Hydrodegradable "clay pigeons" for shotgun shooting may be made from the usual mixture of pitch, powdered limestone or chalk if a special starch additive is included. Exposure of such a "clay pigeon", or fragments thereof, to water causes swelling of the starch particles and consequent fragmentation of material into, eventually, a fine sludge which is ultimately absorbed into the ground.
 
 

G J L Griffin, World patents including US patent no. 5,460,383 October 24, 1995