As mentioned above, most of the common sculptor's hand tools are ancient. In this section we look at these traditional tools for working stone and minor modern variations on them, such as the tungsten carbide tipped versions. Where it matters, the use of tools on soft and hard stones will be distinguished. Specialized tools for lifting, moving, and cutting stone are discussed in section Lifting and Handling Chapter, and in Dividing Chapter.
Ultra hard tungsten carbide and similar "hard-metals" became widely available in the 1930's, and quickly revolutionized most technologies that required cutting hard or abrasive materials. Until the invention of these alloys, hardened steel was the only practical material for sculptor's tools, and the forging and maintaining of carving tools was an important part of a sculptor's training. Tools needed to be repaired or replaced frequently, and a sculptor needed to know how to forge and grind tools, and how to harden and temper them to suit the material at hand.
Tungsten carbide is not exactly a metal itself, but one of two similar metal-like compounds of tungsten and carbon (tungsten carbide, WC or tungsten semicarbide, W2C) that are produced industrially as fine gray powders. The powdered tungsten carbide is sintered in a matrix of cobalt metal (sintering is an alloying process in which finely powdered metals are fused together by extreme pressure, rather than by being melted into a true solution.) Tungsten carbide prepared in this way is the hardest metal-like material in common use. Only certain gemstones and silicon carbide (Carborundum) are harder, and unlike tungsten carbide, those materials can only be used in the form of abrasives, not as cutting edges.
Carbide tools are somewhat more expensive to buy, but cheap in the long run, as they outlast steel by as much as a hundred times over, and allow you to work hard materials in ways that would otherwise be difficult or impossible. They also require far less frequent maintenance, and the maintenance they require is generally simpler. For industrial use, carbide tipped tools have almost entirely replaced steel for cutting hard or abrasive materials like stone, as well as for many less abrasive materials like wood. However, in stone carving, carbide is not always an exact substitute for steel. Carbide is more brittle than steel, and cannot be ground to as acute an angle without becoming excessively breakable. This limitation gives many carbide tools a significantly different, working feel than their steel counterparts. For granite and similar stones carbide the way to go in virtually every case, but for softer stones, for many of tools, the sharper cutting angles that steel permits give deeper penetration and a better result, more easily, for many applications.
Steel tools are usually forged from a single piece of steel, with the business end being hardened by heat treatment, and the rest left in a softer, annealed state. Carbide-tipped tools, on the other hand, typically consist of a mild-steel tool body tipped with a tungsten carbide insert, or inserts, attached by brazing, i.e., soldered using brass. For chisels and similar tools, the carbide is usually set into a notch or drilled hole in the steel body of the tool.
Carbide is darker than steel, and the brass joint line between carbide and steel is usually visible, so carbide tools are easy to recognize. When these tools fail, it is usually either at this brazed joint or by shattering of the tip itself. Either mode of failure may or may not be repairable, depending upon the manufacturer and the cost of the tool. Small carbide burrs for grinders are sometimes machined from a single piece of carbide.
When grinding steel cutting edges on a wheel, it is important to keep them cool by quenching frequently in water or oil, and limiting the time they are in contact with the wheel. This is because the temper of the steel begins to change at as little as 425 degrees Fahrenheit, making it is easy for the heat of friction to "burn" the tip, leaving it uselessly soft. If, when grinding, you can see the metal near the tip change color (not glow, but actually turn yellow, blue, or purple) you have removed some or all of the effects of prior hardening.) It is more or less impossible to burn the steell when sharpening a tool by hand on a whet stone, which can also be more convenient for touching them up while you work. Wheels that run wet also obviate this danger.
With carbide, it is exactly the opposite: quenching while grinding can ruin the tool. This is because while heat does not temper the hardness of out of carbide, its extreme hardness makes it prone to being shocked by abrupt temperature changes. All materials expand or contract in response to temperature change, so when any hot tool tip is dunked in water, the wave of cooling moving through it causes adjacent regions of the metal to struggle against each other as they change size at different times. This does not affect high-carbon steel, because it is slightly elastic, but carbide is so rigid that these stresses can cause internal fractures to form, weakening it, just as a wine glass can break if it is dunked in hot water. Fortunately, most carbide can withstand very high temperatures. Multi-purpose carbide drills will bore through any combination of thick steel, concrete, plaster, and pretty much anything else, even granite, without the need for a liquid coolant.
