If you want to divide up a big block or save a waste chunk of your work-piece that's large enough to be useful, there are several options: the stone can be sawn, split with a pitching tool or tracing tool, or split with wedges.
Sawing stone in the studio is usually done with a circular saw, using a diamond or abrasive disk. Before you even consider sawing stone, understand that unless you have wet-sawing equipment, the operator experience is pretty much like slowly pouring a sack of plaster into an electric fan.
Masonry cutoff saws look something like an angle grinder, but are configured for sawing. A heavy duty angle grinder may be used if has the proper shielding for cutting. In a pinch, an ordinary carpenter's circular saw can be used for cutting a slab, but the size of the wheel that can be used is limited to under eight inches in diameter. The nominal size is misleading--an eight inch wheel has only a four inch radius, less the thickness of the machinery for driving it, leaving an acutal cutting depth of only a couple of inches. A heavy-duty 16-inch cutoff saw with a diamond wheel can be rented for about $60 per day. Gas and electric varieties are available.
The gear housing on a masonry saw also uses up a lot of the potential depth of cut, but these tools can accommodate big wheels. Even so, a 14 inch cutoff saw only has at most a four-inch depth of cut. The limited depth of cut may not be as bad as it sounds, because it is not always necessary to cut all the way through the stone. Often, the stone can be cut deeply from both sides and then broken the rest of the way.
With the right wheel, these saws will cut through any kind of stone. Diamond wheels are what you want for stone, but you can also use abrasive wheels made of hard fiber with silicon carbide particles embedded. Diamond wheels are metal with a crust of diamonds. Whatever kind of wheel you use, make multiple shallow passes, increasing the depth each time.
There also exist more exotic (and more expensive) masonry saws that cut much deeper. These devices can usually be rented.
When sawing with a grinder or cutoff saw, you must match the RPM rating of the disk to the speed of the saw. Each disk has a design limit at which it can be safely operated due to the centrifugal force generated by the spin, and the drag from contact with the work piece. Some of these saws have water fittings, and some of these can run either wet or dry. Wet is much better if it is practical to use in your studio.
If a stone is not too large, it can often be divided using a heavy chisel called a tracing tool, or with a splitting hammer, which is really just an oversized tracing tool on a stick. Stone masons do this routinely, sculptors less so, because of the risk wasting stone with a bad break. The main advantage of splitting over sawing or wedges, is that it leaves a raw stone face will be unmarked by the saw or drill. Thus, it might be preferred for making bases, or in cases where part of the block will be left uncarved. This kind of splitting works very well with hard stone such as granite.
A tracing tool is used to score a line into the stone, preferably all the way around the block, by progressively harder blows on the intended cut line, until the bottom of the traced like is continuous, and not broken by irregularities in the stone surface. After the cut is fully "traced," the tool is placed the traced groove in the middle of the block and hit hard, with a heavy hammer, to split the block. The cut must be far from an edge, preferably in the middle of the block; if it is too close to one side, the break will often head off in that direction.
Sedementary and metamorphic stones, usually retain a memory of the layering of original sediment layers. The direction in which these layers lie is called the bedding. The stone separates more easily in the bedding direction, so it is important to break either with the bedding or perpendicularly to it. Attempting a break at any other angle will rarely give the desired result.
Despite their appearance, which seems uniform in every direction, many igneous stones also often also have a direction in which they split most easily, known as the "rift." They may also have a secondary direction in which they split more easily, usually perpendicular to the rift, and known as the grain. These properties may be caused by the shrinkage of the stone as it cools, with the cooler side being up, and the hotter side being down, and with plutionic rock, from the release of the enormous pressures found miles below the surface. The rift usually corresponds to the vertical direction in the earth, and the grain to the horizontal.
The tracing tool can be used to split even very hard stone, but it can be damaged by using it on a non-flat surface, because uneven contact puts too much strain on the edge. This is particulaly a risk with carbide edged tools. If you must splitting from an unsawed face, the traced line must be made gradually with light taps, until a continuous straight edge is reached. When the bottom of the traced line has become a continuous line the tool can be hit harder.
