Simply brush off the mud and dirt after the cast has had a day or more to harden. Cover the negative cast with a film of grease, vegetable shortening, or petroleum jelly. As soon as the mixture begins to set up, take the greased negative cast and carefully make an impression on the hardening plaster mixture. This will have to be done before the plaster mixture gets too hard, or the plaster will crack from the pressure exerted from the negative cast. With a little practice you can determine the necessary consistency for creating a positive cast that is a duplicate of the original track.
Precautions Take great care if the track is in dry, dusty ground or sand.
It may not be possible to acquire a good cast under such conditions. When attempting a cast in such a case, pour the solution directly into the track, but from a point as close above the track as possible. It should not be dropped in with any force. Tracks in snow also are difficult to cast. Even a fairly cold plaster solution tends to melt the bottom of the track and it can be frustrating to find that the batter has poured right on through.