Choose a resin epoxy glue, or multipurpose construction adhesive that's suitable for your application. Use a liquid epoxy for large open cracks, and epoxy putty or construction adhesive for surface patching.
Clean the surface of the urn thoroughly with soap and water. Wipe it completely dry with paper towels. Sand the area to give the adhesive good surface contact. Cut a piece of hardware wire larger than the crack in the urn and set it aside.
Work adhesive or epoxy into the crack, if possible, following the package instructions. Take care not to expand the damage. Create a brace by tying a nylon rope around the urn to pull the crack tightly together. Insert a metal rod under the rope to form a handle. Twist the metal rod to tighten the nylon rope further. Tuck the end of the rod into the rope to hold it in place.
Tape the hardware wire over the crack on the inside of the urn. Spread adhesive over the hardware wire and crack. Work it into the hardware wire with a putty knife. Allow it to dry completely before removing the rope brace. Touch up the patch with paint.