For large blocks, a splitting hammer can be used in place of a tracing tool, with one worker holding the hammer head in place, and the other hitting it with a maul. The splitting hammer is normally used to trace the line, but a tracing tool can be used first, and the splitting hammer reserved for the break. As with the tracing tool, increasingly solid blows are used along the line of the split, then a final heavy blow in the middle divides the stone. The splitting hammer is primarily a mason's tool, and would rarely be used by a sculptor.
In some varieties of stone, the grain in the direction the bedding is so pronounced that the tone cane be split into thin sheets using wedges. When breaking such stones in the bedding direction, no holes are drilled. Instead, sharp wedges are driven directly into the stone, and the resultig crack is propagated across the stone by following along it, inserting more wedges. Stones that can be split this way are usually sedementary, such as slate, and some varieties of limestone.
Many variations on this kind of splitting exist for different kinds of rock, Most of these techniques are used more by builders and stone masons than by artists.
Splitting with wedges is faster and easier way to divide a block than sawing, and can be used even on enormous blocks or boulders. With a roto-hammer, wedges, and three-pound engineer's hammer, a single person can easily split a stone the size of a Dodge van. Use of this method goes back to ancient times, and it is still widely used.
The basic technique is very simple:
When drilling the holes, clear the hole frequently by withdrawing the drill as it goes deeper, so that the waste does not become impacted and cause overheating of the bit or the drill itself. As always with carbide, do not use water or other coolant.
Feather-and-wedge sets come in a wide range of sizes, for holes ranging from three eights of an inch to an inch and a half. Half inch is a good size for most studo applications. Increase the force of the blows slowly to build expansive force uniformly across the line of holes. Align the wedges so the expansion is across the intended break. The picture below shows the crack propagating across the stone. The feathers should start out as deep in the hole as they will go, If you are using the right sized drill, the amount of wedge sticking out will be correct. The drill hole should always be correct for the feathers, because with the wrong size hole, the feathers will not diffuse the pressure as well, increasing the chance of a crack going off in the wrong direction. You can safely use the wrong sized wedge if necessary, by shimming out with a piece of steel.
Don't just tap each wedge the same number of times; listen to the pinging sound they make to gauge the pressure. The pinging will change tone as the crack starts--if there is a sudden deadness to the tone the break has started; move to another wedge. You can break even a huge stone without swinging the hammer very hard. The 3/8 inch wedges illustrated below required about an eight inch swing with a two-pound hammer to break the stone.
Notice that the line of wedges stops short of the edge at the bottom of the picture. This is because there is very little stone behind the break line on the left side. A wedge there might result in damaging that end of the off-cut.
Ordinarily, the holes are only there to provide a place for the feathers and wedges, rather than to weaken the stone, like tearing along the dotted line. If the stone is sound, and you are splitting the correct direction, the holes don't need to be very deep. The after view of the break show how shallow the holes were.
Before deciding on where to split, inspect the stone very carefully. Sedimentary and metamorphic stones often have a visible "bedding" direction--the natural grain of the stone as defined by the original sedimentation. Stone cleaves easily in parallel to the bedding direction. It also tends split predictably perpendicularly to the bedding. Attempting to split stone at intermediate angles leads to disaster, because because the line of the break will try to jump to follow the bedding, instead of following the plane defined by the holes.
Normally, you only need a wedge ever six inches or so, but if the stone is sketchy, you can use closely spaced, deep holes to remove a lot of stone. If you are using a lot of drilling to weaken the stone, one thing never to do is to also drill from the side. It seems like it would be a way to remove more stone, but it has the effect of stopping the propagation of cracks, making the stone harder to split, and giving the break more chances to wander off course.
The picture below shows a stone with a successful break on the right, and a disastrous break on the left. It's easy to see part of what went wrong: the the direction of bedding is clearly visible by the plane on the top and the parallel plane visible at the lower left, with a large hole penetrating it. The intended cut jumped repeatedly to bedding planes. Notice that the successful break is exactly perpendicular to the bedding direction. Note also that the bedding planes are neither parallel nor perpendicular to the sides of original sides of the block, which are the top and bottom.
Breaking with wedges can be used with stones of almost any size, and works on hard or soft stone. This technique is often used when clearing ground or excavating, to break up boulders.
