The processing of stones
prehistory
The processing of stones, minerals and fossils has a long tradition. As early as the Stone Age, people made tools out of hard stones such as flint, jasper and the like in order to make knife blades, weapons, etc. in the simplest way possible by manually shaping these stones into the appropriate shape for various purposes. As early as the Middle Ages, colored stones, semi-precious stones and precious stones were processed into jewelry and decorative items. Processing, i.e. grinding and polishing these stones, was a complex process at the time, as everything had to be done by hand in the beginning. Guilds were formed which specialized in stone processing. At that time, only real minerals, colored stones, semi-precious stones and precious stones were processed, as synthetic production was not possible. The polished stones were already very valuable in the past, as they were used in jewelry, crowns, garment clasps, etc. and were not accessible or affordable to everyone. The grinding and polishing process, later carried out using water power and sandstone wheels, was closely guarded by the grinders and was a strenuous and time-consuming process. Magnificent exhibits such as thin-walled bowls, cups, cameos and much more can be admired today in the Gemstone Museum in Idar-Oberstein. Considering the simple tools used at the time, these artists deserve the utmost respect.
Present
Today the situation is somewhat different. Almost anyone can learn to work with river stones, pebbles, minerals, precious and semi-precious stones and fossils if they have the right tools. Thanks to a large selection of machines for mineral and gemstone processing, this activity is now very popular as a hobby and also for professional production, and is accessible to a large number of interested people. The information below is based on our own practical experience, i.e. from practitioners for practitioners, and is not the result of scientific research.
How do we process stones, minerals, semi-precious and precious stones and fossils today?
This answer depends greatly on the aim of the cutter and the size, quality, etc. of the stones. Some produce a flat cut or a slice of the stone with a surface grinder in order to "bring out" the inner structures of the stone, others round the stones off to their natural shape using a spirator or tumbler, many grind pendants, cabochons, ring stones and various jewelry using wheel grinding machines or cabbing machines, some cutters refine the beautiful raw gemstone with many surfaces (facets) using a faceting machine, balls, sculptures and various objects are also cut from the stones and much more.
Depending on the grinding objective, there are a variety of processing options that can be considered. A distinction is also made between whether stones are ground by hand or machines (machine grinding) are used.
How are stones, minerals, gemstones, etc. cut by hand?
There are a number of tools available for this purpose. Here is an excerpt:
- Grinding on a glass or metal plate with loose abrasive grain with the addition of water (Sica, aluminum oxide, corundum,
- Sanding with a sanding block using wet sandpaper (Sica equals silicon carbide) with water
- Grinding with diamond hand pads with water
- Grinding on a diamond grinding disc (“lap”) with water
- Diamond-coated tools such as diamond files with or without water
In some countries, foot-operated treadle sanders are also used (similar to a foot-operated sewing machine...) with sometimes very nice results.
In many cases, the stones are separated or formatted (cut) before the grinding and polishing process in order to achieve the desired results. This often saves a lot of grinding work. Cutting actually involves "grinding through" the stones - so-called cutting. Diamond cutting saws with electroplated diamond saw blades or sintered cutting blades are usually used for this. Various machines are used for this purpose.
Here is an excerpt
Stone cutting saw Goliath 450
Mitre saw "Alligator"
Wet angle grinder
Wet angle grinder
Wet angle grinder Makita
Band saw Inland DB-100 including dia cutting band
What is the best way to cut the stones?
In most cases, they are held securely with both hands and guided through the rotating saw blade or cutting belt (in the case of band saws). (In compliance with all relevant safety regulations!).
Or the stones are firmly clamped in a stone clamp and then guided through the saw blade manually or by means of a feed (weight or electric feed). (In compliance with all relevant safety regulations!).
The best saws for your project
Diamond cutting saw GERMANY 250
Stone cutting saw Goliath 450
Cooling oils and additives are used as cooling agents. Water alone is the worst choice, as it does not cool the saw blade sufficiently in the cutting area. Cutting additives that are mixed with water have proven to be a cheaper option. Very hard stones are usually cut using pure cooling oil. Please read the data sheet, as it may contain substances that are harmful to health and the environment. Only cut with a breathing mask/face mask!
