Let me preface this by saying there are many, many methods of opal cutting that are effective and will result in a perfectly acceptable finished cabochons or carvings. I have developed some techniques, over the years that have worked well for me. I would like to share my personal methods with you in the hopes that it will save you some time and frustration (and also produce gems you will be proud of). You may want to, almost certainly, expand on these techniques and develop a method of opal cutting that works well for your style and types of equipment.
Basic Non Motorized Equipment
Some of this is a repeat of the dialogue on our home page, but I would like to go into this topic in a little more detail. When I was 24 years old and stationed about the aircraft carrier USS Saratoga (1969-1973 for any old salty sea dogs who may have been in the same position) I had some free time on my hands and was homesick for my lapidary equipment in Richfield Minnesota. I wrote to my parents and had them send me an unused 8″ 220 grit silicon carbide grinding wheel, various grades of wet/dry sanding cloth, some dop sticks, some dopping wax, and some tin oxide polishing powder.
I ordered some opals from W. H. (Bill) Walker. Bill was an opal-dealer in the late ’60’s and early 70’s. I heard some very colorful stories about Bill Walker in Coober Pedy, from the old time miners and buyers. He made several opal-buying trips to Australia during that time and is quite a legend.
I would study the stones carefully for maximum fire and direction and decide which side would be the back, then grind the back of the stone to make it flat and attach the stone to the dop stick for better control. Then I would carefully dop the stone by heating gently (to make it adhere better to the dopping wax) and fix it to the melted dopping wax (on the dop stick) forming the wax onto the opal base in a smooth symmetrical fashion.
I would use the flat side of the grinding wheel, laying on a towel on a table, by hand to flatten the stone back and rough shape the stone once it is on the dop stick. It is a slow process and I didn’t use much water…just enough to keep the dust down and clean the wheel occasionally…it was a dusty job. Final shaping was done using the 220 grit wet/dry sandpaper and then going to 600 grit paper (final sand and pre-polish with the old 600 cloths) and then polishing on a piece of old felt blanket with tin oxide. It was very labor intensive, but the control was amazing and you seldom cut through a fire layer!
Motorized Equipment with Diamond Abrasives
Sorry, but I don’t have any new Genie’s, Pixies or Titans (although they would make wonderful opal and gem cutting machines…maybe some day).
I have converted several machines, which I purchased new, (several years ago) from silicon carbide abrasives to diamond abrasives. It really was quite a hit or miss proposition to find the right combinations in diamond abrasives vs. silicon carbide and come up with a workable solution to give a decent polish and not cut too fast or too slow.
First of all I use my Hi-Tech 8-inch unit to flatten the back of the stone (once you have studied and oriented the stone). I use a 180 grit for rough grinding for the back and then go to a 600 grit wheel to give it a partial polish.
I then use 220 diamond grinding belts or wheels for rough cutting and shaping the tops of the larger stones. With smaller stones (less then a carat) you may want to use a 320 grit sanding belt for rough shaping. Small stones cut very fast and before you know it there is nothing left…so beware. After rough cutting is finished I go to a 320 grit sanding belt (I personally like Raytech True Circle belts). I then go to a 600 grit belt for some final shaping and sanding (and to put a small bevel around the perimeter of the stone base…to prevent chipping when setting or to compensate for any extra solder the goldsmith may have left around the inside of the bezel) and then to a 1200 grit belt to get rid of the 600 grit scratches.
For pre-polish I use either an 1800 grit belt or a 3000 grit belt. Most of my wheels on the cabbing unit’s are expanding drums to make switching of belts simple and economical. They either (1800 grit or 3000 grit) seem to give a great pre-polish. I then wash the stone and dop sticks well to remove any contaminating grits and it’s off to the leather disc with tin oxide for the final polish. I am old fashioned as many people use 14,000 grit diamond and 50,000 grit diamond…I guess I like my tin oxide on soft leather discs. And as long as you keep the wheel wet, with a little mist from a spray bottle, there is really no heat buildup. It gives a great polish. Note: December 30, 2005…I broke down and bought an 8″, 50,000 grit diamond sanding disk, for the end of my 8″ unit, and have been using it for an extra bright final finish on opal cabs (after polishing with a tin oxide/Linde A mixture). I have to admit it DOES give a great final polish to my opal cabs and I really can tell the difference…the old adage is wrong…you can teach old dogs new tricks!
