High quality sculptor's tools for marble and other softer stones can be made by hand. One good reason is to save money; another is to get exactly the tool you want. It's also fun and satisfying to do.
The first requirement for sculptor's tools is good steel. You can make a great looking tool from any kind of steel bar stock, but much of this kind of metal will be mild steel, intended for structural use. Mild steel is soft, and does not harden well with heat treatment. A better source of steel for new tools is old tools. You can pick up old mason's chisels and punches for next to nothing at flea-markets and garage sales.
Some of these can be tuned up and used as-is, or they can be reground without changing their basic shape. Others can treated as raw material to be forged into some new shape. One great source for heavier tools is worn out bits from pnumatic demolition hammers, which turn up a lot. These are hexagonal bars of high-quality steel, up to an inch thick, and often already forged into something close to a usable shape on one end, either a point, a chisel or a spade.
Flat pry-bars are a good source of high quality steel for smaller tools such as chisels and rodels. The steel is good and they are already the right thickness. Large screwdrivers can also be reworked into fine chisels and claws.
If you have some metal of unknown provenance, there are rules of thumb for evaluating it. If it rusts, it's some kind of ferrous metal, which is a good start. If a magnet will not stick to it well, it is not carbon steel, and probably won't work. The metal should get very hard when heat treated. You can test hardness by how easily it is scratched with a file, or dents when struck with a carbide punch. A quick test for high carbon content is to lock it in a vice and put an angle grinder to it with the lights low. Iron and low carbon steel will make relatively long straight sparks, while high carbon steel will tend to make sparks that break into a lot of branches. With old chisels, the mushrooming of the head can indicate the carbon content. Mild steel tends to roll over without breaking, while high carbon steel (good) separates into chunks as it mushrooms.
The following three concepts are necessary to understanding how to make hardened steel sculpture tools.
Annealing: This is a process by which steel is softened, and the internal stresses caused by previous heating and cooling are relieved. Steel is annealed by heating it slowly, allowing it to remain at the desired temperature for a period of time, then cooling it slowly. Annealing makes it easy to work at room temperature.
Hardening: Medium to high carbon steel can be hardened by heat treatment. Somewhat simplified, the process for hardening carbon steel is to bring it to the critical hardening temperature, which is about 1375 degrees Farenheit, then cool it very quickly. At the correct temperature, steel will glow cherry red. What goes on inside the steel is complex, but in a nutshell, the quick chilling prevents large crystals from forming, leaving an internal structure of tiny, very hard, needle like crystals.
Tempering: The kind of hardening described above is all-or-nothing, and leaves the metal so hard that it is prone to shattering. To make the steel useable, the hardness must be tempered with another heat treatment. When the hardened steel is heated beyond a certain point (nothing close to cherry red) it will start to lose its hardness and become less prone to breaking. Once this critical temperature is reached, the higher the temperature, the softer the metal will get. The tempering starts at tempertures as low as 400 degrees.
A forge can be improvised in the yard with bricks. The lining must be firebrick, as regular bricks will not stand the heat. If the brick is to be mortared together, it must be done with refractory mortar, because ordinary mortar can burst explosively when heated, as can any kind of concrete. Firebrick and refractory mortar can be obtained at any masonry supply house that sells supplies for fireplace building, or from a potter's supply house. An iron pipe laid in a gap in the brick can serve as a port for blown air. A hair dryer or heat gun can be used for a blower. A mixture of soft coal and charcoal will give a very high sustained heat. If a forge is too much of an investment, you can skip the whole thing and make do with a MAPP gas or acetylene torch, or even a large propane torch.
You will also need an anvil, heavy hammer, and tongs and gloves for handling the hot metal.
Assuming you have a steel bar, you can make it into a chisel as follows. First, anneal the stock. Steel begins to glow a barely-visible dull red at about 750 degrees Farenheit. At about 1375 degrees it reaches full cherry red. Beyond this is a brighter red, followed by a progression of colors: salmon, orange, yellow, white and incandescent white, you don't want to go beyond cherry red. If it gets too hot, the finished tool will be fragile no matter what you do later.
The tool should be placed in the coals and brought to a cherry red over a period of at least a half hour, and allowed to remain at this temperature for at least another half hour. If the steel need only sawing, filing, and grinding, you can let it cool now. It should be allowed to cool as slowly as possible by moving it a little way out of the fire, but keeping it buried in the hot ashes.
