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How to break rocks for tumbling
How to break rocks for tumbling is a question that comes up since small sized rocks are not always available in the gemstone material you want. Listed below are a number of ways to break your rock into suitable sizes for tumbling. You can prepare the rocks yourself with a rockhammer or rock pick, small sledge. Be sure and wear eye protection/safety glasses to protect your eyes from flying rock chips. A heavy bag, such as one made of denim, will help corral the chips and can be made by sewing up an old Levi pants leg, a gunny sack, or a doubled up old pillow case. Fold the bag opening under, so no rock chips will fly out. Wear gloves when crushing stone, especially when handling and removing the material from the bag, since the edges may be sharp. Some like to break the stones in a box, which will help corral the chips. If you have a corner to work in, this will also help corral the chips on at least two sides, while you hammer the chips on a hard piece of steel, andiron, etc. If you have several pieces of rock to break up, only place rock of the same hardness in the bag. You won't have to hit obsidian as hard as a piece of agate and you don't want the softer material completely crushed. Breaking rocks individually works well also. Place the rock flat side down. Hit the highest point with your hammer or small sledge, and keep doing this until the rock particles are the size you want. Do not discard the very small fragments which break off, but use them in the rough grind stage. Use them in your tumbler over and over again because they speed up the tumbling process. All of these small chips are rubbing the grit on your stone in a multitude of places which speeds up the tumbling process and makes a smooth, shapely stone. (To get a mental picture of what is going on in the tumbler, try picturing marbles in your tumbler. How many places are being rubbed with grit when the marbles rub against one another during tumbling? The small stones or filler will go between the larger spaces and help speed up the process.) Some rocks are more fragile or brittle, and for these stones, it may be best to slab in the approximate thickness you want, then with a hammer and chisel, break these slabs into sizes close to your preference. For small freeform slabs of harder stones, (agates, jaspers) drop a slab a short distance from a concrete floor. These will make nice small tumbled slabs for pendants. If you see a fracture in the slab, place a small chisel on the fracture, and hit the chisel with a hammer to break the slab. If no fractures are visible, lay a portion of the slab on another slab and hit the angled slab with a hammer and the downward pressure will break the slab. If the slabs are obsidian, a carbide wheel for cutting glass can score a cutting line on the slab, then use pliers to break the scored slab with downward pressure. Another method is to press or slightly bounce a slab on the scored line on the edge of a steel or hard table and the slab should break along the scored line perfectly. For tough to break slabs, score the slab, then place the slab in a heavy duty bench vise, with the scored line, just visible above the jaws, then tap the slab lightly with a hammer until it breaks. Remember, a rounded or rough rock chip will tumbler faster than a small flat slab, so be prepared to spend a little more time tumbling in the fine grit and polish stage. Have fun!
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