The diamond saw blades should also be sharpened from time to time, as the diamonds on the saw blade become rounded and the saw blade no longer cuts properly. Various sharpening agents/stones are used to make them sharp again. Saw blades should not be left in the coolant-additive mix for a long period of time, as they can be attacked by rust and can then cause damage or even injuries during operation! Therefore, drain the coolant so that the saw blade is no longer submerged in it. Alternatively, a circulation pump system can be used. The maximum speeds specified by the saw blade manufacturer must also be strictly adhered to. These must not be exceeded!
Conclusion: the better the cut, the less grinding work you have to expect. The quality of the cut (roughness of the surface) depends on various factors:
- Saw blade
- number of revolutions
- Feed speed (how fast is cutting done)
- Coolant
The right accessories
Coolant Cool 1:1, various quantities
White oil
Saw blade sharpening stone, aluminum oxide
Sharpening stone for dia. saw blades
Diamond saw blade AM sintered
Diamond saw blade, premium
How are the stones cut?
Machine grinding with sanding sheets (coated with silicon carbide. “Wet sandpaper”. Wet grinding
Bonded, inexpensive abrasive is used here, which is applied to light cardboard sheets (also available in hardware stores). It is cut to disc format and applied (glued) to a flat carrier plate with an adhesive (spray). Worn sanding sheets can be removed and the carrier disc can be re-coated. The results are impressive, as the quality of the sandpaper is consistently good. It should be noted that the coarse grains are operated at a low speed (150-300 rpm) in order to become flat. The disadvantage of wet sandpaper is that these sanding sheets wear out relatively quickly and become blunt, which leads to higher costs for harder stones. The sanding time is also longer compared to diamond sanding discs (“laps”). If you proceed carefully, clean results can also be achieved with this method.
Polishing is carried out on various media such as felt, cork, leather, plastic discs with diamond polishing pastes and polishing oxides. When polishing with cerium oxide and other oxides, speeds of 150-200 rpm are recommended while keeping the polishing disc moist.
Dry sanding with water sandpaper is generally not recommended due to health risks!
Mechanical grinding on wet - belt sander
A method that is much more common in the USA and South America than in Europe. It uses Sica or diamond grinding belts with the addition of water to cool and flush the ground material. This method is also used as dry grinding, but this is a health risk.
Grinding with sintered diamond grinding wheels, wet grinding
This method has its advantages, especially for planing after cutting, as it is quick. The service life of the grinding disc is long to very long depending on the thickness of the coating and can therefore also be described as economically efficient. It should be noted that the disc must be worked on from time to time with a diamond dresser in order to remain as flat as possible. Many grinders avoid this method, especially when using several sintered discs, as this entails considerable costs. These sintered discs are more commonly found with facetters.