If you want to forge a new shape, while the steel is still cherry red, it can be beaten into shape on an anvil with a two or three pound hammer. When hot, it can also be pierced and trimmed with chisels relatively easily. Keep it hot while working it, and when finished, let it cool slowly.
When the tool has cooled to room temperature, it is ready to work into to shape with any kind of metal working tools: files, saws, drills, cold chisel, or grinder.
The new tool will be uselessly soft. To harden it, heat the region to be hardened back to a full cherry red, allowed to remain at that color for a period of time, and then, abruptly quench it in cool salt water.
Don't just toss it in--immerse the front half of the tool fully, then quickly pull it out except for the tip, repeating several times, without ever fully withdrawing the tip from the water. The idea is to even out the temperature transition over a good length of the tool, so that the fully hardened region fades into the annealed region as gradually as possible. Sharp differences in cooling rate make the tool prone to breaking. The tool should not be allowed to cool in the water to below 200 degrees. When it approaches this temperature, allow it to air cool slowly.
The salt water can be prepared by mixing 3/4 pounds of rock salt in a gallon of water. The mix is not critical, and table salt will do just as well. Brine chills hot steel almost twice as fast as plain water because it boils at a higher temperature.
If the metal to be quenched is high-speed steel, stainless, or some other alloy steel, oil should be substituted for brine. Salt water can cool these metals so quickly that tiny internal cracks form, weaking the metal. Oil will not draw out the heat as quickly, reducing the tendency to crack. It also does not harden the metal as effectively, so there is a trade off that is particular to both the alloy and the type of coolant.
When the steel has fully cooled, touch it to the belt sander to clean the steel at the cutting end. Don't bear down on the sander, just touch it lightly, because you don't want to heat the steel at the tip. The idea is just to clean so you can see bare clean steel for the next step.
The steel will now be so hard that it is fragile. Tempering softens it to a controlled degree, making it much tougher. Softening occurs at much lower temperatures than the critical hardening temperature. The degree to which the steel is tempered depends upon the temperature to which the steel is raised at this step, but unlike hardening, it does not depend upon the abruptness of the cooling--only on the maximum temperature it reaches. The temperature to which the steel at the cutting tip of the tool is raised is estimated by watching the color changes on the clean surface of the steel. These surface color changes correspond to changes in the crystalline structure of the metal that occur at specific temperatures for a given alloy.
All the tempering changes occur at well below the minimum 750 degrees at which steel begins to glow a faint red. Note that the color changes associated with tempering have nothing to do with incandescence; the metal actually changes color, and it keeps the color after cooling. The progression moves from coolest to hottest with: pale yellow, straw, golden, brown, brown with purple, purple, dark blue, bright blue, and finally pale blue. The corresponding temperatures range from 425 to 610 degrees. At pale yellow, the steel will remain quite hard; at pale blue, most of the hardness will be gone.
One way to temper the steel in a controlled way is to apply heat well behind the tip with a MAPP gas, propane, or acetlyene torch. As the metal heats, the clean steel will start to discolor near the flame in the sequence above, and you can see bands of color move up the shaft towards the cooler tip as the metal heats. As soon as the desired colored band reaches the tip, instantly plunge the tip into cold water to freeze the temper at the desired level, then slowly insert the rest of the tool. It is necessary to keep the tip in the cold water, because heat will continue to migrate up the shaft of the tool from the heated section even after the source has been withdrawn, but you still want to cool the rest of the tool evenly, without abrupt changes in any one area.
Note that by this process, no part of the chisel ever reaches anywhere near the critical temperature for hardening--thus, there is no danger of re-hardening annealed areas; all it can do is soften the already hardened metal.
To reiterate, the tool tip will remain maximally hard up to about 435 degrees. Thereafter, the hotter the tip gets, the more it will be softened. The straw or gold for hard stones, and something closer to the blue range softer stones. The hardness depends on the alloy, as well as the temper, so trying the tools on stone will be the best guide to the appropriate hardness for your purpose.
When the tool has cooled to room temperature, do a final grinding, preferably with wet wheel. Just as tempering softened the steel by the deliberate application of heat, it is easy to ruin the tip by inadvertently heating it on a grinder. Grinding wet avoids this danger.
Electric powered wet grinders are great, but the inexpensive, manually cranked ones are fine for stone tools. They cut quickly, because unlike ordinary bench grinders, you can bear down continuously with no danger of burning the steel.