Hard as it is, even carbide eventually wears down and needs to be sharpened, or even reground. For light touchups a, diamond hone is good, first the coarse side, then the fine. A flat, fine-grit silicon carbide bench stone works well too (all other stones are too soft.) When using the flat stone, you do want to use oil, not to keep the tool cool, but to keep the stone from clogging.
It's actually easier to regrind carbide than it is to regrind steel, because you can do it continuously withou overheating it, but there is one gotcha. Grinding with a fine silicon carbide wheel should be followed by polishing with a fine diamond hone. Carbide is somewhat like glass, in that it tends to break more easily where it is scratched. Polishing denies cracks a place to start, making the tip stronger. For the same reason, use the diamond hone to put a very slight champfer on the sharp edges, and to likewise take the corners off the ends. The tool will not be quite as sharp initially, but the chamfer leaves no razor edge to crumble and cause roughness that would give cracks a place to start.
With steel, you do not want to hone the edge to a mirror finish as you would with an edge for cutting wood-the edge left by a fine grind stone is good enough. A very fine edge will crumble or roll over immediately, leaving the tool less sharp than if it were left raw.
Hammers for stone carving come in a variety of shapes and sizes. The traditional sculptor's hammer, called a square hammer or club hammer , is just a rectangular block of steel with a short handle. This is the default hammer you'll be handed in a sculpture supply shop. It's main virtue is that it looks very traditional. A more polished looking version, also with a short handle, is known as a lump hammer. Lump hammers are used in the trades for driving cold-chisels, masonry nails, etc, and thus tend to be mostly available in heavier weights. See the pictures above for comparison.
Engineer's hammers look like miniature sledge hammers and have longer handles than club hammers. They are not good for general carving, but come in handy for special purposes such as driving wedges to split stone.
Round mallets, are tapered cylinders of steel with short wooden handles.
Hammer choice is a matter of personal taste, but the round mallet has a lot of advantages for all-around carving. They're comfortable, pretty, and the compact design makes it easy to hold in a position that minimizes repetitive strain. These come in several sizes from one to three pounds.
A one or two pound hammer is good for general carving. How you swing varies with the task. Driving a pitching tool takes a healthy swing with a heavier hammer such as an engineer's hammer--two and a half to three pounds is typical. Finer carving can be done with small taps of a one pound mallet using a swing of a couple of inches.
How you hold your arm and hand is very important, especially on the back swing, Even when making light taps, try to move the elbow and shoulder rather than the wrist--you get much more power and better control. Carving requires swinging the hammer much more often than work in the building trades, so even if you are used to using a hammer, it's easy to develop a repetitive stress injury to the big tendon on the back of the wrist. This is caused by lifting the hammer improperly on the return stroke. When drawing back the hammer, hold it so that the the thumb side of the wrist, and not the back side, holds the weight of the hammer. In other words, don't pronate the wrist when pulling back the hammer. Your wrist is much stronger in this direction. If your position makes this difficult, for instance, because you're hammering vertically down, try lowering the workpiece, or standing on a platform to get your body higher relative to the point of impact.
It's easier to increase the force you're applying by speeding up the hammer head than by using a heavier hammer. Even holding the hammer just a littler further from the head adds a lot of energy over the duration of the swing.
One important class of mallets are those which have non-metalic surfaces: either wood, plastic, or stacked leather. Wooden mallets tend to spliter and break quickly when used on steel tools, but are can be used with socketed chisels that have wooden handles. Also, metal hammers will quickly ruin wooden handles. Wooden handled stone carving tools are unusual in the United Staties, but are still used in Europe for softer stone.
Mallets made of plastic or stacked leather soften the impact, and are said to less tiring than metal. Chisels intended for use with these mallets have distinctive wide heads, shaped like mushroom caps to spread the force, so as not to tear up the mallet faces. Conversely, these chisels should not be used with steel hammers or mallets, because the heads are often hardened to retain their shape, and will fragment rather than deform.
