Cultured marble is polymer resin mixed with marble dust or other whitening agent, such as calcium carbonate. Casting cultured marble involves how much of the additive you want to use; if you're casting a figure with fine detail, too much additive will prevent the resin from filling all the small voids in the mould. If, on the other hand, you're casting a simple shape like a column, using plenty of stone additive will give you the look of real polished stone.
Apply mould release to the mould if recommended by the manufacturer. Urethane, latex and silicone moulds typically require mould release when used with casting resin.
Measure both parts of the casting resin into a clean mixing container as directed by the manufacturer. Many casting resins are combined in a 1-to-1 ratio, but some use small amounts of hardener added to a much larger quantity of resin.
Mix the two parts of the resin together thoroughly for two minutes or as suggested by the manufacturer. Scrape down the sides and stir contents up from the bottom with the stirring stick.
Stir marble powder into the resin a little at a time until you reach the desired colour and consistency. Mixtures for highly detailed moulds should be liquidy; mixtures for simple shapes can be much thicker, up to the consistency of peanut butter.
Pour the resin mix into the mould, a little at a time. If the mix is thick, gently agitate the mix in the mould after each pour to settle the mix into the mould.
Lightly heat the exterior of the mould with a hair drier to prompt bubbles to rise to the surface.
Allow the resin to fully cure for 72 hours at room temperature.
Remove the casting from the mould. If the mould is rigid, twist the casting slightly to free it. If the mould is flexible, carefully peel it away from the casting.
Several colours of marble powder are available as casting mixtures. You can also use granite, copper and brass powders to create the look of granite or metal sculptures. If you need a fairly liquid casting mixture, you can whiten the resin before adding the marble powder; this will give you the desired colour without having to add a high proportion of solids to the mix.
Most casting resins emit harmful vapours; do your casting in an area with good ventilation. Be careful when heating the mould; high temperatures can melt some flexible moulds. Do not attempt to remove the casting for 24 hours after pouring the resin or you will damage the casting. Resins with powders mixed into them tend to produce bubbles, which can ruin the casting if they form at the surface of the casting. Be careful to work slowly and gently to heat the mould to remove bubbles before the resin cures.