Grinding with grinding drum, spirator (tumbler)
With such devices, the original shape of the stones is changed slightly to significantly. This depends on the size of the abrasive grain and the grinding time. The ground and polished stones are referred to as "baroque" ground. Not only raw stones but also pre-ground pendants, stone slabs and pre-cut stones can be ground and polished. The average processing time is 2-4 weeks, depending on the hardness of the stones. The stones are filled into the grinding container with the addition of loose abrasive grain (powder form) Sica or aluminum oxide, corundum, and water. The filling quantity should not exceed 2/3 in order to achieve proper mutual abrasion. With the spirator (tumbler), the grinding/polishing process is faster, as these devices work with a high vibration speed and the stones grind each other down more intensively in all directions. Many grinders also add ceramic pellets or other material to which the abrasive grain binds during the grinding process. It is also important to add around 1/3 of small “driver stones” (approx. 1-2cm pieces of marble or quartz chippings (depending on the hardness of the tumbled stones to be ground) which are responsible for the constant movement of the mass in the spirator. The mix of stones, grinding powder, ceramic pellets and water filled into the spirator must constantly rotate slowly in a circle and at the same time also show a rolling movement. The amount of water added is an important criterion on which depends whether everything in the spirator rotates well or not. If you fill in too much water, the amount will soon stagnate and vibrate on the stand, which will not lead to success. Therefore, with a 2.5 liter spirator, you start grinding by adding 1/16 - 1/8 liter of water - and do this in portions. Then observe whether the mass rotates and rolls after a while. If necessary, add a little water with a spray bottle. Monitor the process every few hours and add A LITTLE water if necessary. After 24 hours, everything should be cleaned and if more abrasion is desired from the stone, grind again with the same (but fresh) amount of grinding powder (approx. 3 tablespoons). Then carry out the same process with the next finer grain, etc. When the tumbled stones have finally been finely ground with grain 1200 (depending on the type of stone, etc.), a thorough cleaning is required again, as they can now be polished. Many grinders use their own polishing container for polishing so that no coarse grain from the previous grinding passes can scratch the stones. Cleanliness and cleaning between the individual steps are the top priority here!
Spirators
Spirator Model 106
Spirator Model 108
Spirator Model 113
Spirator Model 114
Spirator Model 121
Spirator-9
Polishing process:
You fill the finely ground stones with a suitable polishing agent, a little water and possibly the addition of polishing ceramic pellets into the polishing container. The small driver stones, which have now also been finely ground, are also put into the container so that the necessary rotation and vibration can be set in motion. Some grinders also use polishing additives such as pieces of walnut shell, small wooden cubes and much more (their secrets).
The polished baroque stones can then be set, provided with a pin loop or used loose, etc.
Spirators are also used in various industries for deburring, rounding and polishing.
In this context, the flat aspirator should also be mentioned, on which stones (which have already been cut) with loose grain are ground and polished under vibration. This process takes a lot of time, but it produces good results, especially if the support surface in the tub is made of hard metal (the usual aluminum tubs, on the other hand, wear down after a while and a slight hollow forms where the stones no longer lie flat). It is also helpful to weigh down the stones or stone slabs with weight (putty or similar) so that more contact pressure is created and the abrasion is increased.
Machine grinding with loose abrasive grain (Sica = silicon carbide) on cast iron, metal disc
An old, still very good method for processing minerals. Cast iron discs are usually used for this, which have a slight incline towards the middle (or not).
These discs are sometimes also grooved, which helps to separate the removed grinding sludge more easily. Here, work is carried out at a low speed (approx. 100-180 rpm) to prevent the applied abrasive grain from being thrown off. A water/abrasive grain mixture is applied to the stationary faceplate and distributed using a clean brush. You start with a coarse grain (K80-K120) and apply good hand pressure, and grind the stone flat using a circular motion. The faceplate and stone (and also your hands and fingers, etc.) must then be carefully cleaned so that you don't carry any of the coarse grain over to the next, finer grain. After a few grit sizes, you reach grit 600, 800 or even 1200. Then you polish at a low speed (150-300 rpm) using a damp felt polishing pad, for example with cerium oxide (for the entire quartz group, among other things). Due to the wide range of applications, surface grinding machines with speed control such as our EMSCH 200 RPM are recommended.
This method is still often used by agate grinders, who achieve perfect results with it. Every agate grinder knows the procedure with grinding time, increased contact pressure and the associated cleaning work.
We have replaced this process with our own developed special quartz/agate diamond grinding pads for ourselves and now also for many customers and achieve (almost) equivalent results after 3 grinding passes + polishing. Some grinders who use our own pads no longer polish because the shine after the 3rd pad is enough for them. We therefore give preference to these special pads because all of the disadvantages are eliminated.
Universal/surface grinding machine EMSCH RPM
Special pad, 3-step, Ø 200 mm
Special pad, 3-step, Ø 125 mm
Special pad, 3-step, Ø 150 mm
Special pad, 6-step, Ø 150 mm
Dynamic 200 mm
Agate grinding set, brown 150 mm
Agate grinding set, brown 200 mm
Special pad, QU, Ø 125 mm
How does machine grinding of cabochons work?