"Dead blow" hammers, can be made of either metal or plastic, and also tend to be easier on the user because the hit harder for their weight. These hammers are hollow, and contain several ounces of metal shot, which absorbs much of the bounce, resulting in more energy being delivered to the tool, and less to the user's arm.
The punch, or point-chisel is the workhorse of stone carving. A punch is just a metal rod, sharpened and hardened at one end, and left flat on the other (striking) end.
For marble and other medium-hardness stones, the cross section of the sharp end is usually square, with the four flat sides coming together in a gentle curve. The curve has two purposes: the sides come together at a relatively wide angle behind the tip, which makes it stronger where it is under the most stress, while slightly further back, where strength is not so much of an issue, the punch has less curve. The taper lets it go slightly deeper in the stone before the width bursts the stone, the deeper the penetration, the wider the chip. A corollary to this is that the lower the angle, the more mechanical advantage in the sideways pressure.
For granite and other hard stones, punches are usually tungsten-carbide tipped. These punches have stockier shafts with a tip ground to a wider angle. Use of the punch on hard stone does not rely on penetration in the same way that it does on marble and limestone, but rather, tends to be used for knocking off bumps and corners. When used straight in, they tend to crush more than penetrate. Thus, carbide punces work best on hard stones but tend to work poorly on marble and limestone.
Another punch type that is often used on hard stone, but not soft stone, are those with round carbide tips. These penetrate stone, especially hard stone, more easily than square tips, but the round shape tends to simply poke a hole in soft stone, rather than wedge away chips. The Trow and Holden company makes an interesting variation on the round punch that consists of a round punch on one side, and a sledghammer-like face on the other. The point is not swung at the stone, but placed on it, and the hammer head struck with a second hammer, similarly to a bull hammer or a splitting hammer, except that one person wields both hammers. The flat face can also be used as a hammer itself. This tool is usually used on hard stones such as granite.
Round-mouth punches are used for soft stone. The round mouth tapers to a narrow, slightly curved edge, which broadens the area of stone it bears against.
The following subsections describe the three major techniques for using a punches.
Obliquely to the Stone: Rapid removal of stock without bruising the stone. To reduce soft and moderately hard stones like limestone and marble quickly, in a controlled way, hold the point at an angle to the surface of the stone and drive it across the stone with repeated hammer blows. Each blow pops off chip, plowing a trench across the surface. Row after row, in parallel, lowers the entire surface. See the picture below The same technique is used to remove high spots.
The inflexibility of the stone, combined with its low tensile strength, causes the outward pressure of the penetrating point to be relieved by blowing out a chip on side of the chisel where the stone is weakest, which is ordinarily the outer side.
Thus, the square profile of the marble punch is functional. Properly held, a flat side, not a corner, should face outward, in the direction the chip should fly. The flat side spreads the outward wedging force evenly over a large area of the surrounding mass, delaying the breaking of the the chip. This allows the outward pressure to build up enough so that when the chip pops off, the stone above the tip is thick enough to pull out a wide area of stone from around the tip.
A round punch would not work as well for this stroke because the round side of the punch concentrates the outward wedging force, instead of diffusing it, breaking the chip prematurely. The same thing happens if you hold the punch rotated by forty-five degrees, causing a corner, instead of a flat side to face outward.
The punch is held at about a forty degree angle to the work and struck solidly with a hammer or mallet--ideally, one blow, one chip. To reduce a broad area, don't pick the punch up each time, but instead just drive it along forming a furrow. Don't bother to measure forty degrees; you'll feel and hear when the angle is correct. Too steep and the punch will ring, but no chip will fly; too shallow, the point will slip, scratching the stone without biting in.
The pictures below shows the angle of attack and a pile of chips. The white lines in the illustration show the path of the punch across the stone. Note the wide separation between them, which indicates the width of the chips that are thrown.
Perpendicular to the Stone: Rapid removal of stock where bruising is not an issue. The picture below shows the use of the punch straight in. This stroke can be used either at the edge of the block, in which case it will result in a big chip breaking away, or directly into the side of the block, in which case it causes a cone of stone to be blown out all around the point where the punch enters the block.
