It is highly advisable to evacuate the air trapped in the mixing process by evacuation.
If no vacuum chamber is available, you can also proceed as follows:
First, a small amount of silicone is mixed and applied with a brush as a 1-2 mm thick layer on the original. Allow this layer to rest at room temperature until the surface is substantially free of bubbles and has crosslinked. Then the remaining amount of silicone is mixed and thus poured onto the mould.