People have been shaping stones for millennia. As a natural material, stone varies widely, and a range of methods and tools can be used to achieve a similarly wide range of purposes. This article will focus on basic techniques that are appropriate for landscape uses and building structural walls with stone. These techniques can often be applied to other types of stonework, such as veneer, as well.
For the purposes of shaping, there are two main types of stones: level bedded (including sandstone and slate) and irregular (such as granite and basalt). Level bedded stone will tend to break easily into flat sheets or plates. Sedimentary stones will almost always be level bedded, as are many metamorphic stones. Igneous stones, formed through the cooling of lava, are irregular, because they don’t naturally form flat sheets or plates, but they may still have a grain that affects how they break.
Stones are strong in compression, but weak in tension. Stones are also weak in bending and twisting forces (brittle). We can use this knowledge to make sure we’re applying force to a stone in a way that takes advantage of these weaknesses. How a stone is supported will greatly affect how it breaks, or if it breaks.
Common Types of Shaping
Splitting is done by applying a force through a wedge-tipped hammer or chisel. The stone on either side of the chisel’s tip is in compression, while the rest of the stone will be in tension, where it’s naturally weak, and this will allow the stone to split. Splitting is best applied in the direction of the bedding plane or with the grain of the stone. Use Stone Busters, a tracing chisel, and a trimming hammer to split stone.
High-spot removal is critical for fitting stacked stones together, but it can be difficult, because directly hitting a bump on a stone puts it in compression, where it’s quite strong. Using a point concentrates the force, causing chips to break off. Using a directional point at the correct angle makes it possible to split off larger pieces. Use a directional point, a point, a mason chipper, and Stone Busters to remove high spots.
Dressing generally refers to final shaping, done with lighter taps, to square up stones or create decorative texture on their faces. The light taps are essentially wearing away or slightly crushing the surface of the stone in a controlled way. Use a directional point, a point, a mason chipper, and a tracing chisel to dress stone.
Bending works well for diminishing the size of flat or long stones. Stones supported at either end and struck in the middle, or struck on an overhanging end, undergo a bending force that will cause most stones to break reasonably easily. Bending is usually achieved by striking several places along the break line. Use Stone Busters, a Quarry Buster, and a trimming hammer to bend stone.
Trimming is the process of working the sides or edges of the stone. This can be done with shearing force, which relies on stone’s brittle nature to work, but it can also include splitting forces. Trimming is typically done perpendicular to the bedding plane or the grain of the stone. Hitting from the end of some types of stone is much more effective than hitting from the flat side. Use Stone Busters, a mason chipper, a trimming hammer, and a brick hammer to trim stone.
Tracing a line is the practice of repeatedly hitting a stone in a line, often on more than one side. This creates numerous microscopic cracks in the stone. When the cracks connect, the stone breaks. Start with light hits and gradually increase the force of your strikes. Tracing a line is precise but slow; it requires many hits. This technique works better with some stones than others, but should be used only when needed, due to the effort required. Use Stone Busters, a trimming hammer, a tracing chisel, and a Quarry Buster to trace a line.
Types of Tools
Having high-quality tools when working with stone will make shaping faster (and certain shaping possible), improve your accuracy, and reduce maintenance time. With good skill and experience, however, basic tools can also be used effectively.
Carbide-tipped tools are commonly used by professionals and hobbyists. The carbide keeps the cutting edges sharp far longer (these tools often only need sharpening once a year), and they will work well on very hard stone. However, they’re much more expensive, easily ruined by improper use, and require a special grinding wheel to sharpen. Steel tools are less expensive and are easy to sharpen, but they only work well on soft and moderately hard stone and often require sharpening every week.
Trimming hammers come in many shapes and sizes. Often, they have a head with a square end for trimming and a blade at the other end for splitting. The square end will be used 80 percent of the time or more, so some trimming hammers don’t have a blade at all. A 2-to-3-pound hammer offers good versatility.
Brick hammers are a type of trimming hammer. These lightweight hammers work well with smaller and softer stone. An 18-inch-long handle will give these hammers a lot more power, despite their light weight.
Hammers designed to be struck by another hammer, such as T&H Stone Busters, are a game changer. T&H makes a wide range of these hammers, and there are a few made by other manufacturers. The T&H Stone Busters have the precision of a chisel and tremendous striking power, making them the most versatile of these hammers. They’re used by placing the blade of one hammer on the stone (as a chisel) and striking it back-to-back with the other. This eliminates the need for precise aim with a single hammer. The horizontal and vertical blade arrangements allow for good ergonomics without having to reposition the stone or yourself. The Quarry Buster is a scaled-up version of the Stone Buster that’s meant to be used by two people: One person holds the Quarry Buster on a stone, and another hits it with a sledgehammer.
A directional point is a chisel with a 3/8-inch-wide tip. This tool is ideal for high-spot removal, because it concentrates force in a specific area. The small chisel tip allows for quicker material removal than regular points and often does the work of an angle grinder faster and with less dust and noise. The Rebit PKM 25 and the T&H bull point are the two directional points available. True points concentrate force even more than directional points, but tend to remove material more slowly and with less control.