For sawing I use my newest piece of equipment (2 years old) a Diamond Pacific TC-6 trim saw equipped with a MK-1000 6 inch by .006-inch blade. I purchased the saw through Kingsley North Lapidary supply (whom I recommend for equipment and general lapidary supplies…they are timely and helpful, and courteous). The blade is very thin so you don’t lose much opal) so watch your fingers and it is also stainless so rusting is not a problem. I like to use Lube Cool 4800 for a saw coolant…a 16 ounce container goes a long way in a small trim saw especially when diluted 10 to 1 for sawing purposes.
A Little About Dopping
I used to use dopping wax to hold my stones on the dopping sticks, but after cracking a few heat sensitive opals (and having a few fly off of the stick and smash into little pieces) I decided I needed a new approach.
First I tried sodium silicate or water glass (as a glue) but found it too thin and difficult to remove. Just for the heck of it I tried white wood glue. It was thick and held the stone in place with never letting go. The only problem was it was not waterproof. I solved that problem by adding a coating of clear fingernail polish over the well-dried wood glue. I placed the stone with a drop or two of glue on the stick and centered it carefully) and placed the stone and stick vertically in a flattened pad of children’s modeling clay and let the stone dry overnight. I then coated the glued portion with the fingernail polish and ten minutes later I was ready to cut. This takes some extra patience when waiting for the glue to dry overnight, but with a $500.00 (or more) stone in the balance it may well be worth it.
To remove the stone I use acetone or fingernail polish remover to remove the fingernail polish and set in a glass of water overnight and the stone usually just falls off the dop stick in the morning. Be sure to clean the back of the stone with soap and water to remove any extra glue or you may re-glue the stone to a tabletop.
If you have any questions about techniques, equipment or opals in general don’t hesitate to call. Evening hours 406-651-4947 (8:00 AM to 5:00 PM Mountain time). We like to visit about opals in general and are for the most part, very friendly, so don’t hesitate to call!
Treating fractured opals
6/25/13: This procedure works well for Australian opal only. I wouldn’t recommend Opticon for Welo opal as I’ve heard that it causes to stone to become permanently milky although I’ve yet to try it myself. The talk among Welo opal cutters is that cyanoacrylates or Superglue work well for treating fractures. But again I have no experience here…so I would recommend trying this on a small stone first instead of experimenting on your best opals. If you have some luck, please let me know so I can pass it along in this format.
Last winter I had a client come to me with a gorgeous full-fire chunk of rough opal that he had purchased during a trip to Australia 15 years earlier. This piece was thumb shaped and weighed 17 grams. He wanted to surprise his wife with a pendant and earrings, made from this stone, to celebrate their 20th anniversary. Unfortunately the stone was so cracked, I wasn’t sure I could slice it into 3 pieces. I told him I might be able to treat the finished cabs with Opticon and he agreed he had nothing to lose. After successfully slicing the stone, I carefully fashioned it into 3 matching cabochons. The Opticon process turned out better than I’d hoped for. The pronounced fractures were almost invisible and a beautiful opal was saved.
Here is the procedure I use:
Clean cabochons thoroughly with acetone, so the resin can penetrate well into the opal. I use a 1-pint, wide mouth, Mason jar with a canning type sealing lid. Pour about 1/2 inch of resin into the jar and gently immerse the cabochons in the Opticon resin. Screw the cover on the jar loosely. Immerse the jar in a small saucepan with warm water and bring to a boil. Simmer the jar, with the resin opal mixture, for about 1 hour.
Remove jar from water; tighten lid and set-aside until cool. This will generate a vacuum in the jar and draw the resin into the cracks. Repeat boiling and cooling cycle again for thorough resin penetration. When heated the resin becomes very thin…almost like water…allowing the resin to penetrate into any cracks.
After 2nd boiling cycle and when mixture has completely cooled, remove opals and lay aside on paper towels. Wipe resin mixture from opals. Don’t clean with any solvents. A slight amount of resin is needed to be present in the fractures for the hardening process.
Place opals in 1/2 pint jar (or larger) and drip enough hardener on the opals to completely saturate. Leave in hardener, at room temp, for about 1/2 hour. Remove opal from hardener jar and clean well with denatured alcohol on a soft cloth. Do not soak the treated stones in alcohol or acetone after this procedure or the Opticon in the stone, which are camouflaging the cracks, will be removed.
