Actually, in many places the next of kin sign an authorization for cremation which specifically states that the metallics (dental appliances, artificial joints, heart pacemaker wires and other implant leads, etc.) are either commingled with the cremains (ashes) or else separated out and "disposed of" by the cremator or funeral home. Actually, none of these metals will melt at the 1800 degrees (F) that most of these ovens operate at.
This is a great secret of the funeral industry - the percentage of families who give this a second thought is obviously tiny - and this is a wonderful source of secondary (untaxed and undeclared) profits to the industry. Very touchy subject in the industry, but the people who cremated my mother were quite open about it when I requested the metallics be returned to me after cremation. They also told me that when they remove a cardiac pacemaker they leave the platinum wires in the body, and they are frequently thrown away or sold as scrap metal - they were not even aware they were platinum!
I added a chapter to my book entitled "Mortuary Metals", for those interested.
Regards - Scott Andrews recyclebiz.com