by Donald Clark, CSM IMG
One discipline that is common to all forms of lapidary is how to take large pieces of stone and reduce them to useful sized pieces. If you are in the field, you might be able to drop them off a precipice, and then retrieve the pieces at the bottom. This method has two disadvantages. One is that there is rarely a convenient cliff to drop them off. Second is that the method is extremely wasteful. You have no control over how the pieces break and you are not likely to find them all.
Lacking a handy cliff, you will have to resort to mechanical methods. The quickest way to reduce a large rock to smaller pieces is with a rock hammer. As obvious as it sounds, many people slave over their saws when a few quick blows from a rock hammer would do the job.
Hammering rough does not allow precise control over the size pieces you get, although it is much more accurate than the dropping it off a cliff method. If the rock you are working on has a fracture, you can usually break it along that line.
Cutting a kerf in the stone will give you greater control. A kerf is a shallow saw cut, usually less than an inch deep. You can create a kerf almost anywhere you need it. Place a chisel or large screwdriver in the kerf and give it a solid blow with the hammer. The rock will split in two under the kerf. While the technique does not always give you a clean, straight split, it is surprising how well it works.
The above methods are only useful for cabbing material of moderate value. You would never want to hammer an expensive piece of rough where the loss would amount to much money. Nor would you want to use the technique on something fragile, like opal or calcite. Expanding fractures would reduce the useable areas and value of the material too much to be worth the savings in labor. For these materials, saws and tile nippers are called for.
Lapidaries use slab and trim saws. The difference is the size blade they use. Trim saws have small, thin blades that remove a minimum of material. Slab saw blades are thicker, because they are designed to do heavier cutting.
Trim saws use 4 to 6 inch blades that run between .004 and .012 inches thick. The cost will vary between $25 and $50.
Diamond blades are like a revolving finger nail file. They will not cut fingers, but they will give you a creative manicure if you are not careful. The exceptions to this are the very thin blades of .004 to .006 inches. At this size they will cut fingers. They are also much easier to bend, or dish, than a heavier blade. So unless you are cutting very expensive material, it is best to use a slightly thicker blade.
Slab saw blades run from 6 to 36 inches with thickness of .025 to .200 inches. A diamond blade will cost between $30 for a small, medium quality blade, to well over $1,000.
Other than size, the main difference in price is the amount of diamond on the blade. How much you need depends on how often you are going to use it. A hobbyist, who just cuts an occasional stone, can get by with any of the good quality blades. These have the diamond abrasive rolled or bonded on. A professional shop, which uses their saws constantly, would best be served with a sintered blade. Sintered means the rim has diamond throughout it, not just on the surface. Because there is so much more diamond in these blades, they cost considerably more. However, the value is there because they last so much longer.
Beware of very inexpensive blades. Some of them will only cut three or four stones and are not worth the money.
Not all slab saws use diamond blades. An older style is called a mud saw and they are still available as used equipment. A mud saw has no abrasive attached to the blade. Instead, the blade runs through a troth of abrasive, which is usually silicon carbide. This is the “mud.” It carries some of the abrasive with it on each pass.
The primary advantage of a mud saw is economy; they cost much less than one with a diamond blade. Their disadvantage is that they require more maintenance. The abrasive breaks down with use and the saw must be shut down and recharged periodically. Properly set up and maintained, a mud saw works superbly.
When selecting and operating a saw you need to make sure the stones are fed straight into the blade. If you feed the stone to the blade at an angle it will bind and possibly bend, ruining an expensive blade.
With new slab saws, this should not be a problem. However, on a used saw you should not take it for granted. Check the feed mechanism to make sure it runs perfectly parallel with the blade.
With trim saws, you feed the stone in by hand. To do this properly, stand so your primary eye is in a direct line with the saw blade. This way you can see if the blade starts to bend, indicating you are not feeding the stone straight. Keep the table around the blade clean of debris, as bits of rock can deflect the stone. With small pieces, it is helpful to press them into a piece of cardboard, and slide the cardboard into the blade. On some occasions, you might even want to embed your stones in wax or plaster.
In all cases, you are instructed to hold the stone against the table as you slide it forward. No one has ever recommended hand holding the stone as you feed it to the blade. It is simply too difficult to feed straight and the chance of damaging the blade gets much higher.
That is what every saw manufacturer recommends. However, when trimming facet rough, it can be extremely difficult to both lay the stone on the table and feed it in a direction that removes a bare minimum of useless material. The price of good quality facet rough tends to be quite high and sawing off more than necessary is an expensive proposition.
