Lapidary Techniques | CRATEX Abrasives

Gemstone that hasn’t been cut and polished is called a ‘rough’, and gemstone that has been lightly hammered to remove fractured material is said to have been ‘cobbed’. Abrasives most frequently used in lapidary work are diamond and silicon carbide.

Diamond is used for rougher grinding of wide variety of materials, while silicon carbide is used for cutting softer gemstones. Other compounds used for polishing gems are aluminum oxide, tin oxide, chromium oxide and cerium oxide.

There are several common techniques used in lapidary work: sawing, grinding, sanding, lapping, polishing and drilling.

 

Sawing

Sawing is performed on thin, high-speed rotating round blades that are usually made of copper, phosphor bronze alloy or steel and are impregnated with diamonds. There are several types of saws that are frequently used for cutting through gemstones: slab, trim and faceter’s trim saws. Liquids (such as water or oil) are used to prevent the blade and stone from overheating and for washing away cutting debris.

 

Grinding

Wheels used most often for grinding (i.e. shaping gemstones into preform) are silicon carbide or diamond wheels. Lubricant or coolant is used to prevent overheating and to remove debris.

While sawing is a technique used for removing large amounts of material faster, grinding will provide more control, and, unlike a saw, it will allow you to cut curves. Coarse grits are used for faster material removal and are used for cutting rough stones. Finer grits are used for final shaping and sanding and for grinding delicate material.

In case you use a coarser grit on a delicate workpiece, it can shatter it. To get an idea of the grinding wheel gutting power either practice on a scrap material first or always start with a finer grit.

 

Sanding

Sanding is a technique used between grinding and polishing, but it doesn’t require as much water as the grinding wheels. It is performed with finer abrasives to remove scratches created by coarser grinding wheels.

For round stones it is best to use flexible surfaces (e.g. sanding belts), as they will protect the round curves and prevent the creation of flat areas.

Make sure you wash your hands and stones before sanding, because any hidden grits left by the previous technique can cause deep scratches and you won’t be able to polish them out.

Sand the stone until the grinding scratches are removed and the stone gets a satiny look. Always wash the stone thoroughly before proceeding to the next stage.

 

Lapping

This technique is similar to grinding and sanding, however, it doesn’t use single or multiple point cutting tool, but it cuts by way of loose abrasive action. Its purpose is creating flat surfaces or spherical contours on a gemstone and improving its surface qualitative values, i.e. making it smoother.

It is a passive form of grinding preformed at low pressure, low speed with very low removal rate (from 5 to 500 microns are usually removed from each side) and is almost always a wet process. The surface achieved with lapping is dull, non-reflective (matte) surface.

The technique requires the use of loose abrasive powders as the grinding agent. “Slurry” or “lapping slurry” is a mixture of abrasive and cutting fluid and the correct ration is important for obtaining consistent, repeatable surface roughness. The material that needs to be lapped will determine the type of abrasive, and the amount that needs to be removed will determine the abrasive grain size.

There are a lot of lapping compounds out there and the prices vary considerably, so you’ll need to do some research, try a few out until you find the compound that suits your needs best. For example, lapping of very hard materials, like carbides, some ceramics or sapphire will require some diamond or boron carbide, while medium-hard materials, like harder metals and some aluminas, can be lapped with silicon carbide.

In most cases lapping is performed on flat surfaces (not spherical shapes or contours) such as one side of laps, rotating or vibrating flat disks most often made of cast iron, steel or copper-bronze alloy. Since the abrasives used can charge the lap plate surface, lap plates should be softer than the workpiece material, while the abrasives should be as hard or harder than the workpiece material.

 

Polishing

Polishing is a technique used for ultrafine shaping of the stone and achieving a mirror-like finish that is a reflective surface. In other words, it is what enables the reflection of the light on the surface in case of the non-transparent materials, or the refraction through the stone in case of the transparent materials.

Polishing is a “wet” process performed by using a fine or sub-micron abrasive particle combined with some sort of a liquid. Some applications require thorough water rinsing during the end of the process with the purpose of removing polishing debris and preventing the staining of the surface.

Loose abrasive doesn’t have to be used, as usually a pad containing abrasive grits is utilized in the process. Due to the usage of the pad and a slurry the surface tension is quite high if compared to lapping.

Polishing removes very little material, so the surface must be well prepared and already of high-quality before this process (pre-polished surface is often a surface that was previously lapped).

 

Drilling

Lapidary drilling is making small holes in through gemstones in the purpose of hanging them by a cord or creating a thread of stones. It is done either with a small rotating rod with a diamond tip mounted in a rotary tool, such as Dremel, or used in a drill press. The or ultrasonic or vibrating drills are extremely effective, but rather costly, so it might not be your prime choice.

It is recommended to always use a coolant, as that will increase the life of the product and the value of your workpiece. Never force the drill bit, rather let the diamond do the cutting. Always drill on a slow to moderate speed, because if you drill on a high speed, you’ll rip off the diamond plating. Important is also to frequently take the drill bit out of the hole and clean it with water to remove debris. Plenty of water during the drilling is also recommended, just take all necessary safety precautions first.

Mark the drilling points on the both sides of the stone and start cutting under a slight angle until the drill bit starts to bite into the stone. Only then you should start straightening it up in the direction you want it to go. The stone should be gently pressed on the drill bit, and you should allow the water to reach the drill tip constantly during drilling, so use up and down motion.

Never drill right through the stone as you might end up with a crater. Instead, drill about half to 2/3 of the way, flip the stone over and start drilling on the other side through the marked point. Your precision might not be excellent right away, but you’ll get the sense of lining up the holes over time. After you finish drilling, the hole will have sharp edges, which could damage the stinging material, so gentle grinding or smoothing is required.