Chisels come in a wide array of sizes and shapes. The wider the blade of the chisel, the more control over the direction of the split you’ll have, but the more diffuse the impact will be. A 1-1/2-inch-wide chisel seems to be a good balance of control and power for many stone types. The sharpness of the angle greatly affects how the chisel works. A tracing chisel with a 90-degree angle is great for tracing a line, but when sharpened to 70 degrees or so, it becomes a fantastic splitting tool. However, it can then become damaged if used against the grain on hard stone. A T&H mason chipper has an offset design that makes it ideal for trimming, despite its blunt angle, because you can drive it deeper without causing spalling.
There are a variety of hand guards and chisel handles on the market for reducing the possibility of hitting your hand by mistake. The T&H Chisel Wizard is particularly good, because it’s adjustable and has a secure grip. Plus, it’s possible to hit angles with it that would be impossible when gripping the chisel directly.
Note: Quality stoneworking tools can be quite difficult to source. All the tools in this article are available from The Stone Trust, a nonprofit organization with a mission to preserve and advance the art and craft of dry stone walling.
Reading the Stone
While detecting the bedding plane of some stones is fairly obvious, seeing the fine grain, hidden cracks, and imperfections can take a lot of experience. Pay attention to the details of the stones you shape so you can become familiar with the telltale signs of the stone type you’re working with. Shaping perpendicular to the grain can be quite difficult with many stones, and some stones are simply too hard, brittle, or unpredictable to be worth shaping.
Gaining practice and experience is the best way to become successful when shaping stone. Different types of stone react differently to different tools and methods. Skill is gained by paying close attention to the details and results of your attempts to shape stones. Modify your approach, tools, or methods when you don’t achieve desired results. Getting strong enough to have a fast, accurate swing with a 3-to-4-pound hammer is critical to efficient shaping with most stone types. There are dozens of additional hand and power tools, methods, and techniques for shaping stone, but the physics of how stone shaping works stays the same.
One of the best ways to advance your knowledge of shaping stone is to take a hands-on course. Courses offered by The Stone Trust will allow you to gain hands-on experience with a wide array of tools and stone types. Many additional courses are available to show you how to build with the stones you shape.
It’s All About Angles
The angle of the tip of the tool you use and the angle at which it strikes the stone will have a dramatic effect on whether the stone breaks as you intend. When you strike stone directly with a hammer, the angle of the swing will also affect the break. In general, when trimming with chisels or Trow & Holden (T&H) Stone Busters, the tool should be angled in toward the center of the stone to drive the break in and prevent “spalling,” or unintentional breaks.
When you’re removing a high spot, the point or directional point should be held close to perpendicular; 60 to 85 degrees is a sufficient angle.
Safe Stone Shaping
Successfully shaping stone is fun and satisfying, but, like any activity, it has its own set of risks. The list below isn’t meant to be exhaustive, but covers some important points:
- Only use hammers and chisels that are intended for stoneworking. Woodworking tools can chip or shatter when used on stone, creating a dangerous situation. Never use a wood chisel, hatchet, ax, or maul on stone.
- The square or rectangular ends of hammer heads are for striking stone only. Never use a brick hammer as a chisel! Hammers meant to be struck by another hammer are distinguishable by a round chamfered end on the head, such as those on the T&H Stone Busters.
- Always use proper personal protection equipment (glasses, boots, etc.). Stone chips can easily fly 30 feet or more, so pay attention to your surroundings.
- Never swing a hammer directly toward someone. The hammer can slide out of your hand, or the head can come off.
- Avoid breathing stone dust. Fine stone dust, especially that created by power tools, is harmful to breathe. Always wear a proper respirator when you use power tools, or when you’re creating a lot of dust using hand tools.
Rules of Thumb
- The bigger the stone, the bigger the hammer. For small or thin stones, a 22-ounce brick hammer can work well. But on boulders, 16-to-20-pound sledgehammers are commonly used. If your hammer makes a high-pitched clang when it hits the stone, or feels like it’s bouncing right back off, then it’s probably too light.
- The speed of the swing is more important than the weight of the hammer for success. Swing fast, rather than putting pressure on the hammer as it strikes.
- 50 light hits don’t equal a single hard, fast strike. Many light hits will do nothing, or will vibrate a stone apart, rather than breaking it.
- How a stone is supported underneath is a critical factor in how and if it breaks.
- Using hammers alone is much faster and more powerful than chisels, but to use hammers with precision takes great skill. Chisels are more precise, but slower.
- If you make no visible progress after 3 to 5 strikes, stop and reassess how you’re hitting, the tool you’re using, and whether the stone is supported correctly.
Brian Post is a Certified Master Craftsman, examiner, and instructor with the Dry Stone Walling Association of Great Britain. He’s also a licensed landscape architect, the executive director of The Stone Trust, and the owner of Standing Stone LLC, a company specializing in stonework.