This is very effective on limestone or marble, but it should not be used anywhere near the intended finished surface with marble unless the surface is to remain unfinished, as it bruises the stone deeply.
Bruising: The pictures below shows the characteristics of such bruising in a sample of the same hard white marble the demonstration piece is carved from.
The right half of the picture on the left shows the effect of the punch straight in. The triangular punch mark just above the label B was directly into the freshly split face of the stone using a moderate hammer blow--about eight inches of swing. Note the white halo around the pucture mark.
The punch was also applied in the same way just above labels A and C, and the stone subsequently chiselled away to show the depth of the bruising. Approximately 3/8 of an inch of stone was removed from the original surface at mark B, and about 1/2 of an inch at mark C, and the surface smoothed. Note that in both cases, cloudy areas of bruising are still visible.
A claw chisel with five teeth was pounded directly into the surface at the top of the block in the same relative locations, with approximately the same force. The marks are clearly visible directly above the B punch mark at the top of the block. With the force spread among five teeth, the bruising is more superficial. The same depth of cut left a barely visible trace on the left, above A, and completely removed the the bruising on the right, above C.
On the far left side of the left hand picture, an area spanning the entire block from top to bottom was bushed with a pneumatic bush hammer. The bushed stone shaved away in the middle and bottom. The middle section is shaved down about 1/8th below the bottom of the deepest craters, and the lower section about 1/4 inch. Note that the middle section still shows many bruises, while the deeper cut has almost entirely removed the hammer marks.
The right hand picture shows a cross section cut through three straight-in punch strokes, similar to the ones shown on the left hand picture. Two punched craters on the top are labeled A and B, and the one on the side is labled E. The area of stone that has been cut down is about two and a half inches square, and has been smoothed with #220 grit wet/dry paper. The stone was moistened to show the surface better. Note the white clouds below and around A and B, to a depth of about 1/4 inch below the deepest point under the punctures. These are the bruises seen from the side. There is a thinner rim of white under E.
Sub-surface cracks marked D and C can be seen radiating out from A and B in both directions. At least three parallel white flaws show up under C. There are also some very faint, white, crack-like flaws extending down into the stone below and to the right of A, going at least three times as deep as the white cloud.
Note that crack D, on the left, is open, as can be seen from its dark color, which is residue of the black sandpaper that has gotten into it. The multiple cracks marked E did not actually open the stone, as can be seen from their pure white color, but still permanently disturbed the crystal structure of the stone. If the stone were to be cut away horizontally to sufficient depth to remove the cloudy areas, these cracks would still appear as white dots.
Another interesting feature of this example is how different the marks of A and B are from E, probably because of the different directions of the impacts. The grain of the stone is plainly visible, running diagonally from lower left to upper right. Therefore, the direction of impact for A and B differs from E by 90 degrees with respect to the grain. Punch mark E removed the most stone, yet seems to have produced the shallowest cloudy area. The white region under E is also thin and parallel to the broken surface, rather than puffy cloud, and there are no obvious cracks. The lesson here is that the depth of bruising from an impact in one direction is not a reliable indicator of what you will find from another side of the stone.
Carbide v. Steel: Steel punches are not hard enough for granite; carbide is a necessity. For softer stones, carbide punches and steel punches have very different working properties and feel. A carbide tip may be just as sharp as a steel tip, but the angle of the faces coming together is greater for carbide. The wider angle means that the point doesn't penetrate the stone as deeply before the chip is blown out, causing the tip to gouge a narrow groove across the stone, rather than a wide furrow. Thus, you won't get good results in marble and limestone with a carbide point using the oblique stroke.
The pick, a hammer that comes to a faceted point similar to a punch, was once a very common tool for roughing out marble and limestone, but is not often used by modern carvers. It is swung to remove a lot of stone quickly and is often illustrated among the tools of Medieval and Renaissance sculptors. It is essentially a punch and a hammer in one, and like a punch, can be used either obliquely or straight in. The popularity of picks with stone masons is regional. In some places they are still a common tool for dressing stone, probably because the local stone is easy to work with that tool. Picks with heads weighing up to several pounds are available in both steel and carbide.