Use tweezers and rubber gloves when handling stones. Opticon hardener and resin will cause skin irritation (I have heard some very scary stories), so I ALWAYS wear rubber gloves. If the resin or hardener accidentally gets on your hands, alcohol is a good solvent, then wash thoroughly with soap and water. I keep old resin and hardener in the mason jars and re-use whenever I treat opals. This way an 8-ounce package of Opticon will last almost indefinitely. 8 ounces of Opticon sells for about $22.00 from Rio Grande 800-545-6566. Any questions or suggestions give us a call at 406-651-4947.
Introduction to Opal Carving and Contouring
Here is a text that was designed for to be a lesson plan for my opal cutting classes and eventually turned into a magazine article for Rock & Gem magazine. Hope it is of some use in fine-tuning your opal cutting skills.
On one of our buying trips to Australia, Darlene and I were lucky enough to attend a two-day opal carving course taught by Stuart Jackson, a master opal carver and cutter at the TAFE on Hutchison Street in Coober Pedy. The brief synopsis presented here is loosely based on that carving class with tidbits thrown in from personal experience. Keep in mind this is not the only way to carve opal, just one method that has proven comfortable and that works well for me.
EQUIPMENT: MOTOR UNITS
For opal carving and general jewelry work, I prefer the small, motorized hand pieces that fit comfortably in your fingers…almost like holding a pen. Marathon Micro-Motor System is available from Rio Grande and Gesswein. This unit has a 35,000rpm motor, foot control and runs on either 220V or 110V. It uses a small, powerful motorized hand-piece that fits comfortably in your hand. The price of the micromotor units have come down and they are now competitive with flex shaft units. An alternative is the Foredom SR Flex Shaft System which is also available from Rio Grande. This unit has a forward /reverse selection that is useful to keep debris from flying into your face. It has a collet based hand piece (44T) that is not as convenient as a chuck-based hand piece (#30). These flex shaft units run from US$175.00 (SR) to US$400.00. Have a look at a couple of catalogs and decide which units are best for you. Rio Grande can be reached at 800-545-6566, Gesswein at 800-243-4466, Kingsley North at 800-338-9280 and Contendi at 800-343-3364 . When carving, I use two units with a different cutting point in each hand piece to decrease the time required to change points.
EQUIPMENT: CARVING ABRASIVES AND POLISHES
For roughing out the opal I like to use the small, nickel-plated, 150 grit diamond points available in 20/30 piece assortment sets with many different sizes and shapes. They are very inexpensive and last a long time on the relatively soft opal. The inexpensive points are sold, in most cases, in coarse grits. Lasco Diamond Products (800-621-4726) has a great assortment of different grits (and shapes) of diamond points available. The big advantage in using finer diamond (400 & 600 grit) point wheels is that deep scratches can be smoothed out much quicker than using the Cratex wheels. On the Mohs hardness scale opals rate 5.5 to 6.5 while quartz gems rate 7.0. After I have roughed out an approximate design with the diamond points, I switch to course Cratex wheels of various shapes to finalize the carving design. Then I am ready to begin the smoothing process. I store the wheels in labeled individual organizer boxes to avoid cross contamination. Use one box for each different grade of abrasive. I work my way down from course to medium to fine and then extra fine. The Cratex wheels are soft and lose their shape in a relatively short period of time. But fear not, they can be used down to the nub by reshaping when spinning against the cut on an old bastard file. This name is not a reference to the character of the file. After the Cratex wheels have done their job and there are no more deep scratches seen when finishing with extra fine (check carefully with your Optivisor to avoid repeating this process after you have reached the polishing stage and it becomes very obvious), I switch to diamond compound on small felt wheels of various shapes. Apply the compound directly on the felt buffs-no extender fluid needed. After finishing with extra fine Cratex I switch to 1200 grit diamond. I then progress to 3000 grit, 14,000 grit and then to 50,000 grit. If you have some perfectionist blood coursing through your veins, go on to 100,000 grit and see if it makes a difference. Cleaning the stone and your hands well between diamond grit steps is essential to a good polish. Be sure to keep your felt/diamond buffs of different grits, separated from one another, by storing in labeled individual organizer boxes.
INTRODUCTION: WHY CARVE?
