over the whole piece so that the 'blob' parts on top are not trapped. This cloth is just cheap dish cloth from a high street household store, but it is very absorbent so it will soak up the latex enabling a thicker mould to be built up quicker:

Once everything has dried, we will be left with a thick latex and cloth mould. However, this will still be quite flexible, but at this stage we also have the opportunity to add a rigid support jacket. The support jacket will fit perfectly over the latex mould to hold it straight while we cast from it. For this example we will make the support jacket out of modroc plaster of bandages:

Modroc plaster of Paris bandages are built up over the main part of the mould. Cut the bandage roll into pieces, make each one wet in a bowl of water, try to get as much excess water out as possible, and then layer them up all over the piece. Around 4-5 layers were used in this example:

Once the plaster bandages have dried out thoroughly, they can be removed from the latex part, and the latex can be peeled from the clay sculpt. This should leave you with a rubber mould that fits back inside the rigid plaster bandage support jacket:

At this point we have finished making the mould of the piece and we are now ready to cast from it. Of course you could just pour plaster or some other casting resin into the mould and make a solid piece. However, in this example we are going to make a rigid and hollow shell from Jesmonite AC100 and glass fibre fabric. This will be much lighter than a solid object and also be very strong due to the reinforcement.

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.

Firstly we have have mixed up a small pot of Jesmonite as directed. This was measured out with a set of scales and mixed by hand. For large amounts of Jesmonite over 1Kg it is recommended to mix it with a blade in a power drill. This is largely so it can be mixed properly before it starts to solidify which takes around 8-15 mins. However, in this case we used a teaspoon handle.

The Jesmonite thixotrope was added until the resin was no longer runny. This should be added at 2-6g per 1Kg of mixed resin, so this is a very small amount. In order to get this right, the handle of the teaspoon was dipped in the thixotrope about 2cm and then used to mix it into the resin - that's literally all you need.

A nice thick coat of thickened Jesmonite AC100 is applied to the mould, making sure that it went into all the details. You may wish to mix and apply several thickened coats until you can no longer see the rubber mould through the resin, allowing each a few minutes to set in between:

The next coat we will apply will be a thin glass fibre fabric called surface tissue. This weighs approximately 30 grams per square metre, so it is very lightweight as easily conforms to the surface. This coat will bond to the initial gel coat and make a rigid shell which we can apply a thicker fabric to without cracking the gel coat when pressure is applied. The surface tissue is damped down all over with more mixed Jesmonite resin, this time with no thixotrope added:

The next step is to apply some proper reinforcement. Once the surface tissue coat is dry to the touch the next layer of thicker fabric can be applied. In this example we are using Jesmonite Stitched Quadaxial mat. This an open weave multi-axial glass fibre mat which is easy to wet through with resin. Although the manufacturer of Jesmonite recommends only using Quadaxial mat, other glass fibre fabrics can be used. One layer of Quadaxial mat is applied and wet through with mixed Jesmonite AC100 resin, shortly followed by a second layer:

Allow at least an hour for the whole thing to set. Jesmonite AC100 has an active catalyst so it will set solid even in thick layers, it is not reliant on air drying. However, it will be softer for the first 24-48 hours before full strength is achieved, so if you need to sand, drill or trim the piece, this is the time to do it.

Once it has all set, you can remove the plaster bandage support case and peel off the latex mould to reveal the piece which should be exactly the same as the original sculpt. Of course the mould can be used time and time again to produce multiple pieces the same:

The excess can be trimmed from the piece and it can be generally finished as you wish. 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.