The Cutting Process - Boulder opal
Buying rough boulder opal parcels and finishing the stones can be a rewarding hobby or business activity. This is a summary of the process that I follow.
Please observe the following safety points.
Electricity and water don't mix - some of these steps use wet sawing and grinding machines. Always make sure there is no risk of electrocution with your machines.
Eye protection is required when using the machines for sawing, grinding and sanding or polishing the opal.
Breathing protection is required to prevent silica dust particles from entering your lungs when sawing, grinding sanding or polishing. When dry sanding or polishing opal, use and "H Class" vacuum to extract the dry dust from the work area.
Long hair and loose clothing should be restrained or removed when using rotating machines.
1) Buying the rough Material. Purchasing boulder opal in its natural form is a real challenge because you can't actually see exactly what you are buying. Most miners will use a large brick saw to cut the boulders to expose opal veins.
2) Blocking the Parcel. Use a large diamond saw (like a wet cutting tile or brick saw 8-16 inch diameter) to cut the large boulders into smaller pieces - about the size of your fist. Try to make cuts that are perpendicular to the lines of opal, to intersect the opal veins at 90 degrees. If you cut parallel to the veins, there is a risk of the saw blade passing through an entire vein and destroying it. Cut off any boulder that has no opal veins in it.
3) Facing the Parcel. Use a 6 inch wet saw blade with a continuous sintered rim to cut the opal veins out of the host rock. Use the lapidary saw blades that are very thin and leave a nice smooth cut surface. You need to decide which will be the top of the stone and then make sure you leave some host rock on the bottom of the stone. Generally it is best to have the top with the more convex shape facing up. Try to expose the edge of the opal vein all of the way around the outside of the stone and then remove as much of the ironstone from the top as you can, without removing the opal colour bar.
4) Coarse wet grinding. Use a coarse grinding wheel to remove as much ironstone from the top as you can. I use a 400 grit wheel that is resin bonded diamond. The wheel is 4 inches in diameter at 3000 RPM and using the edge of the wheel enables grinding to follow most of the undulations of the opal vein, but will not get down into small concave areas.
5) Fine wet grinding. Use a bullet shaped grinding bit to remove smaller spots and any concave areas of ironstone from the face. I use a 400 grit grinder that is resin bonded. It has a 20 mm diameter at about 4500 RPM. Be careful about how much ironstone you are trying to remove. Some ironstone spots actually go all the way through the opal vein to the host rock, and by trying to grind them away, you will actually just grind more opal away.
6) Coarse dry sanding. Use a flexible diamond sanding disc to dry sand the opal face. I generally use 1 inch diameter discs at 26000 RPM. Remove any remaining ironstone spots and smooth out any uneven dips and ridges and scratches from the wet grinding. Dry sanding opal allows you to see any cracks or flakes in the opal vein that disappear when the opal is wet. This step is where you actually get to see for the first time what your opal will look like when it is finished. Most of the opal I sell as rough faced material is at this stage.
7) Trim and shape. Mark any major cracks with a pen and cut along them with the 6 inch wet saw to end up with smaller stones that don't have cracks. Trim the shape of the stone to pre-form the final shape. Cut the backs down to be just a little bit thicker than what the finished stone will be.
8) Grind and shape. Dop the stone by using PVA glue to stick the stone to a small length of timber dowel. Dopping the stone gives you more control over the piece as you work on it in theses final steps. Use the wet diamond grinding wheel to refine the final shape of the stone, blending in the straight lines, curves and corners of the stone to make a shape that is pleasing to the eye.
9) Medium dry sanding. Use sandpaper discs with grits in the range of 180/240/320 depending on the size of the opal to sand the top and sides of the opal. I use 1 inch diameter discs of Hermes WS Flex 18 silicone carbide sandpaper at 26000 RPM. Use the coarser grits for larger opal pieces and the finer grits for smaller pieces. Remove all of the scratches from the coarse dry sanding and if the opal vein is thick, use this step to round off the edges of the opal vein.
10) Fine dry sanding. Use sandpaper discs with grits in the range of 400/600/800 to sand the top and sides of the opal. I use 1 inch diameter discs of Hermes WS Flex 18 sandpaper at 26000 RPM. The 600 grit sandpaper is the grit that I mostly use for the pre-polish finish. Remove all of the scratches from the medium dry sanding.
11) Polishing. Polish the opal face using Opol polishing powder mixed into a smooth paste and applied to a natural felt polishing wheel. A 3 inch cylinder will polish most areas of the opal face, but a smaller wheel of about 1 inch will ne needed to get into smaller concave areas. The top of the stone is now finished.
12) Finish the back. Remove the stone from the dop stick by soaking it in warm water. Now dop the stone using PVA glue with the back facing up. Finish the back by grinding and sanding it to the required finish. Remove the stone from the dop stick.
13) Clean the stone. There will be residue from the Opol polish and also the dopping glue. Use an ultrasonic cleaner and a microfibre cloth with methylated spirits to clean the stone. The stone is now finished!