Q: I attended one of your seminars last year, and you referred to stones as brittle. That isn’t really true of all stones, is it? Some stones are actually very strong.
A: Brittle is not the opposite of strong. Brittle is the opposite of ductile, and weak is the opposite of strong. They are two different things. Strength is the ability of the stone, in a given sectional dimension, to carry a load. All other things being equal, a stone with greater strength can carry greater load, resulting in greater stress, prior to fracture. Ductility or brittleness describe the behavior of the material prior to fracture. In steel, which is a ductile material, we can observe significant elastic deformation prior to yield, and rarely actually see a complete fracture, unless there is fatigue. Most masonry materials, including stone, are brittle. They exhibit very limited deformation, in many cases not even visible deformation, prior to a sudden and complete fracture. They can indeed be very strong, but when they break, they do so suddenly and without warning. That is a brittle behavior. You can actually do a quick experiment of this to demonstrate the difference. First, purchase two Hershey’s chocolate bars – the traditional ones with the score lines dividing them into rectangular sections. Leave one at room temperature, and place the other one in your freezer overnight. Then unwrap them and break each along their scored lines. Notice how the warm bar will bend and deform a bit before it breaks, while the chilled one will break without giving you much warning, and it will usually break more cleanly along the scored line. Furthermore, the chilled bar will actually take more force to break it than the warm bar does. Therefore, the chilled bar is stronger, yet more brittle than the warm bar. Before you ask—yes, the other benefit of doing this simple experiment as that you get to consume the test specimens after the experiment is complete!