Mary or Hollybagger2 I would use nails.
You've got to be careful not to crack the castings; you might try using:
~ 5 minute epoxy, you can get that from Radio Shack;
~ Liquid Nails
~ Dow Corning Silicone I caulk, which is usually used for tubs, but will cure using ambient moisture and form a good adhesion on porous surfaces.
Be careful with those originals; in fact you might make a mold off them first, then cast them in a resin which is much more stable that old plaster.
the problem with gluing plaster to anything is, it dries as pretty much a compacted powder. it's like trying to glue wood to flour. the glue will adhere to the outer layer of the plaster, but beign that it is naturally dusty, the small particles will just detatch.i've heard that some people will add bits of fly screening deep into the wet mould so they're sticking out, aand glue those because they hold deeply into the plaster.
HI. i know this question is from the dinosaur period but the good answer was to make a mould from silicone and make another cast and cast wall plugs in it for the wood or buy those sticky back foam picture hangers but varnish the back of your plaster casts first.
I have been making plaster cast of hands and attaching them to a wood base, for 25 years and I place a piece of metal like a large washer to the wrist of the cast, just before it sets up. when the cast dries, I have the metal to wood, instead of plaster to wood. The casts have stayed on the wood for 25 years. I don't know any other way to do it. there doesn't seem to be any glue that will hold,I have tried everything. Hope this helps.
When you have the metal washer in the plaster do you then glue the metal to the wood?
if you make plaster wall ornaments it is better to cast wall plugs into the wet plaster ornament. you can then screw wood into the dry plaster ornament. then screw the wood into the wall. so this method will work for new castings. or cast wire into the back of your wet plaster casting. or use large paper clips. bend one end to cast into your plaster. as with glue i would glue the piece onto a piece of ply wood with epoxy or any other resin as it will glue and seal the plaster back onto the wood. then drill holes and nail the wood onto the wall.