Green sand is both a product and a process for casting metals. It is the most commonly used casting process. Green sand casting is suitable for all alloys. In this process, molten metal is poured from a crucible into a one-use, two-part mold, which is opened and recycled after the metal solidifies.
The mold consists of a box with a top part -- the cope, and a bottom part -- the drag, filled with green sand except for a cavity of the desired shape, which is introduced by using a positive mold and compacting the green sand around it. Green sand is mostly silica sand, with a bit of water and binder, such as bentonite. The metal is poured into the mold cavity down a shaft called a sprue and the air and excess metal is pushed out through a riser.
The term green sand refers to the moisture in the sand, which must be fairly carefully regulated for proper results. Too little and the mold will crumble. Too much and the molten metal will spatter and pop, or even explode.
The resulting metal piece has a rough texture that is typically finished and two or more hunks of metal attached that are cut off and reused where the sprue and riser(s) channeled the metal and air.
Many people use green sand techniques to cast aluminum parts in home-built forges in their garages. A traditional hackerly introduction to working with metals is with the building of a furnace and the casting of lathe parts. By the time you've got a working lathe, you're an authority on home-foundry and are ready to start machining. Wood is a relatively common material for positive model pattern making, but anything can be used. I've worked wood and clay into patterns as well as using manufactured objects "as is."
While green sand casting is great for those with avocational interest -- due to low startup cost, ease of workability at home, and broad appeal, it is a widely used industrial process as well because of the flexibility, stability, simplicity, and rapidity of production.