[Lilly]

what kind of glue works best to hold plaster together?
for example 2 halfs of an egg mold
im not sure if elmers glue would work for this.

 

[Ugly Joe]

I would recommend a polyurethane glue.
(Gorilla Glue, or Elmers Ultimate Glue...I find the Ultimate Glue to be just as effective, but considerably less in price)

It bonds to practiaclly any surface (glass? not all that well), and once cured (about 6-8 hours) it is sandable and paintable.

I DO highly recommend clamping the piece, however, as polyurethane glue expands as it cures - you may get some that oozes out of the joint, and will need to be scraped/sanded off.

...but that will be a permanent joint.

 

[Lilly]

Ugly Joe cool, thanks -i'm going with the gorilla then if i can't find the ultimate

 

[ScareFX]

Ugly Joe is on target. I started using the polyurethane glues only about a year or two ago ago but I've found them to be very effective for gluing most surfaces.

 

[multipleseans]

I don't know if this will help you, but I took a mold making class ages ago.
When a piece would break we would soak the broken pieces in water for about an hour, then mix a small amount of plaster ( mix it rather thick, and add table salt to make the plaster set quick). Use this mixture to join your broken pieces.