However, shrinking a cast is a different ballgame altogether. You may consider using materials like alginate, wax or latex rubber that shrink during curing. However, the shrinkage is not proportionate and you will only end up with a deformed version of the original.
What you desire is uniform and balanced reduction with the details staying proportionate to the reduced size!
This is still possible by using special solvents. For this purpose, the original mold should be resistant to solvents or coated with a solvent resistant spray to protect it from the harsh effects of the excess solvent. Blend the silicone rubber with the solvent to get a thin mix. This will be flowy and take considerable time to cure. Once semi-done, demold quickly and leave it to dry.
The solvent evaporates to gradually shrink the cast in a proportionate manner all over. While the level of shrinkage depends on the amount of solvent added, it generally takes 3 days for the cast to reduce to 85% of the original. Leaving it for 7 days will further shrink the size to about 80%. You can make successive casts to get further shrinkage.
Some artists substitute the solvent with odorless mineral spirits that again tend to evaporate, leading to proportionate shrinkage.
Similarly, some water based urethanes are also available that successfully reduce the size of a cast to scale.
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