Hardening and tempering can be accomplished together, with a single heating, as follows. The tool tip is heated to cherry red, just as in the hardening process described above. Then the last half inch or so of the tool tip is quenched in water for about three seconds, making it maximally hard. The exact time will depend upon the mass of the tool. The tool should be moved up and down in the water a little to spread the cooled region more smoothly, but keep the tip immersed for the whole three seconds.
The tool is then taken from the quenching bath and quickly rubbed on a block of stone to scrub a region of steel at the tip clean so that the color of the bare steel can be observed. The mass of hot steel just up the shaft will quickly heat up the tip again, just as the application of the torch did in the preceding method. As before, when the desired color reaches the tip, quickly quench the tool to cool it, again, plunging the remainder in and out to smooth out the temperature transition.
This method works because the relatively large mass of steel behind the thinner cutting end can hold a lot of heat while still remaining well below the critical temperature for hardening. This method is easier to control on larger tools--small tools heat and cool quickly, narrowing the working margin.
Claw Chisel Claw chisels can be cut from salvaged mason's chisels after they are anealled. Mason's chisels are ideal because they are fairly thin, but other kinds of chisels can be reground too. Anneal the chisels first, then clean up the end on a bench grinder to get it square and straight. Grind the sides to give it a long smooth bevel. Near the edge, increase the angle somewhat, so sides don't come together at too sharp an angle. There are many ways to shape the teeth, depending on what you want. A good way to get triangular teeth is to cut straight into the edge with a fine toothed hacksaw to separate the teeth, then finish them up with a knife file or triangular file. To get chisel-like teeth, you can also use a narrow warding file instead of a hacksaw to separate the teeth. A warding file is narrow, rectangular in cross section, and has cutting teeth on the edge, as well as both sides. They are made for cutting slots. The separation only needs to be about 1/8" deep.
When the tool is fully ground and finished, re-harden the steel and temper it to a level appropriate to the stone you intend to work with it.
Hand Sets and Heavy Chisels A decent hand set or trimming tool with a carbide edge can cost a couple of hundred dollars. If you're a stone mason, using it all day, it's worth the money, but these are just nice to haves for a sculptor. But you can make make heavy chisels of steel that work just as well as carbide on marble. Old bits from power demolition tools like pneumatic jack hammers are perfect. You can saw them down to size, anneal them, and regrind them into whatever chisel you want, then harden and temper them.
Regrind the edge to be flat and square.
Rasps and rifflers are made from tool steel blanks, annealed to be as soft as possible. Forge the blanks to a rough shape by heating and flattening them on the anvil as described above. The steel needs to be ground and sanded shiny smooth before the teeth are cut, or they will dull quickly.
Prepare a punch from a piece of good tool steel, such as a machinists punch. The shape of each tooth will be formed by the outer top shape of the punch. Carefully grind the end to a triangular cross section that will raise teeth in the desired shape when tapped into the soft metal at an oblique angle.
You must use a very precise touch to keep the teeth of even height. The teeth do not need to line up evenly. The punch gives them their final shape-nothing further will be done to shape them.
Finally, harden the working end of the rasp as described above. Do not temper it, but try to get the temperature transition to the handle as smooth as possible.
To make your own bush hammer, anneal a suitable sized block of steel as described above. Old hammer heads can be converted, but test them for carbon content--Some hammers are made from low carbon steel that will not harden well enough. A piece of car or truck axel is probably a good candidate.
Cut the annealed block square on both ends with a hacksaw, and clean up both faces with an angle grinder and sander, testing with a try-square to make sure they are flat and square. Score cross hatching into the face to mark the divisions between the pyramids. Saw into the face along these lines to a suitable depth with the hacksaw, and then use a large coarse triangular file on the cut to turn the squares into little pyramids, which will all be the same height if the face was flat when you started.
When you've got it right, lock the block in a vice, and use a drill press to bore a large centered hole for the handle. Widen one end of the hole slightly using with a die grinder with a conical silicon carbide or aluminum oxide stone--either will work. Then harden the face as above, and temper it to very hard. A handle can be whittled fresh from hardwood or cut down from a broken hammer handle.
If you are really into it, you can make the hammer of ordinary mild steel, and then harden the teeth by a process known as "case hardening." This is a general term for various processes for soaking carbon directly into the metal to form a "case" of high-carbond steel around the tool. The case is only 1.0 to 1.5mm thick, so it is probably good only for hammers with smaller teeth. Consult blacksmithing resources for more details.