...Mould it with thickened Silicone RTV

This How-to will tell you how to mould an item in Silicone RTV using the thickener additive. This process is sometimes easier than boxing up the item for Silicone moulding of larger pieces, and also makes a more efficient use of the Silicone moulding compound.

The cure of Silicone RTV rubbers can be inhibited by other chemicals such as latex, and sulphur found in some types of clay. Please check compatibility before moulding. This silicone is NOT safe for use directly on skin or for use with food products such as making ice cube moulds.

For this example we are going to make a Silicone mould from a clay sculpt which is the bottom jaw for a monster mask/prop. This is item is sculpted in Oil Based Clay which is sulphur free, so we know we will have no issues with the Silicone curing:

We have added some extra clay which will stop the Silicone from running down the inside of the sculpt, and this gives us a clear edge to the piece. Since I've had latex on my cutting board before, we covered it with tin foil to make sure we have a clean surface to work on.

Read the manufactures instructions and data sheet before using this product.  Wear gloves and goggles, and use the product in a well ventilated area, or wear a suitable respirator/face mask. The catalyst for Silicone is harmful, so the entire kit should be kept out of the reach of children and animals.

We are now going to mix up our silicone. We mixed roughly half of a 1Kg kit at a time using a gram scale to get the right amount of catalyst:

The catalyst is orange in colour, whereas the base Silicone is white. This enables you to see when it's properly mixed - you should find the whole thing is a uniform salmon pink colour once it's mixed. It's recommended to pour the mixed Silicone into another container so make sure it's mixed properly - otherwise you can never get the base Silicone that's stuck to the side of the container mixed with catalyst. In this case we just mixed it in one container, but if you do this be careful if you scrape the last bit out of the pot because it probably won't be mixed with catalyst.

Here's the mixed Silicone, you can see that it's still quite runny - most of it would just run off if we applied that to the piece right now:

Next we mix in the thickener. This should be added at 3-5% in small amounts until the Silicone is the desired thickness. The main difference you'll see here is that the Silicone is much less runny and mainly stays put:

Now it's time to spread the Silicone onto the item:

Make sure you spread this into every detail and smooth away any trapped air. For this piece we covered the entire sculpt with one fairly thin layer of Silicone:

You can get away with one coat of Silicone for some pieces, but in this case we mixed the rest of the kit up and applied a second coat. We waited around an hour before doing this so the first coat had started to cure, but Silicone sticks to Silicone whether it's cured or not, so this could be done at a later stage:

This silicone has a working time of 90-120 mins and cures in 22-24 hours, according to the manufacturer. We found that it had lost it's tackiness and set through after around four hours, although it won't achieve it's full strength until 20+ hours.

24 hours later...

Now the Silicone has cured it's time to make the mother mould. This is a rigid support jacket which will support the Silicone mould once it is removed from the piece. We did this one in Modroc because it was quite small, although you could use fibreglass, Jesmonite, or just plaster. First we made a dividing wall out Oil Based Clay - it has two 'keys' in it so it always fits back together correctly, and we start to apply the Modroc:

We covered the first side with around 6-7 layers of Modroc until it is covered and looks like it will be strong enough:

The Oil Based Clay dividing wall peels away in one piece:

We applied petroleum jelly to the exposed edge of the plaster wall - this is crucial to stop the second half from sticking fast to it:

We then made the second side of the mother mould with some more Modroc:

Silicone gives incredibly good release for most moulding and casting products, so it was easy to separate the Modroc mould halves after the Modroc had set:

The Silicone mould peeled right off the sculpt:

We used a small clamp to hold the Modroc mother mould back together, and then inserted the Silicone mould back inside. All ready for casting / laminating in your choice of material - we'll be doing this one in Jesmonite and glass fibre matting: