...Mould it with Silicone RTV the Simple Way

This How-to will tell you how to mould an item in Silicone RTV using the most simple method.

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 top jaw set of teeth 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.

Note that the original sculpt was made by firstly pushing the Oil Based Clay into a fibreglass head which is why there are odd ridges and marks in the base piece. However you'll see that all these details get picked up in the moulding process:

The piece is boxed off into a container made from some more clay. This will give us something to pour the Silicone into to stop it running away. Make sure you use firm clay for this and make sure it's stuck to the base properly so it doesn't leak.

Other possible methods for this are to use an ice cream carton or yogurt pot for smaller pieces, or even to build a box out of Lego bricks to make it the most efficient shape:

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 all of a 1Kg kit with all the catalyst supplied:

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 it is fully mixed:

Now it's time to pour the silicone into the box. The recommended way to do this is to pour it into the lowest point first so that no air bubbles are trapped as the level rises. As you can see any air bubbles will rise to the surface AWAY from the piece we are moulding:

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 we can just remove the clay. There's no need to making any rigid supporting mother mould in the this example because we have one solid block of rubber which will stay in shape:

We can now turn it over the remove the clay sculpt. Oil Based Clay releases very well from Silicone so we can almost pull it out in one piece, just a couple of the teeth got left behind which weren't too hard to get out:

Here's the empty mould ready for casting in resin:

Although it looks a funny shape, in fact it's a perfect replica of the original sculpt, and it still fits the card template that the original was sculpted around:

Now we can cast the piece in resin. We will use Jesmonite for this which is poured into the mould, we used a small brush to make sure it goes into all the features and there is no air trapped. Again any air bubbles rise to the surface AWAY from the top of the piece as it is being cast 'upside down'.

After the Jesmonite has set, we left this one overnight, it can be removed. Again Jesmonite releases very well from Silicone so it practically slid out:

And here's the piece next to the lower jaw we made in the example with Thickened Silicone: