Q: How do you give the cut edges of stone a cleft look?

A: Peter Chappelow, who owns Half Moon Stone Works with his brother, Thomas, in western Vermont, responds: In the article “Stone Veneer on a Foundation” (Aug/19), my brother and I mentioned that the project designer opted for a bluestone cap for the top of the veneer after we had begun applying the veneer. Then, once we had installed the cap, a further decision was made to spall the cut ends of the cap to give it a rougher appearance, more like the face of the cap.

Usually considered a defect or problem in brick or concrete that needs to be repaired, spalling occurs when water enters the porous surface of the masonry and causes the outer layers to flake or pop off, leaving the rough interior surface of the masonry exposed. Freeze-thaw cycles can exacerbate spalling. In this case, we needed to force the spalling, and we didn’t have the time that it takes Mother Nature to create natural spalling.

With a porous stone like bluestone, the process of spalling (also called “thermalling” or “flaming”) can be used to rough up smooth-cut surfaces. First the stone is thoroughly wetted.
With a porous stone like bluestone, the process of spalling (also called “thermalling” or “flaming”) can be used to rough up smooth-cut surfaces. First the stone is thoroughly wetted.

First, we soaked the areas to be spalled—in this case, the ends of the cap—with water. Bluestone is a very porous stone, so we held a wet rag over the cut end of the cap for a few minutes to let the water soak into the stone.

The mason applies heat with an acetylene torch. The heat turns the water to steam.
The mason applies heat with an acetylene torch. The heat turns the water to steam.

While the stone was still wet, we blasted it with an acetylene torch. Heat from a torch rapidly turns the water beneath the surface of the stone into steam, which makes the surface layer pop off. We repeated the wetting and heating technique as many times as necessary until we had achieved the amount of spalling for the desired rough look.

The rapid expansion of the water causes the outer layer of stone to pop off. The alternating wetting and heating process is repeated until the desired amount of spalling is achieved.
The rapid expansion of the water causes the outer layer of stone to pop off. The alternating wetting and heating process is repeated until the desired amount of spalling is achieved.

Spalling (also called “thermalling” or “flaming”) is a dangerous process because the flakes of stone can come off the surface with a lot of force. Safety glasses should be worn at all times—by the person with the torch and by anyone else helping or working nearby. And because we did the spalling after the cap was installed, we also needed to protect the adjacent trim surfaces. While one of us worked the torch, the other held a steel trowel against the trim to protect it from the heat, rewetting the end of the cap as needed.

For nonporous stone such as granite, a wide chisel can be used to flake off the outside layer of stone. This process—known as “pitching”—is more delicate and can be performed on a wider variety of stone.

Photos by Tim Healey