Since opal carving or contouring is so labor intensive, I usually reserve opals that show no promise for traditional cabochon cutting for this procedure. There are several reasons that opals may not be suitable for traditional cabochon cutting. The most common reason is that the shape of the opals may be very irregular-thick on one end and thin on the other or thick on the ends and very thin in the center. Another reason found in very top gem grade material is that you may need to contour the stone to save every carat of material available in the rough stone. This may allow a creative gem cutter to convert an opal that was estimated to finish out into a $1,500 gem into a stone that sells for $2,000. One more reason to carve an opal is to create an object of art. This art object may have a structure completely different than and not dependent on, the shape of the stone or available fire pattern, and the stone is used only as a medium for the final project (this won’t be covered in this article, but it would not be difficult to utilize these techniques to perform this function). Another reason is to be able to shape a stone to follow an irregular or undulating fire layer, to cut a concave face on a boulder opal seam or to remove an irregularity or flaw (such as a sand pocket) from the middle of a stone by only removing a small amount of opal. Mastering these techniques is relatively simple and allows a gem-cutter much more freedom in selecting rough and allows one to greatly improve the final yield from a parcel of opal!
TECHNIQUE: STUDYING YOUR OPAL
After I have selected a piece of rough, I study the stone to determine how best to begin carving. Keep in mind factors such as sand pockets, irregular surfaces, undulating fire lines, directionality of the fire and a final design that is pleasing to the eye. When you have exposed the heart of the opal by grinding away any opaque surface layers, begin planning on removing any extra potch or sand pockets while planning for an eye pleasing final design that uses the fire lines or fire patterns to enhance the beauty of the gem. Yes, what you are thinking is correct-this is the hard part! Some of my favorite shapes are geometric, swirls, waves and shapes that suggest natural objects without too closely resembling them. This will remove the burden of trying to exactly reproduce plants, animals or scenery-something that may require the skills only available with formal art training.
Begin grinding away the colorless skin of the opal using your diamond bits or cabochon cutting lapidary wheels. When you have exposed the heart of the opal begin planning on removing any extra potch or sand pockets while planning for an eye pleasing final design that uses the fire lines or fire patterns to enhance the beauty of the gem. Yes, what you are thinking is correct-this is the hard part! Some of my favorite shapes are geometric, swirls, waves and shapes that suggest natural objects without closely resembling them. This will remove the burden of trying to exactly reproduce plants, animals or scenery-something that may require the skills only available with extensive art training. With a shape or design in mind begin grinding with the small 150 grit diamond plated points. Keep in mind that the ball tips and the cylinder shaped tips are used the most so several kits of the same shapes are needed. They are inexpensive and at the time of writing this article, varied from US$7.55/ kit (20 piece) to US$10.45 (30 piece). These points cut fast and can be used wet or dry. I know that sounds like blasphemy-dry cutting opals, but there is very little heat build-up with the small points. I have yet to have a problem. Dry grinding opals releases silica dust into the air and inhaling silica dust causes silicosis. Wearing a dust mask would be a very good idea! Also, eye protection is important as the Cratex wheels are not designed to run at 35,000rpm and may fly apart if you accidentally use too much speed. Motor operated hand pieces (or flex shaft units) have a bad habit of getting away from you due to the effects of centrifugal force and friction. You may end up grinding an area on your project that was not meant to be ground. To prevent this from happening, you must keep both hands resting on and well supported by the bench or bench pin. When you have roughed in your design to the approximate shape, change to the coarse Cratex wheels and remove the scratches produced by the 150 grit diamond points. You will also be finalizing the shape of your contoured or carved opal. Continue on with medium, fine and extra-fine Crates wheels removing scratches from previous wheels and fine tuning the shape. Inspect your work well with magnification between steps, to be sure that scratches from previous Crates wheels have been removed. Keep in mind that you don’t want to use high speed with the Cratex wheels or they will come apart. Another gotcha with Cratex wheels is they produce a lot of heat, so if you are cutting dry, check your work often for heat build up and keep the speed down. Before progressing to the diamond compound pre-polish/polish stage, give your work area a general clean up. Dust away any abrasive from the Cratex wheels and wipe down your workbench with a wet cloth or paper towel to remove all debris. Wash your hands and your work!
Another method that works well is to rough shape you opals with 320 or 400 grit metal/diamond points. They will not produce as much chipping and deep scratching as the 150 grit diamond/metal points will. Then switch to 280 grit Nova points which should clean up your work nicely, leaving a slight sheen on the carving to better show you where you still need to concentrate your carving and smoothing efforts. Continue using progressively finer stages of Nova points until you reach the pre-polish stage. Thanks to Phil Alderslade in Tasmania for this tip!
