Grinding & Polishing A Stone Bowl



From Hand Selected Field Stone To A Finished Stone Bowl, by Matthew Gregson

Selection” being the key, let me explain … 


Why field stone?  The rounded presentation lends its self to becoming a stone bowl.   In addition to the vast variety of colors. All of our selected rocks start in sand pits.  Left there by the glaciers that once engulfed and scoured these mountains.  These rounded rocks are however a direct result of collisions with millions of jagged, large & harder remnants of these mountains before arriving in the sand pits as the glaciers receded. Compromised structure comes with the territory careful selection many pits & pure volume help to produce a quality stone bowl or vessel.       


The selection, my wife and I arrived after a soaking rain at a sandpit. Every shape size pattern & color imaginable thousands of rocks observed, hundreds rolled & examined for beauty & structure.  If the selection is deemed worthy its ported with a select few companions to our car & loaded to go home.     


For the following steps, to avoid being long winded about the unforeseen I’ll attempt to put this in layman’s terms.  


The dissection begins grinding stone, first by cutting with a diamond blade on a tile saw, or wet grinder, the bottom presentation.  This is the first time the structure can truly be discerned. 


 If all is well, my wet grinder receives a convex cup wheel.  The outside edge is beveled with water pressure dialed in as to not to escape the centrifugal pull of the spinning wheel.  This water pressure is constant with all of the following steps so that the water is always in contact with the surface being worked by the diamonds; needed both for stock removed and life spam of the tooling. 

 Now with one wet polisher with a rigid backing pad, the other with a flexible backing pad, the stone receives a Velcro™ polishing pad made with some plastic polymer agent imbedded with 30 grits or 50 grit diamonds (depending in the quality of the cut, or what the rock calls for).   The rigid pad is used for the flat; and the flex for the bevel in stages to 50 grit.   The stone is dried between grits form here on in to assure that the water doesn’t mask any problem areas before the grit is advanced prematurely.   Then, back and forth, the bottom receives 100, 200, and then 400 grit.  


Now comes some rubberized padding work station clearing of debris so that the bottom doesn’t get scared in following steps.  The rock is now flipped on its newly found bottom on the rubber pad.  The wet grinder now receives a diamond blade.  A series of relief cuts are made so that there is between 1/8” to ¼” max distance between each cut to produce the basin. 


Next VERY carefully wielding a rock hammer and cold chisel, I remove the debris being careful not to let any force direct outward. 


Now the grinder receives that convex cup wheel again!  The slurry of debris at the bottom of the cuts sprays 360 degrees everywhere, as the cuts are erased and the inaccuracies are shaped and smoothed.   I turn down the variable speed grinder and lift my sights on the sharp edge produced by concave resin.  With more control, slower speed and proper water flow, the edges get rolled.  I am constantly moving the rock for positioning issues, angles of attack, excess water build up, etc.  


Now that the distracting lines from the relief cuts are gone, and depending on either or depth, shape, size or structure the relief cuts, the chisel and grinding may be repeated as the rock and artist calls for.  Taking note that as the rock loses weight, the stone may need more bracing to avoid flight, via the spinning tooling inside it.  Once satisfied with the well-shaped basin, the wet polisher is applied with 30 grit for more accuracy in the formation of the curves and erasing the roughing from the cup wheel.  Adjusting speed and water flow once again addressing the edging as well, 50 grit is applied and steps repeated. 


Now is the time I put the tool the side, and fill the basin with water.  Then with 60 grit carbide water proof paper, I shape the edging while avoiding clogging by repeatedly dipping the paper in the water.  Next, I apply 120 grit on the stone; then back with the polisher inside the basin with 100 and then 200 grit once again.   The hand shaping continues with 220 grit as it provides more consistency in shaping of the stone. 

 Note:  Because of the softer nature of carbide vs. diamond and slower speed of operation, any chipping of crystal structure, scratches and ridges caused by the 30 and 50 grit rather hungry diamonds, are removed. 


Now again back with the polisher and 400 grit!   The inside of the basin gets polished.  The edging now is polished with finer diamonds so the handmade curve should hold their shape.  While the tools still got 400 grit, work space is cleaned again and its prime time to see to if any external work done is still good; if not, I fix it.  From here on in (for efficiency) every surface tooled receives the same grit still drying; i.e. from 800, 1500, 3000 and 6500 grit between steps to avoid back stepping.   At this point even after dried, every tooled surface looks wet.  Most rocks are done unless the specimen chosen was Jasper, Agate or some other rare beauty whereby I’ll need to polish with 8,500 and 10,000 grit.  Rocks that host crystals that spit light showing the ‘Schiller effect’ and are polished to 13,000 grit! 


Whatever the rock’s chosen finish grit is and drying for the final time, the stone receives a liberal application of Aqua Mix™ (a professional grade sealer due to its ability  to be buffed completely dry).  With excess removed first with paper towels and then with cotton cloth, the stone is vigorous rubbing which leaves the sealer in only the pores with no smearing.  This is to keep the rocks natural color and keep foreign colors from penetrating the rock.

 Now it is time for the stone’s exhibit!    Follow us on our web site for shows coming near to you, or on-line ordering.   Our shop is open daily!