Preparing the Mold

• 1). Wash the interior of the mold with soapy water using a gentle scrub brush. This will remove any residue from previous castings.
  
  
• 2). Rinse the mold and allow it to dry.
  
  
• 3). Brush the interior of each mold piece with a thin coat of petroleum jelly to act as a mold release. Wipe away any excess with a paper towel. You need only a micro-thin layer. If the mold is made of rubber, use a spray-on mold release instead of petroleum jelly.
  
  
• 4). Bolt, clamp or strap multiple piece molds together. If your mold is a single piece (such as flat molds for flagstones), skip this step.
  
  

Creating Solid Casts

• 1). Mix a large batch of ready-mix concrete according to the instructions on the package. It should be thin enough for pouring but not watery.
  
  
• 2). Pour the concrete into the mold, filling it to the top. Take note of how much concrete you use so you can make just enough on future castings.
  
  
• 3). Tap the sides of the mold with a rubber mallet to free any trapped air bubbles. These will rise to the surface and break. If your mold is flat and shallow, tap the table around it to get the same effect.
  
  
• 4). Scrape the concrete at the mold opening flat with a piece of scrap wood if needed.
  
  
• 5). Allow the concrete to cure for 24 hours.
  
  
• 6). Release the concrete from the mold, either by peeling it off in the case of soft molds such as silicone or by prying the mold off in the case of rigid molds such as fiberglass.
  
  

Creating Hollow Casts

• 1). Mix a small batch of concrete following the instructions on the package.
  
  
• 2). Pour the concrete into the mold.
  
  
• 3). Slush the concrete around the mold, coating the inner surface evenly. If needed, scrape the area around the mold's pour spout flat with a piece of scrap wood.
  
  
• 4). Allow the concrete to cure for three to six hours.
  
  
• 5). Mix additional batches of concrete and slush them into the mold, gradually building up the thickness to at least 1 inch.
  
  
• 6). Allow the final layer of concrete to cure in the mold for 48 hours. The extra time will allow for additional hardening as a hollow cast is much more fragile than a solid one.
  
  
• 7). Carefully remove the mold from the concrete.