Concrete is defined as a plastic material because it can take the form we give it.
Depending on the task, free plastic molds are all around us and awaiting our imagination.
At times it is worth buying an item, say a plastic bird bath, simply because of its form which in the long run could save lots of money while creating something original.
While a plastic bird bath might have the base and bowl easily used as a mold,
the pedestal could provide some small old world fluted columns but
these would prove difficult to extract.
Other interesting molds are the plastic granite boulders sold at
home improvement centers like Home depot.
They cost about $ 60.00 each but the cost of a real boulder would be more.
This boulder mold, which is very hard, could produce multiples
and planning would be needed for
the process of extraction, handling and faux.
The plastic of these granite boulders is very hard
but one could saw the mold in half
and produce 2 half rocks
either using them singly or by joining them
like joining two half spheres into one single concrete ball.
When using the same plastic mold over and over it might be wise to wash it with dish soap and a brush or clean it with paint thinner, a small Chinese bristle brush and a paper towel. This will remove the leftover of the oily release agent as well as remove any concrete debris left behind which might end up on your next composition.
Apply the mold release agent spartanly for a smoother concrete finish.
A heavy spray of the oil will inhibit the concrete to reach all edges of the mold and will cause more of a texture if desired.
Depending on the composition and the forms used, say vinyl or wood garden edging to make pavers, the use of a form release agent would not be required.
In the following one of a series video, a very nice gentleman explains how to make garden concrete spheres.