Now begin pre-polishing with 1200 grit diamond on hard felt points. I prefer bullet shaped points available from most lapidary supplies or Lasco Diamond Products. A note of caution: Some of the felt buffs have metal rivets exposed on the tops. If this metal should come in contact with your opal it will deeply scratch your work and make you very sad-beware!! Continue on with 3,000 grit diamond compound on felt buffs, then onto 14,000 diamond and then to 50,000 for a high polish. Inspect CAREFULLY with magnification between each step to be sure you don’t have deep scratches remaining. As I said earlier, you may try 100,000 diamond for the ultra super high gloss polish. OK, this is the moment of truth. Put on your Optivisor (or equivalent) and relying on good lighting, closely inspect your work. At this point scratches will stand out on the highly polished surface. You may need to repeat a few steps using diamond grit to remove scratches (or even to the Cratex wheels). Don’t feel bad. This is pretty much routine and part of the process, because it is very difficult to see scratches until you reach this stage. At least that is what I have been telling myself all these years! Opals are one of the most beautiful of God’s creations! After spending a day cutting gem grade opals I can still see the beautiful multi-colored flashes of fire when I close my eyes at night. These delightful stones are unknown millennium in the making. If we can find a way to gently transform more of this precious stone into finished works of beauty, instead of dust in the bottom of a grinder pan, it is surely time not wasted.
REFURBISH YOUR WORN OUT DIAMOND BELTS
I just hate to throw away perfectly good diamond sanding belts with no useable diamond abrasive remaining. The belt integrity seems fine but the diamond abrasive is gone. With diamond powder so inexpensive, I thought with a little ingenuity, I could make a good working belt out of an old worn out belt. And I did! I believe this procedure would work for Nova wheels as well.
OK…Let’s get down to it. I clean the belt thoroughly with acetone. Read the warnings on the acetone can, it is dangerous stuff. Place the belt over an expanding drum or old grinding wheel (just to give it a solid backing) and suspend on a dowel between two stacks of books or boxes so the belt/wheel hangs free and can be rotated easily by hand.
I use Hughes Epoxy 220 made by Hughes Associates out of Wayzata MN. The epoxy hardens without being brittle and mixes well with the diamond powder. I use the small plastic measuring cups for liquid medication doses that are available in drug stores, for measuring and mixing. Mix 1teaspoonful equal amounts of hardener and resin in the small cup. Then dump in a 5 carat vial of the appropriate grade of diamond powder and mix thoroughly again. This is not 5 Minute Epoxy, so you have plenty of time to mix. Mixing is very important here as small lumps of abrasive will lower the effective grit of your abrasive and cause the finer abrasive grades to cause scratches. Mix-Mix-Mix.
Apply the epoxy-diamond mixture to the belt smoothly using the disposable acid flux brushes pictured below. Apply in a thin coat so there are no lumps or thick areas. The epoxy sets up in one hour (hardens in 8 hrs) so you must keep rotating the belt every few minutes to keep mixture from running on the belt. A heat lamp or warm desk lamp greatly reduces hardening times, but don’t get the belt too hot. One teaspoon (and 5 carats diamond) is enough for 1-6in X 2in belt. I usually have a little left over. Ten carats is about right for a 8in X 3in belt. I would toss the used brushes so you don’t cross contaminate although they can be cleaned with acetone and reused for the same grits or coarser.
I recently finished a 600 grit belt and it cuts faster than a new belt just out of the box. I have an older 220 grit diamond-sanding belt, that I refurbished 2 years ago, and it seems as though it will never wear out.
NOTE: Even if a thick coat is applied uniformly, it tends to run, giving you a bumpy belt (though they are certainly still useable, I have a bumpy belt that has been used for several years and is finally beginning to smooth out as the diamond wears off). I now use 2 thin coats (1/2 the diamond powder per coat) rather than 1 thick coat, to create a smoother belt and give your wheel extended life.
Ethiopian Welo Opal: Cutting Tips
Updated 1/3/2017: The new opal deposit in Ethiopia, in Wello Province in the north of the country, is producing spectacular crystal opal displaying very bright play-of-colors in a variety of patterns. The opals in this new deposit, discovered in 2008, are nodule-like stones in an unusual variety: where some stones resemble top Mexican fire opal with excellent play-of-color, others resemble subtle dream-like stones from Brazil and many resemble bright crystal opal from the Andamooka opal field of South Australia. It’s easy to become enamored with this opal.