The ancients quarried huge blocks for construction and statuary using wedges, often in combination with chiseling. The piece to be quarried would be undercut with chisels, then split off using wedges. Marks from this procedure can still be seen in ancient quarries. Plugs of olive heartwood were sometimes substituted for metal wedges. The dry plugs were hammered in tightly into a row of holes or into natural cracks in the stone, and water poured over them until the swelling of the wood burst the stone. An interesting modern version of this is called Dexpan, which is a cement-based powder that is mixed into a slurry and poured into pre-drilled holes. As the slurry sets, it expands, cracking the stone after about 24 hours.
When breaking large stones, be sure to place lumber and/or lifting chain before the pieces fall, as it may be difficult to get it under the stone later. Sometimes it is useful to tie rope or chain around the stone before splitting it, to control the motion of the split pieces, particularly if one cannot be sure which piece or pieces will fall over.
Stones often have visible flaws and veins that affect the strength of the stone that will influence the line on which it wants to split.
The left side break, shown above is like a museum of bad splitting. Not only was the bedding direction ignored, but a visible crack went unnoticed which would have resulted in a subsequent carving disaster even if the break had worked as planned.
The picture immediately above shows a close up of the wildly-off split shown in the picture of the two breaks There is a narrow region of dirt and green algae on the edge extending down from the point where the three planes of the stone come together indicating that a crack had long been present there. The crack can be seen to continue towards the upper left through the unbroken region on the original outside of the stone. This type of flaw, even if algae does not give it away, can often be spotted by spraying the stone with water. Flaws are revealed as lines where the water is sucked into the crack. Tapping with a hammer can also be a clue--just as you can hear that a kitchen bowl or a plate has a crack, by the dead sound, you can often detect a crack in a block of stone.
There are times when a block is too thick to saw through, yet breaking with feathers and wedges isn't the ideal choice either. For instance, if the break plane is intended to be the flat bottom of a piece, then, between the diameter of the holes, and the wavering face of the break, you may have to remove a significant amount of stone to flatten the break plane. Another reason you may want to saw is that you are not confident of how cleanly the stone will break, either because you cannot ascertain the direction of the bedding, or because experience tells you not to expect nice straight breaks with this type of stone.
A large saw, cutting from both sides, will not achieve more than a total of eight or ten inches of cut depth. However, even if cutting from both sides will leave several inches of uncut stone, sawing can still allow you to finesse this situation. You can cut all the way around the block as deeply as your saw allows, then break the core of uncut stone in the middle by wedging.
The narrow wedges made for use with feathers are not right for this job--they may simply dig into softer stone without spreading it, and they are prone to damage, especially on hard stone, because they won't have the protective supporting sheath of the feathers.
A wide mason's chisel with a tapering blade makes a perfect wedge for this job. (Some mason's chisels have blades that are parallel, except for the beveled edge. You can use such a chisel, but it should be ground to a taper first, as described below.) Mason's chisels only cost about ten dollars new, and are easy to find at flea markets. If you are dividing a large block, and need several wedges, you can make them from 1.5 or 2.0 inch by 1/4 inch steel bar stock. Grind the taper with a silicon carbide cup stone mounted on an angle grinder. Use a hacksaw to cut the wedge off the bar, and repeat, until you have enough wedges. If you are grinding down chisels for this purpose, beware that the heat of grinding will probably burn the steel, ruining them for use as chisels.
To break the stone, raise it up on a supporting strip of wood placed directly under the cut on the bottom, so that the weight of the block will work in your favor. Strike your chisels or wedges into the saw kerf, tapping gently at first, and building up the force slowly, as you would using the feather-and-wedge method. Wedge only above the core of uncut stone, not close to the sides, as this risks breaking off chunks. The wedges may crumble the stone at the edge--this is expected--but the wedges will quickly seat against the deeper stone. If your wedges are too thin, shim them out with strips of galvinized sheet metal, aluminum flashing, or any similar material, bent into an L shape, and hung over the side of the kerf.
If the stone does not break right away, get the wedges in tightly enough that they ring when struck, then give it some time before hitting them again. Repeat until the stone breaks.