How do I use silicone?

Silicone is available in many grades of hardness. Rubber hardness is described by a number ranging on a scale from 0-100. This is called a ‘shore harness’ number. The higher the number, the harder the material.

There are two main types of silicone used for mould making and casting:

  1. -Condensation cure

  2. -Addition cure

Both are very different and are not compatible with one another.

There are two important things to remember about silicone:

The first is that in order for it to change from its liquid state to a solid we must add its relevant chemical catalyst.

Each brand and type of silicone has it own specific catalyst and percentage, so always check the correct catalyst ratios before you start mixing. I have highlighted below some of the main differences between the two main types of silicone rubber.

The second and most important point to remember is that this catalyst must be mixed into the main body of silicone completely. Because of the silicone’s extremely thick nature it is a difficult, laborious and lengthy process, and it is difficult to know when it has been mixed enough because there is usually no change in colour.

Tip: Adding pigment to the catalyst is a great indicator of how well the two parts are mixing. Every compound material has different pigments designed specifically for it, so ensure the pigment you are using is compatible with condensation cure silicone before you start.

Avoiding Mistakes with Silicone

Remember, preparation is the key to obtaining a professional looking finished product! So that extra bit of time you spent mixing your silicone is a sound investment.

‘Doing it properly the first time means not having to do it twice!’

When mixing silicone, it is absolutely critical that the mix is consistent, if it has been incorrectly mixed, the silicone cannot ‘vulcanise’ properly (become its solid state or ‘cure’) and will either take several days to cure, cure with patches of liquid silicone running through it or it might never cure at all.

Another common mistake is the use of inappropriate release agents. Silicones do not bond to many surfaces so usually no release agents are required at all, but if you are advised to use a release agent, for example when making a multi-part silicone mould, petroleum jelly is best.

Another important rule to remember is that silicone only bonds to other silicone of the same type.

If the silicone is contaminated by an incompatible material an effect called ‘inhibition’ may take place, this is always easily recognisable by a slimy coating on the surface of your mould or casting, which will never cure fully. This effect is more commonly seen when using addition cure silicones because of their more temperamental nature, but can affect condensation silicone too.

Any cured silicone may also cause inhibition on any uncured silicone surface, depending on the type in use, so it is essential an even coat of a suitable release agent forms a barrier between any adjoining surfaces.

It is not recommended that silicone moulds be used to cast silicone parts, because it is difficult to ensure a complete seal of the surface especially with when dealing with complex shapes and fine detail.

If a rubber component is required from a silicone mould, a polyurethane rubber should always be the preferred casting material; or vice-versa.

Silicones are available from Bentley Advanced Materials

Prices vary: £15.00-£30.00 p/kg for most types

What is Silicone?

Silicone’s are highly durable materials with excellent resistance to chemical corrosion, fire and extremes of temperature. They are also pliable, waterproof and electrically insulating.

It is not surprising therefore that silicone has a large number of uses in everyday life such as rubber tubing, tyres, mobile phone keypads, electrical tape, bicycle handle bar grips and bath mats. Fabrics coated with silicones are used to make sails, parachutes, and hot air balloons. Cosmetic products and medication, moisturizers, creams, lubricants and oils are all made possible by this highly versatile silicon-oxygen polymer called silicone.


  1. Condensation cure silicone-

- Good for most mould making and casting applications

- Usually added at a ratio of 5%, but may vary

  1. -Good for prosthetic work

  2. -Uses a tin-based catalyst

  3. -More resistant than addition cure silicone

  1. Addition cure silicone-

- Good for most mould making and casting

  1. -Special effects applications and paints

- Available in a food safe and skin safe version.

  1. -Use 10% platinum-based catalyst, but may vary

  2. -Available in a transparent version


Addition silicone skin over plastic lighting unit


Skin-safe addition silicone face mould


Silicone mould taken from a plaster cast


Two-part, condensation silicone skin mould with a fibreglass jacket or mother mould


Transparent addition cure silicone split mould


 Silicone life-casting


Silicone open moulds


Some objects require the whole shape being broken into separate components and moulded according to their shape and size