Professional strong plastic, you can use it over 1600 times. For re-use purpose, spray the mold and take down, then you can use it again after the first stepping stone, so you do not have to wait. The thick level of concrete can be varying depends on your requirement of garden stepping stones. A wetter mix of concrete for concrete golden molds is used to get garden stepping stones with ground / lower level, and a stiffer mix of concrete is used to get garden stepping stones with higher / stone level. Concrete molds work well in all situations and create a very well garden stepping stones.