As a rough dealer, I learned to feed stones into the blade by hand at whatever angle was best for preserving material. First, either find a line on the surface, or mark the stone where it needs cutting. Now, the main trick is to keep your eye in line with the blade. Hold the marked line on the stone straight to the blade. Feed it in gently and watch the saw blade constantly. If you see any bending, make a gentle but immediate adjustment.
There are also occasions where you need to saw a kerf in a stone that is too large to feed through your saw. The only way to do this is to set one end of the rock on the table and a side on the blade. Then gently rotate it forward until a suitable kerf is cut in place. Again, keep your eye in a straight line with the saw blade and make sure it doesn’t bend.
Do this at your own risk. If you do not have a keen eye, a steady hand, and an adequate attention span, you will ruin your blade.
Trim saws will work and last well without any lubrication other than water. However, that is not necessarily recommended. At the minimum you should add a rust preventative. There are commercial products available where saw blades are sold. These usually have a lubricating properties as well, which is all to your advantage.
Slab saws have much more stringent requirements. There are saw oils on the market that are excellent, if a bit on the expensive side. If you are unsure what to use, get an oil that is specifically designed for lapidary use.
Lapidaries have used a variety of lubricating solutions over the years. One of the most popular is a combination of kerosene and motor oil. While it serves its lubrication purposes well, it is both toxic and flammable. There are water-soluble oils that are designed for use in machine shops. These do away with the flammability problem and are much less toxic. Several people like to use automotive anti-freeze.
Bear in mind that all of these substances are toxic to breathe and there is always mist when the saw is in operation. When setting up a saw make sure there is adequate ventilation. If you are using a flammable solution, outside is best. You certainly do not want the fumes collecting in a closed room while you are away! I have seen some excellent workshops set up in a carport or under an awning.
Before throwing out a blade because it no longer cuts, try cleaning it by sawing a common brick. It is amazing how much this can extend the life of your blades. It removes tiny particles of grit that have accumulated between the diamonds. Harder stones, like agate will not do the same job.
Periodically, you will also need to clean the sump of your saw. Fortunately, it doesn’t have much odor, because it is one of the nastiest jobs you will ever undertake. You need a large can or bucket to hold the residue, scrapers and rubber gloves to protect your hands.
Open the valve, or tip the saw to remove the lubricant. If you run it through a filter, (coffee filters work,) you can reuse it.
There will be a layer of sludge left in the bottom that needs to be manually scraped out. Any flat scraper will do, but a putty knife works particularly well for getting into corners. You do not have to remove every last bit of sludge, but the more you get out, the longer it will be before you have to clean it again.
Now all you have to do is to find a home for the mess you removed from your saw. The stone residue is nicknamed “the plumbers best friend” because it takes so little to plug up a drain. So that method is out. Most likely it is mixed with a toxic petroleum product, so you should find a hazardous waste disposal site. These are getting easier to find and many now exist at the local dumps.
Tile nippers are available in all hardware and building supply stores. Their usefulness is limited, but in the right circumstance they are excellent.
Nippers are used almost exclusively for removing small amounts of material from facet rough, or edges of cabbing material that has already been slabbed. Simply place the blade of the nippers on the area to be removed and give them a squeeze.
This is usually faster and more efficient than using a saw. There is no preparation or clean up afterwards. On fractured gems, you can press on the fracture and cause it to finish splitting all the way through. This will save you more material than the most careful sawing, as the fracture may be a curved line. A saw can only cut in a straight line and you always lose the width of the blade.
This is an excellent method for preparing tanzanite for faceting, where removing fractured areas is one of the most common steps. With the nippers you will frequently get a large piece with maximum recovery, and a smaller piece for melee. If you use a saw, the primary piece will be smaller and you will lose the small gem altogether. When the material is this valuable, all of it is worth saving!
The coarse wheels on a cabbing unit are also helpful for reducing rough to a useful size and shape. Indeed, that is what they are designed for. A saw is faster for removing large amounts of material, but a grinding wheel will give you more control. You can cut curves with a wheel, where you are restricted to straight lines with a saw.
You must be careful when grinding delicate material. A coarse wheel that is ideal for jasper would shatter an opal. It can also open up dozens of fractures in stones with perfect cleavage. Always think about the stability of your gem before taking it to a grinding wheel. If you are in doubt, either practice on a piece of junk, or start with a finer grade. Certainly do not drop your opal off a cliff.