The pictures above, shows a small pick with a bush hammer on one end. This is a popular small format for student use, but it is more useful as a bush hammer than as a pick because it is too light for roughing out. The pick end gets used mostly as a one-point bush hammer. Teachers sometimes recommend this tool for a student's first carving, on the grounds that the crudeness of it forces the student to keep the carving simple, and pay attention to the nature of the stone, rather than to the carving tool, but it's basically a toy. On the right is shown a three-pointed hatchet and pick combination modeled on the one in Vasari's [Vasari 07-2] book on technique. This tool is almost unknown today.
Punches and picks clear stone away quickly, but they leave a very rough surface like a plowed field. To get to the next level, a toothed chisel, or claw chisel is used. The most common types act exactly like a row of punches bound together. The claw takes away less stone, but in a much more controlled way, leaving a surface that is still rough, but is much more uniform.
Tooth chisels, also called claw chisels, like punches, come in both steel and carbide-tipped versions. The carbide holds up much longer than steel but typically cost about three times as much. Carbide tooth chisels handle more similarly to their steel counterparts than do carbide punches. They require no sharpening, and last much longer, so even at three times the price they are a good deal. The drawback of carbide tooth-chisels (aside from the expense) is that the carbide tips tend shatter or detach from the steel very easily if you drive the tool head into a tight space, squeezing the teeth from the side even slightly. They have very little strength in this direction, and no power to cut into the surrounding stone, so teeth are often break or get wrenched out.
The flat claw chisel is a common variant. The teeth of flat claws are more like miniature flat chisels than punches. The edges are and aligned with line of cut. They take a finer cut and leave a distinctive notched pattern in the stone.
Tooth chisels are not solely for rough work. Many sculptors have used the distinctive texture they leave as a finished surface. The pictures above shows the surfaces left by various tooth configurations.
Michelangelo relied heavily on claw chisels, and frequently left large areas of the sculpture with an extremely rough clawed surface that was often only a degree finer than that left by the punch or pick. He also used claw chisels to more delicately, to texture surfaces to control the play of light. The picture above shows Michelangelo's use of the fine claw as a finish tool.
Bernini, among the greatest masters of marble technique, sometimes used finely clawed surfaces in a way that resembles engraving, to define and emphasize the contours. The pictures below illustrates two subtly different uses of the claw chisel by Bernini. Note the stone on which the dog's foot rests--the marks of the fine claw are the final surface of the stone. The dog's foot is similarly striated, but the final surface has been further smoothed with abrasives. Note that the striations on the dog's foot are essentially graphical rather than sculptural, in that they emphasize the contours of the sculpture and do not attempt to literally model the dog's fur. This piece was executed when the artist was eighteen years old.
Contrast this with the costume detail in the pictures below in which fur is directly modelled with a fine tooth chisel. While both artists control the play of light, the unknown French sculptor has worked on several levels, directly rendering the rich surface of the ermine trim, and subtly taken advantage of the whitened, crushed stone left by the claw tips to suggest the sheen and visual depths of the fur.
Note also the undisguised use of the drill in carving the gaps behind and between the braided ropes.
Tooth chisels are not usually used in carving granite and other extremely hard stones, but a variety called a ripper sometime is.
Rippers are superficially like tooth chisels, but the teeth are like little flat chisels turned vertically with respect to the line of the cut. The picture above shows a ripper in action. Notice that although it is superficially similar to a claw, it works by a different principle. The geometry of the teeth precludes penetrating and wedging, but tend to act more by crushing the stone superficially.
When a stone worked with a tooth chisel is very close to its finished contour, the sculptor will usually switch to the flat chisel. Here, the wedging principle still applies, but the tool has an edge, not points, so it cannot get very far under the stone before the outward force crumbles the waste stone.
Edge chisels are a finishing tool--they are not usually used for removal of bulk stone. The chisel is held at a low angle, struck with the mallet. If the tool is sharp, and the angle low, it will leave a smooth matte surface. If the cut is a little deeper, it will leave a striated, rougher surface.
Droves are very wide edged chisels that typically do not have the two-sided bevel of an ordinary flat, but are bevelled on only one side. The picture above shows an array of chisels and roundels.
