Step 1: Plaster Shere
Get yourself a 12 concrete mold 1/2 round they are made for concrete yard I bought the molds from a store on ebay called gostatue they were US $15.25 each i used them during the summer to make h lf spheres for my drive way edges and walk ways it's almost winter so pouring concrete is not really an option so why not play with them with some plaster
Step 2: You will need:
Plaster of Paris
A few mixing vessels
A Stiff stick and small putty knife
Some cheesecloth or Gauze
A throw away paint brush or two
A drill and 1/8" drill bit
A Debris Blower with a small Nozzle
Step 3: Making the Halves
I put 5 layers in my sphere with one layer the 4th layer being a build up on the edge so that when i meet up the edges they have something to rest upon each other just the brushed edges would be to thin to hold up .
Keep your brush clean between coats plaster will set up under water so you have to keep it banged out real good and while your puting coats on dip the brush in the water once in a while to keep it damp but not soaking wet .