Most of the opal from Welo is hydrophane opal. Hydrophane opal quickly absorbs water and the opals base color appearance changes, becoming very clear. The play-of-color, though not entirely disappearing, fades out. But luckily this is only a temporary problem and when the opal is allowed to air-dry for 3-7 days, almost like magic, the body color and play-of-color return to normal. Most of this opal becomes white when going through its drying process, but it too returns to its normal crystal or semi-crystal state, although it may take a few days longer to become clear again and for the play-of-color brightness to return.
When cutting this material I use Elmer’s Glue-All to glue the stone to a 4mm x 120mm long wooden dowel. Let the stone dry overnight, then coat the non-waterproof glue with a coating of clear fingernail polish to make it waterproof. Avoid using dopping wax: especially if the opal has been exposed to and has absorbed water. It seems cracks can appear near the matrix pockets on the stone. I’m guessing that when the matrix is exposed to water it expands and causes the opals to crack. I recommend removing the matrix with dremels, micromotor units or flex shafts with diamond bonded points using no water. This will be a dusty procedure, so you may want to use some type of vacuum system in the shop. Be aware that this material is VERY heat sensitive. Mostly I use 360-grit as it lessens the chance of cracking. Opal is relatively soft and a 220-grit or 360-grit diamond wheel cuts fast enough and also doesn’t leave the deep scratches (and wont be as likely to chip) that an 80 or 100-grit wheel will. I like to be very stingy with cooling water used on the wheels. If only enough water is used to keep the wheels slightly wet and the dust down, it should be sufficient and prevent the stones from quickly absorbing water and the finished stone will dry quicker. If you are very concerned about your opals cracking remove all the matrix first dry before using water when cabbing or carving.
Once the stone has been oriented, roughed out and the back of the stone flattened on a flat lap, I go to my normal water-cooled method of opal cutting using a 320 (or 280) grit belt, on an expanding drum for final shaping of the stone. And 600-grit will remove any leftover flat spots and coarse scratches-then onto 1200-grit and 3000-grit with a final polish using a mix of tin oxide (or cerium oxide) and Linde A (or alumina A) 10:1 ratio. Many cutters go to 14,000 grit diamond and 50,000 grit diamond for final polish. Finally, remove the fingernail polish coating by swirling in a jar with acetone in it for about 20 seconds (careful acetone is VERY flammable). Soak in a glass of water for 2-3 hours and the stone should fall off of the dop stick. If not let the stone soak overnight. I’ve never had a problem releasing the opal if it is allowed to soak in water overnight.
One weakness these stones seem to have is sensitivity to pressure: they can easily crack with anything more than moderate pressure. Unfortunately the cracking can take place right down the middle of the stone. This isn’t a crazing problem as you might find in Australian or Mexican opals, but a tendency to be very fragile. It’s especially noticed when flattening the backs of cabochons on a flat lap. I’m wondering if these stones might have developed stress or pressure zones during formation that vibration from grinding or heavy pressure may cause fracturing in these zones. So please use caution when grinding and sanding these gems and a slow gentle touch is best, as opals are a relatively soft stone that responds well to even fine abrasives. At times these stones seem to crack for no apparent reason, during the cutting process. To prevent cracking some customers have suggested soaking rough stones, destined to be cabbed, overnight in water. I haven’t tried this myself as I’m not sure that this wouldn’t produce more cracking-cutting saturated stones may make things worse. I believe the jury is still out, but I tend to school of less water rather than more. If you have had good luck with either method (or something different) please share this with me and I will in turn share it with others-please e-mail email@example.com or call 406-651-4947.. But Ive yet to have a finished cabochon crack when soaked in water to remove the Elmer’s glue on the dop stick. I agree this doesn’t make sense, but then this is a very mysterious stone.