Roundels are like flat chisels, but with a slightly curved edge. The curvature makes it possible to finish depressions in the stone smoothly. Rondel gouges are also available.
Sometimes when chiseling marble, the flat chisel leaves behind craters in the stone, marring the mostly smooth surface. What's happening is that the chisel, biting into the stone, gets too strong a grip, and instead of immediately crumbling the stone just behind the edge, a larger chip is pried up in advance of the edge, and it snatches out a some stone from below the intended surface, leaving a crater. The picture below the upper left quadrant shows a relatively smooth chiseled surface marred in several places by pits caused by grabbing. A large pit is seen just above and to the right of the center, and several can be seen just above the lower edge of the chiselled area.
The quadrant below ws surfaced with a claw with carbide points for teeth, and the one to the left was cut with steel claw with chisel teeth. Notice that in both cases, the higher surfaces between the teeth could be described as nothing but pits, but none of them go below the plane defined by the cutting edges.
Flat chisels are available in both steel and carbide. As usual with carbide tools, the angle of the cutting edge is greater, so the tool must be held at a steeper angle, resulting in a less pleasant feel.
Carbide-tipped flat chisels are also used on granite and other hard stones. As with points, the tool acts primarily by crushing the granite rather than cutting it. Chisels specially designed for granite should be used--the cutting edges are thicker and tool generally more rugged.
Cape chisels are a variation of the flat chisel. They have a diamond shaped head and a cutting edge perpendicular to the flat side of the chisel. This tool is used for incising lines in stone. Variations on this chisel have shaped cutting edges. The picture below shows the distinctive cutting end of a cape-chisel. This tool is also used by masons for clearing the mortar from between bricks or blocks, and thus can sometimes be obtained at an ordinary hardware store.
The striking end of chisels are not hardened, and therefore tend to gradually mushroom as they are beaten on. The picture below shows a badly mushroomed tool in front, and what it should look like in back.
Mushrooming makes the chisel harder to strike accurately and the rolled edge eventually breaks up, leaving sharp edges. Keep the ends close to their original shape by touching up periodically with a bench grinder. If you don't have a bench grinder you can lock the chisel in a metal vice and use a small angle grinder to clean it up. Very often flea-market tools are found in this condition. If you are buying old chisels to re-grind into sculpture tools, the mushrooming can be a clue to the quality of the steel. As a rough rule of thumb, deep cracking and broken off chunks around the edge of the rolled steel edge indicate higher carbon content. This is a good thing for stone tools. If some segments have broken off, so much the better. If the steel rolls around little cracking, it is probably a milder steel that will not harden as well when heat-treated. You can verify this by grinding on the bench grinder. Tree like branching sparks indicate high carbon. Low carbon steel tends to make sparks that form long straight lines.
When you know what to look for you will see the traces of this tool in carvings from all eras, from the Classical Period onward. The danger in using a drill is that the eye picks out the regularity very easily, both the perfect roundness and the repetition of the same size holes. Drilled holes are easier to make than to disguise, and over-using the drill often leaves a piece looking like it's taken a load of buckshot.
Manually powered drills have rarely been used by sculptors since power tools became available, but historical examples are illustrated here.
There is no reason to use this tool today, as a die-grinder or flexible shaft tool is more versatile and more effective.
Modern pitching tools usually have a carbide insert for the cutting edge, but steel works fine, if the stone isn't too hard. When using a steel pitching tool on hard stone, you may need to incise a line first with a carbide chisel or tracing tool.
This is an expensive chisel, but you can easily modify a heavy mason's chisel to serve the same purpose by regrinding the edge. A steel mason's chisel only costs about ten dollars new, or you usually can find ones at the flea market. In regrinding the edge, be sure not to burn the steel--work slowly and dunk the tool frequently in cold water. See the section on making tools. for details on hardening and tempering.
Pitching tools are an exception to the wedging principle by which most of the stone carving tools work.
