“What is the difference between concrete and cast limestone pieces…and why is cast limestone more expensive?!”
So I thought that it would be helpful to write a paragraph or two explaining the differences, as both are wonderful materials and each has its place in the garden.
Dry cast limestone ornaments are hand made using a limestone composite. A slight bit of water is added – just enough to hold the composite together (this is the reason for calling the process “dry cast”). The damp mixture is then hand packed into a mould and left in a humid room to cure. Curing usually takes about 3 weeks, during which time the piece hardens and can be removed from the mould. The dry cast limestone is porous and looks remarkably like carved limestone. The rough surface and minerals of limestone also promote weathering and help create a beautiful patina with lichen and moss settling on the surfaces.
The first picture shows a newly dry cast limestone and the second picture shows a weathered version after a year or so.
Concrete, in contrast, is made using a wet cast process; that is, water is added to the aggregate and then the mixture is poured into a mould. The mould is shaken so that the concrete can cover all the details of the mould. A slurry comes to the outer edges of the mould which creates the smooth surface of concrete. At the same time, air bubbles also come to the surface which are often visible. Once the piece has hardened, it is removed from the mould. A smooth surface is less hospitable to the moss and lichen which lengthens the time needed to create a beautiful patina
This picture shows concrete's smooth surface. If you look closely, you might see an air bubble or two.
The cost between the two processes differs as well – dry cast limestone is a hand crafted product whereas concrete is largely manufactured and so concrete tends to be about half the cost of dry cast limestone.