WET CUTTING vs DRY CUTTING
During a recent opal cutting class we experimented by completely dry cutting these stones. We did 5 cabochons completely dry (using 14K diamond paste on canvas for a polish) and 5 wet. Of the wet stones 3 of 5 cracked after wetting. All of the dry cut stones finished up with no cracks. In the next class the following month we tried to narrow this down a bit and only roughed (360 grit grinding through 600 grit sanding) the stones dry (frequently using a dry trim saw to cut bits and pieces off instead of using the grinder and avoiding clouds of dust). We did use water sparingly for the 1200 grit and 3000 grit sanding through polish steps with no cracking. Note: I’ve been using a small amount of diamond compound (in syringes), of the same grit as the wheel, to provide lubrication and better pre-polishing…cutting down on heat and excess wear of the belts. Dry cutting is very dusty, but this limited experiment showed the effectiveness of dry cutting. I am now wondering if some of these stone have much amplified hydrophane properties which cause them to absorb more water faster than the stones silica structure can hold and thus crack. And I’ve also noticed that cracks tend to start at the matrix pockets in the opals. Removing the matrix pockets first with carving equipment may cut down on cracking and allow one to cut cabochons with water?? Perhaps the matrix in the pockets absorbs water, expanding faster than opal, and causes cracking.
Tips from Neil in Queensland Australia: I found your website during a Google hunt on cutting tips for Welo. It’s the most helpful and descriptive source on the web in my opinion. I have been cutting the Welo opals dry ever since with huge success. In the last week, after grading, I have cut and pre-polished over 40 opals without cracking. The only cracks are from the ones missed after trimming and drying. I use your dopping method too, using exterior aquadhere which is a more durable pva woodworking glue. Clear nail polish is still needed.
Welo Opal Cutting Tips from Daniel Bresien: I have purchased small parcels from several vendors over the years and have used the following procedure with about 50% of the stones being cuttable.
1) Grind off all non opal crust. Inspect stone and saw through or grind away any non-opal inclusions.
2) Immerse in water over night. Let dry 2 – 3 days. Repeat.
3) Typically fractures will have already been present and have proceeded through the stone due to the stress of being soaked and dried. OR on inspection you will find pin point inclusion where new fracturing promoted. Mark these inclusions with a permanent marker
4) Lightly grind to remove thin edges and any newly detected inclusions. The permanent marker will soak into the included matrix so you can see what needs to be removed.
5) Soak and dry. Anything that survives can be preformed and cut.
6) After final polish set the stone aside for 6-8 months.
7) Some light crazing may occur 6-8 months after finishing. Inspect your stones under magnification or take a very high resolution photo. Stones showing surface crazing need to go back through the polishing process and should be fine after this point.
I use wet grinding and sanding to shape the opal.
I polish with diamond paste on a belt with silicone.
Effectively I am now cutting one year and selling the next. Cut: 6-8 month inspection. Re-polish: 4-6 month inspection. No crazing;Sell. Crazing: Find another use. Makes great inlay material or aquarium gravel if your wife likes pretties for her fish. 🙂
CARVING WELO OPALS
If you plan to carve this opal (it carves wonderfully) and are using syringes filled with color-coded diamond paste, beware of the color staining the opal. I’ve switched to mixing my own diamond compound (1200, 3000 and 14,000 grits) with a mixture of diamond powder and clear baby oil-it’s even a bit cheaper-and it smells nice. If you’d like a thicker consistency, add a bit of Vaseline to the baby oil and stir well. Or I’ve heard that olive oil, melted bees-wax or Crisco has worked well. If the the oils penetrate into the stone, soaking the stones in Acetone overnight, after the opals are completely carved, has removed all traces of the oils.
I like to carve this material using a combination of diamond/metal bonded grinding points, Cratex points (coarse, medium, fine and extra-fine) for sanding and diamond compound on bullet shaped felt points…dry. When working dry, using these techniques, you won’t have the color change problems associated with working wet. I’ve found that as long as I’m VERY careful with heat, checking the stones temp often, this opal can be successfully cut dry. But it is heat sensitive, like other opals, so care must be taken working without water. And be VERY, VERY careful of dust: which can cause silicosis, very debilitating disease. Of course, if you’re not comfortable carving Welo opal dry, water is an option.
As an aside: I really like the newer Diamond Pacific Nova Miniature Points available from several sources for carving (or the Rock Peddler). These points are diamond powder embedded in a resin and come in a variety of grits from 60grit to 50,000grit. Diamond Pacific recommends using these points with water, but using them dry (and checking often for heat) has worked well for us.
Another carving method that works well is to rough shape you opals with 320 or 400 grit metal/diamond points. They will not produce as much chipping and deep scratching as the 150 grit diamond/metal points will. Then switch to 280 grit Nova points which should clean up your work nicely, leaving a slight sheen on the carving to better show you where you still need to concentrate your carving and smoothing efforts. Continue using progressively finer stages of Nova points until you reach the pre-polish or polishing stage. Thanks to Phil Alderslade in Tasmania for this tip!