This tool is usually used on rectangular blocks. Place the chisel's edge parallel to the edge of the block, at a forty-five or fifty degree angle, and tap it a couple of times first to set the edge. Then hit it hard, once, with a heavy hammer. As the sharp edge bites into the stone, it brings the flat side to bear on the stone adjacent to the cutting line. The flat edge can't enter the stone, so the momentum knocks a big chunk away in the direction of the blow. Note that this is a completely different principle from that of the punch, which either wedges away the stone perpendicularly to the direction of the hammer or in the opposite direction. The picture above. illustrates a pitching-tool. The pitching tool can useful for removing a lot of stone quickly, but it only works on outside corners.
The handset is used by sculptors for roughing out, but is more often used by masons, who use it to give a distinctive rough hewn face to rectangular blocks used in walls. The faces of such blocks that contact other blocks are hewn or sawn square and flat, while the fronts are pitched off with this tool to give the characteristic appearance of broken stone facets. Sometimes a margin is chiseled around the edge of a pitched block to give a clean appearance. The pitching tool is alternately called a hand set. Its action is similar to that of a bull hammer.
A tracing tool, shown below, is a heavier, more rugged version of the straight chisel, with a broader edge. This chisel is used to score straight lines into the flat side of a stone for subsequent splitting. The tool splits stone in two ways--first by weakening the stone along a line by starting a groove in the surface, then by wedging the stone apart. Its action is similar to that of a splitting hammer.
The bull hammer and splitting hammer function more like the pitching tool and tracing tool, respectively, than what we ordinarily think of a hammer.
Bull hammers seen below. have a working edge that is flat on the face and square, with sharp 90 degree corners, much like a pitching tool. The bull hammer requires two workers. The hammer's edge is placed on the stone, near one side and held by one worker. The back side of the hammer is struck with a heavy hammer or maul by a second worker. A single blow divides the stone. The break line can be incised first with a tracing tool. It is important that the edge sits against a flat surface. A curved surface can break the edge.
Splitting hammers shown below. are used used in the middle of a stone, and work like tracing tool. Like the bull hammer, one worker holds the tool in place, and the other hits it with a heavy hammer. Both of the bull hammer and the splitting hammer are of more interest to masons than to carvers, but they can be used effectively to rough out blocks, make stone bases, etc. As with a bull hammer, the full cut line may be incised first with a tracing tool or chisel. As with the bull hammer, it is important that the tool be used on a flat surface, and not on a bump, as the uneven forces could break the edge.
A typical bush hammer, shown below, looks like a meat-tenderizer, the face being cut into a waffle pattern of little pyramids. With the momentum of the hammer behind it, each pyramid can apply tremendous pressure to a tiny point to crush a crater into a struck surface. Repeated striking reduces the surface to a continuous field of craters which are cratered again and again, gradually removing material to any depth, or simply unifying the surface texture. Bush hammers aren't used as often on marble as they are on harder stones, but they are sometimes used for texturing.
It is not necessary to hit hard--a rap is enough. Bush hammers are available with a many different face designs, with fixed or replaceable faces, and in both carbide and steel. Carbide has made steel bush hammers more or less obsolete except for use on soft stone. Chisel-like tools with bushing heads are also available for places that aren't accessible to a hammer.
Bush hammers are useful even on softer stones like marble and serpentine because of the textured, matte surface they produce. Bushing faces for marble often have more numerous smaller teeth. Obviously, bushing should not be used on marble if the surface is to be further smoothed because of the deep bruising. Bushing tools for soft stone are sometimes called frosting tools.
Granite and similar stones are too hard for normal chiseling, and are usually carved by sawing, bushing, and abrasion. Sawing is often used to remove large amounts of stone, and then followed by bushing to do the intermediate shaping. Grinding with silicon carbide or diamond abrasives follows, unless the bushed surface is to be left as is. Four points is the most common configuration for stock removal on hard stone, but Bush hammers are also available with crosses, circles and other shape on the striking face, in lieu of waffling. Entire sculptures can be carved with nothing but the bush hammer.
Bush hammers for granite usually have fewer, larger, points than hammers for marble and other softer stones.
Rasps come in all shapes and sizes but the principle is constant; a rasp is a piece of steel with the surface worked into an array of cutting teeth. Rasps are made from a soft tool blank by driving a hard tool into the surface at an oblique angle to tear up little tongues of metal that will serve as cutting edges. After the teeth are formed the tool is heat-treated to harden it. Unlike chisels, the edge is not usually tempered. Rifflers are small, shaped rasps. They are available many shapes, sizes, and degrees of coarseness.