After a bit of practice and careful work, this is a popular form of gemstone grinding. Often, translucent, interestingly structured, even less valuable stones, semi-precious stones such as various types of quartz, cat's eye, tiger's eye, inexpensive turquoise, lapis, agates, banded, patterned stones or simply stones you have collected yourself are used, as well as valuable stones such as opals, star sapphires, etc. Transparent, valuable types of stone without cloudiness, on the other hand, are usually faceted. Cabochons come in various shapes and thicknesses, and before grinding, it is important to determine where the stone has its greatest shimmer, the best pattern, the best light refraction and the strongest color. To do this, the stone is cut into a disk and the desired final shape is then drawn on it. The shape can also be transferred from templates using a brass or aluminum pen . The piece is then sawn so that a little excess remains after the cut. This roughly cut stone is then manually (if not too small) ground out to its shape on the grinding wheel (usually a hard diamond grinding wheel with grain 80, 100, 120). The stone is then applied to a wooden dopp (stick) with stone putty or using 2K glue and sanded and finely ground using the finer grinding wheels. After using a soft grinding wheel with grain 280, the stone is dried and checked for any deep scratches and curvature defects, as such defects can now be corrected by re-grinding with grain 280. Usually, you grind up to grain 1200 or, better, grain 3000. (Speed approx. 1400-1700 rpm) Finally, the cabochon is polished and removed from the wooden dopp. Stones that have been putty are removed again using a flame and the 2K glue can be removed with hot water. (approx. 70-90 degrees) whereby you have to pay attention to which stones are sensitive to heat and you have to be careful with the flame. (Spirit or gas burner). We also sell an electric putty melter .
CabKing 6"
Wet angle grinder
Wet angle grinder
Wet angle grinder Makita
Wet angle grinder Makita SET
Machine grinding with diamond grinding pads
This innovative method has become increasingly popular over the last 10 years (although it still varies from region to region). Diapads are available in a wide range of shapes and sizes. This is an example of stonemasons who have been using them to wet-work natural stone surfaces very successfully for a long time (FLEX). These pads are used in conjunction with wet hand angle grinders. These pads have also been used for a long time in natural stone restoration (floors, window sills, kitchen worktops, etc.). We made use of this for mineral processing and have been studying their shape, inner workings, binding, diamond concentration and surface structure for over 6 years. Thanks to years of research and testing, we are now able to offer special pads for minerals, gemstones, fossil processing, etc. that produce very good results. All you need is water. We also use these pads on surface grinding machines, for example for grinding agates.
We offer these agate/quartz pads with Velcro fasteners for wet angle grinders (FLEX) as well as glued and pressed onto sheet metal discs, which can then be clicked onto surface grinding machines with magnetic plates. For safety reasons, the magnetized mounting disc should have a centering mandrel so that the pads cannot be thrown off sideways at high speeds. Such surface plates can also be purchased from us.
The conventional diamond pads available on the market (mostly of Chinese origin) are available in a wide variety of designs and constructions to suit the various areas of application. These are only of limited use for mineral and fossil processing, as they often contain too little diamond grain, often no sieved diamond grain and a bond that is too hard, which inevitably leads to scratches.
Labradorite
Labradorite ground with wet angle grinder and diamond pads.
Carving technique
You can also often see "carved" stones that have been wet-worked with a Dremel or other hand-held grinding devices. These tools are particularly popular for grinding opal, as they can be used to grind out wavy, misshapen natural shapes where the opal layer sits, for example, without wasting a lot of the opal's (expensive) color. Hard, galvanized and sintered grinding bodies with pointed, round and concave shapes are often used here to get to the desired areas of the stone. Diamond grinding tips are also used here, such as those offered by Diamond Pacific as NOVA grinding pins. Round-cut diamond grinding foils on soft surfaces are also popular.
Carving set SSP
