Condensation Cure Silicone

    What is Condensation Cure Silicone?

Condensation cure silicone is a commonly used rubber for most mould making and casting

applications. Catalyst ratios vary, but the one we regularly use is added at a ratio of 5%.

It is available as a heat resistant versions which can be used for low-melt alloys like

pewter and lead casting.

Condensation silicones use a tin-based catalyst.

Health and safety tip: The main bulk of the silicone is an inert material, however the toxicity of the various catalyst components can be moderate to high and should be handled with care.

    How do I use Condensation Cure Silicone?

1. Remember, after you put the mixing vessel on the scales you must zero the scales. This will cancel the weight of the vessel simplifying the calculations you will do when adding your catalyst.

2. Work out how much catalyst you will need and add it to the mix. To mix silicone effectively, mix the top layer with the catalyst slowly for a minute or so until the catalyst is stirred into the surface layer of silicone. Now, mix this into the main body of the silicone for 3-5 minutes

3.When mixing remember to scrape all silicone from the sides and bottom of mixing vessel and stir it into the mix. Avoid folding the silicone when mixing, if possible as this will only add air to the mix.

4.Once you are happy that the mix is thorough, ask your technician to check it. If it is OK, you can de-gas it. Place the mixing vessel into the vacuum chamber and wait until most of the air has been removed. This usually takes between 5-10 minutes depending on the amount you are de-gassing.

5.When the bubble popping has slowed and clear patches begin to appear at the surface, the silicone is about done.

6.As always, when you pour your material, start from the lowest point and allow the level to rise gradually. This will help avoid trapping air in the mould. Try not to pour the silicone directly onto the master if you are making a mould.

  1. 7.Once cured, after 6-8 hours, you may remove the master/cast. Do not worry about being a little rough with the cured silicone. It is very strong and with good elasticity, although thin silicone skins may tear if you are not careful.


Improving the life of your mould-

Post-curing the freshly cured silicone in the oven for 4-6 hours at 70 degrees centigrade will improve the mechanical properties of the mould and give it a longer working life.

Spray wax release agents can be applied after every casting to extend the life of your mould. Generally, condensation cure silicone rubbers are biodegradable and will begin to weaken in tear resistance after 12-24 months, depending on level of use.

Enlarging your mould-

Soaking cured silicone in white spirits swells the mould, allowing the user to enlarge objects. This is done by completely submerging a condensation silicone mould in a bath of white spirits. This is left for 2-3 days, then removed and dried off and should be cast from immediately.

It is best to cast as soon as possible because the silicone will ‘weep’ white spirits until it has reverted to its original size again.

Warning: This process will break down the chemical structure of the silicone and weaken the rubber leading to little or no tear resistance.

Often the mould becomes unusable after this process so make sure you have made a casting from the original mould first as a precaution against mistakes.

Silicones are available from Bentley Advanced Materials

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


Avoiding mistakes with silicone.

When making a multi-part mould it is essential an even coat of a suitable release agent forms a barrier between any adjoining silicone surfaces.  If you do not the silicone parts will bond together as one.

Also, 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.

Always ask your technician for help identifying safe materials to use if you are unsure.

Note: Condensation cure silicone only sticks to itself porous surfaces and glass.


Mixing unpigmented catalyst into the silicone


Condensation silicone moulds


Life-casting taken from an alginate mould


Condensation silicone multi-part skin mould


Condensation silicone open mould taken from a clay pressing


Condensation silicone open mould taken from a clay press plaster cast