How to polish stones by hand
Grinding and polishing of rocks and minerals
There are many inconspicuous rocks and minerals that only become interesting with the right cut or polishing.
Above all, stones with weathering crusts, geodes and druses, stones and minerals with banding or patterns can be optically and qualitatively upgraded by grinding and polishing and, for example, prepared for more detailed investigations.
By working on the mineral or rock surface Colors intensified, the mixed parts and their arrangements (structure) as well as structures in minerals are visible.
Intrusive, igneous rocks - e.g. granite, gabbro, larvikite, syenite - and crystalline, metamorphic rocks such as marble, quartzite, amphibolite and gneiss, where the crystalline structure is emphasized by grinding and polishing, are particularly suitable for grinding and polishing.
Minerals that are suitable for grinding and polishing work because of their structure include rhodochrosite, jasper, malachite, aragonite and agate.
Materials and tools
Grinding and polishing are usually two consecutive work steps.
With the grind the material surface is first leveled or smoothed. Finally, since the result is a matte sheen, that follows polish. The smallest scuffs and scratches are removed and the shine is enhanced.
It should be noted that not every mineral or rock is suitable for polishing. Crystalline rocks with dense packing of the aggregate parts, e.g. compact rocks such as granite, gneiss or marble, for example, provide better results than pore-rich rocks such as pumice. At best, a silky shine is achieved on the surface and in the worst case, the stone is destroyed due to the low hardness and the wealth of pores.
Sanding work always starts with coarse-grained sanding sheets, which are followed by ever finer sanding sheets. The Common abrasive sheets and abrasives have a grain size of 80, 220, 400, 500, 600, 800 and 1000.
Grinding work with a 80 grit are suitable for the coarsest work, in contrast to this, smaller grains are used for the finest cuts.
The size of the grain therefore determines the strength of the abrasion.
- The coarsest and coarser sanding sheets (80, 220) are used for pre-sanding
- medium sanding sheets are used for fine sanding (400, 500, 600)
- finer sanding sheets (800, 1000) are used for fine sanding and polishing.
The stone to be worked on determines the change to the next finer sanding sheet during the sanding work itself. If there is no further material abrasion due to the grain size used, the switch to the smaller grain size follows.
As Abrasives come from both natural and synthetic particles for use. The natural grinding materials consist of minerals with a Mohs hardness of 6 or more, most commonly corundum, diamond or quartz are common for this purpose, i.e. the pulverized minerals are applied to the grinding tip (e.g. diamond-tipped grinding heads or corundum grinding stones).
The reason why minerals are used for grinding is the hardness of the minerals mentioned. Diamonds, corundum and quartz are hard minerals that are able to scratch and work on softer stones and minerals. Because of its low Mohs hardness, plaster of paris is not able to work on a significantly harder ruby - the other way round it is.
For this purpose, artificially produced diamonds (so-called industrial diamonds) or corundum are often used, or those specimens are used whose quality is too low for use as a gemstone.
The most common abrasive material used is silicon carbide, also known as moissanite. With a Mohs hardness of 9.5, moissanite is the second hardest mineral on earth after diamond. The reason for the use of man-made or natural silicon carbide as a grinding and polishing agent is the heat resistance of the element. Stone and mineral processing go hand in hand with the development of very high temperatures, which moissanite withstands up to 1,600 ° C. Corundum is heat-resistant up to 2,000 ° C; Diamonds only up to 800 ° C.
The use of abrasives can operated manually or by machine become. Either sanding belts or sanding sheets are coated with appropriate abrasives or are supplied as a ready-to-use powder. In this case, we also speak of a cut with loose grain, which is opposed to the bound grain on sandpaper or sanding belts.
There are further differences in the application. While the powder initially with water to a Grinding paste is touched, the machine-operated cut becomes the right one Sanding belt or sheet attached to a suitable machine. Care should be taken when using the water for the paste. Distilled water is preferred. The deionized water prevents any salt deposits or possible color changes caused by them. Once the paste has been mixed, it is transferred to a base that is moved over the object to be processed in circular movements. If there is no further abrasion, switch to the next smaller sanding size. In order to prevent scratches on the surfaces, it is recommended to rinse the stone or mineral thoroughly with water before each replacement in order to wash away any dusty residues.
Water is also used for machine sanding. The liquid drips continuously onto the stone during the work. In this way, the stone or mineral is cooled at the same time, the abrasion is washed away and the cut appears smoother. Another advantage of machine grinding and polishing is the faster processing results thanks to rotating grinding wheels.<>
A special form of polished stones are Tumbled stones.
The principle of creating gloss and shaping can be compared with pebbles from the Baltic Sea.
Due to the movement in sea water, the mass of rocks, the finest quartz grains and water is constantly rubbed against each other. As a result, edges are rounded, surfaces are smoothed and the stone gains shine.
Tumbled stones as they are offered in stores are produced by two different devices: Drum machines and spirators.
The difference between the two machines lies in the Construction and speed processing. Drum machines are barrel-shaped containers, while spirators are like bulbous vessels.
To drum stones and minerals, they are kept in motion for several hours with abrasive powder and water.
Spirators produce faster results. The tumbled stones rub against each other permanently due to the mechanically operated movement and are individually smoothed and their shape is influenced by the resulting material abrasion.
Grinding and polishing stones - Our recommendations **
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Last updated: September 26, 2020
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