There's a hazy line between rasps and files. In general, files usually have a series of parallel edges machine-cut into the surface, as opposed to the teeth being clawed up from the surface of the steel. But there are exceptions of nomenclature in both directions. A variety of rasps and files are illustrated below.
To use a rasp, simply lay the cutting surface against the work-piece and push forward with simultaneous downward pressure. Avoid pressure on the return stroke to prolong its useful life. Rasps are most used on soft and moderately hard stones such as marble.
One traditional rasp, shown below,is called a stone mason's drag. It is made by setting sections of hacksaw saw blades into a block of wood. The block is pushed perpendicularly to the direction of the blades to rake stone from the surface. They are mainly used to flatten surfaces. The Shinto Corp. makes a modern version, shown below, that is intended for woodworkers, but works on softer stones.
Where an edge rolls softly away from an incision, for instance where skin is deeply wrinkled, use the burin to cut the sharp part of the crease, followed by the inside curve of a woodworker's gouge, to cut the roundness of the skin at the edge of the creases in the skin at the base of the fingers in the marble piece below were done this way. The cutting edge of a gouge used in this way needs to be completely reground so that the bevel faces the outward, rather than inward. The overall bevel should still be the sharp, 30 degree angle of a wood chisel, but the acutual cutting edge of the bevel should be steep, say, 70 degrees. Only about 1/16 or less should have this steep bevel, so that he sharp corner can fit deep inside the incised groove. The inward curve of the couge will help to shave off stone in such a way that minimal or no sanding will be required.
You don't necessarily have to incise the groove first to use a chisel like this. With a little practice you can use the corner of the gouge directly, skipping the first step.
You will have to regrind your tools continually as you work. Even soft stones frequently contain traces of silica other minerals that quickly dull a blade. Don't use good gouges for this--you'll grind them away to nothing, and good ones don't hold up with stone any better than the cheap ones. A good way to carve hair, cloth, and similar complex surfaces is to chisel the overall shape, then do the detail carving without a mallet as described above.
Sharp tools used in this way will leave an finish similar to that of #220-grit sandpaper--fine enough that for many purposes the tooled areas will require no further polishing.
Subtle modulation of flesh, cloth, leather, and similar gently curved surfaces can be accomplished with carpenter's wood chisels reground in this way. The cutting end should first be ground to a curve. You can grind a partial bevel back along the curve if the thickness of the steel will interfere with your view, but it is not necessary.
The cutting edge will be sharp--as describe above--but the edge will be almost a right angle as shown in the drawing above. Buy the cheap, no-name carpenter's chisels that come five sizes in a plastic pack, for five or ten dollars, so you can have a range of curvatures to choose from. By varying the angle of incidence to the stone and the relative angle of the chisel and the direction of push, a continuum of curvatures can be obtained from a single chisel. the skin showng veins and tendons in the illustrated marble hand was modeled this way.
The edges on these chisels must be fresh to work well, so they should be touched up frequently on the bench grinder. They don't need to be honed, just ground with a fine grit wheel. The tools leave smooth surface, about what you get with 120-grit sandpaper, and takes off stone about as fast as a fresh rasp, but leaving a silky smooth surface.
Steel for woodworking is tempered to be quite soft, so unless they are heat-treated, these tools dull within minutes. However, heat treating is trivial in this special case because, as they will not be subject to the stress of hammering, they do not need to be tempered. After the intital grinding to reshape the edge, heat the tips to cherry red using a MAPP gas or propane torch, and keep them at that temperature for a few minutes. Be sure it stays cherry red, but not hotter. You can wrap the shaft near the handle with a sopping wet cloth to keep avoid damaging the joint to the handle. While it is still cherry red, plunge it directly into cold water and keep it there until the entire tool is cool enough to touch. (If just the tip is cooled, heat can migrate down the shaft and temper the tip.) When it is cool, give it a final wet grinding, and them wipe the tool with oil, as it will otherwise rust very quickly. After this treatment you can go much longer between sharpenings.