is there any health hazard to used copper pipes, i.e. mineral deposits, as art/craft material?
Not much of a hazard. Old pipes might have lead joints which are not much of a problem unless you sand them. The dust can be hazardous to kids.
As mentioned, only leaded solder. It would also depend on what the copper pipe was used for before you used it. May not have been only a water line.
In years gone by the solder for copper pipes contained lead. Todays solder is lead free. Other than that there should be no danger. The main mineral on old copper pipe will be calcium.
