Casting the Stone

Cast concrete or "cast stone," as it is properly called, has been in use literally for centuries. Ancient Romans and Etruscans used lime and other minerals to make concrete that was then formed or cast into various shapes, including decorative sculptures for buildings. Even the ancient Egyptians used a similar process to cast some of their sculptures. The quality of any cast concrete depends on the mix of minerals. Traditionally, lime was the major ingredient. However, even the Romans created both low quality concrete using only lime and higher quality concrete -- which added more expensive minerals. Today, Portland cement is the modern choice and cast concrete is utilized on many buildings as a lower-cost alternative to natural solid stone.

Also important to the quality of most concrete mixes is the amount of water utilized. A soupy mix will yield a casting that is soft and vulnerable to wear and damage. A somewhat dryer mix will yield a more solid stone-like quality concrete.

Another important factor in achieving a quality concrete casting is to make sure all of the air pockets are eliminated as the mold is filled with wet concrete. To do that the concrete should be added slowly and shaken down or tamped into the mold.

Making the Mold

Mold-making is another centuries-old tradition. Molds can be complex like those made to cast automotive parts or as simple as the plastic molds that children use to cast clay toys. There are many compounds used to create molds. The two most common are plaster and silicon RTV (room temperature vulcanizing) rubber. While the mold-making process can sometimes be complex and involve many steps, the basic concept is not a mystery. You can learn to do it yourself. The basic elements needed to make any mold are:

• A model -- from which the replicas will be cast.
  
• A mold medium -- plaster is common, as is silicon RTV.
  
• A sealing medium like shellac -- so the model won't get stuck in the mold.
  
• A parting agent or mold soap -- so the casting won't stick to the mold.
  
• A cleaning solution -- to clean up the mold after each casting.