 Cutting
Australian Boulder Opal
by Eugene McDevitt
Intermediate lapidary project.
Back to Part 1: Selecting & preparing rough,
exposing opal.
Part 2: Dopping, Cutting, & Finishing
At this point, your rough should be prepared and your opal exposed. I will
not be discussing using a flexible-shaft machine. While some boulder opal, particularly
high-grade material, requires a machine, in many cases very attractive stones
can be successfully cut using diamond wheels.
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- Australian boulder opal rough
- Dop sticks
- Dop wax or strong glue
- Trim saw with thin blade (rim thickness approx.
0.02")
- Diamond grinding wheels, 80-, 220-, 600-,
1200-, and 3000-grits
- Cutting equipment
- Rough leather polishing disk
- Tin oxide polish
- Lamp with 100-watt bulb
- Safety glasses
- Dust mask
- Old work clothes
For information on supplies, please see the Annual
Buyers' Directory.
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STEP 1. Dopping the Opal
Using the 220-grit wheel (or gently using the 80-grit wheel) prepare the stone
for dopping by flattening its back. Reduce the thickness of the back of the stone
so that it is just slightly thicker than what you think would be appropriate for
the finished stone. If necessary, you can remove more later. Gently place the
back of the stone against the 80- or 220-grit wheel while rotating it in your
fingers.
Remember, you are flattening the back -- not cabbing it. The ironstone backs of most of my stones are probably no more than 5mm thick, unless the stone is quite large or has an unusual shape. Make sure that the back is thick enough to support your opal.
STEP 2.
The stone is now ready to be dopped. The bottom of the stone should be clean and dry. I use one or two drops of glue to attach the dop sticks. Preheat the opal by resting it on the edge of the dop pot for no more than 30 seconds. After polishing, soak the opal in warm water to remove it from its dop stick. Apply light pressure to pop the opal off its dop stick after about 30 minutes. It may or may not come off -- you may need to soak it overnight.
If the back of the opal is porous, use dop wax instead of the glue, in which case you should remove the opals from their dop sticks by placing them in an open bowl, then place the bowl in the freezer for 5 to 10 minutes. Remove the bowl from the freezer and carefully place the blade of a knife along the line between the bottom of the stone and the dop wax. Apply gentle pressure, and the stone should pop off rather easily.
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STEP 3. Cutting the opal.
If you are using a machine that recirculates its water (i.e., the water is sprayed up onto the wheels, dripped down into a pan, then pumped from that pan and sprayed up onto the wheels again), change the water and clean the pans frequently. Chunks of ironstone and opal that were removed with the 80- and 220- grit wheels can splash onto the 280- grit wheel and cause scratches. The same is true with the set of finer wheels. I usually prepare several stones using the 80- and 220- grit wheels and then change the water. Also, I usually cut several stones before I move the stones on to the 1200-grit wheel. |
STEP 4.
Once the glue has dried or the wax has cooled, check your opal for scratches. Think about its natural shape (which might not be the shape you want it to be). Use the 280-grit wheel with light pressure to remove serious scratches from the edges and the top, remembering to keep the stone moving to avoid flat spots and scratches. Stop frequently and wipe your stone clean to check progress. Look at the surface of the opal, not into the opal. After doing this, use the 600-grit wheel to remove the scratches caused by the 280-grit wheel and to expose more of the opal.
As I try to expose more of the opal, I often find stubborn islands of ironstone on the surface and mountains of ironstone that rise up through the opal layer to the surface. This is the nature of boulder opal. Be careful when trying to get rid of those islands -- some run deeper than you think and to remove them completely would severely distort the shape of your stone. Don't worry; these unique patterns add personality to your stone.
STEP 5.
At this point you need to make some important decisions. Look at the overall appearance of your stone, both the opal and the ironstone. Often the combination can be quite beautiful. If you are not satisfied, most of the tricky work is over. If not, or if you think that there is more treasure lurking beneath the ironstone, continue removing ironstone with the 600-grit wheel, remembering to keep the stone moving. The 600-grit wheel can still remove opal and create flat spots and scratches.
You are doing several things during this stage of cutting. You are removing scratches fromearlier stages, removing ironstone from the surface, and making the surface and shape of the stone more pleasing to the eye. Look at your stone and see what it shows you. You might need to go back to the 280-grit wheel and change the shape. Once you have removed the large scratches and are satisfied with the look, move to the 1200-grit wheel.
STEP 6.
The 1200-grit wheel will remove almost all visible scratches. It will not do much to change the shape of your stone, but it can improve its appearance. Use moderate pressure and keep the stone moving. Make sure to cover the entire stone, including the sides and areas of ironstone. Stop frequently and wipe your stone clean to check your progress and make sure that the scratches are disappearing, although the effects of the 1200-grit wheel are not immediately obvious as those of the 280- or 600-grit wheels. The 1200-wheel is also useful to round any sharp edges. Once you have removed the visible scratches, move to the 3000-grit wheel, or if you do not have a 3000-grit wheel, start to polish your stone.
If you machine has a 14,000-grit wheel, I strongly suggest that you not use it to cut boulder opal. This wheel often introduces scratches.
STEP 7. Polishing
To polish, use tin oxide on a clean foam-backed rough leather pad. Lightly wet
it with water from a spray bottle. With the machine running and the pad spinning,
use a brush to apply a well-stirred mixture of tin oxide and water onto the damp
pad. The tin oxide and water mixture is thin enough so that it does not clump,
and has the consisitency of thin gravy.
 STEP
8.
Select a dopped opal to be polished and either gently touch it to the polishing
pad so that it picks up some polish or just touch it with the brush used to apply
the polish and water mixture.
Using light to moderate pressure, polish the opal on the pad. Keep the opal moving by rotating the dop stick between your fingers and by changing the angle between the opal and the pad. Check your progress frequently by gently wiping the surface of the opal. Remember to polish the edges of the stone as well. Keep a firm hold on your stone and do not apply too much pressure. Do not let the opal get too hot. Sometimes the rough leather pad can grab your stone and yank it out of your hands, particularly as the pad dries out or if your stone has an uneven surface or sharp edges.
STEP 9.
It takes practice to determine when you are finished polishing an opal. You want to remove all the scratches as well as make the opal as bright as possible. Keep the pad wet and reapply the polish and water mixture when needed. If the pad gets too dry, it can scratch the opal or cause it to overheat.
If your opal has much ironstone on the surface or if the surface is particularly
porous, be careful. Often the leather pad will catch on the ironstone and either
you will feel a tug on the stone or the pad will tear out small pieces of the
ironstone that could damage or scratch your opal.
STEP 10.
If you are polishing a matrix opal, follow a similar procedure. Often the opal
in such stones will polish normally, but the ironstone matrix will be dull. A
useful technique is to polish the opal as described, then let the pad dry slightly.
Apply more pressure while you keep the stone moving, so that you can feel the
stone get warm. Remember, you are polishing the matrix, not the opal areas of
your stone. Be careful -- gently heating the stone on your polishing pad can improve
the finish of the matrix, but too much heat can damage the opal. This technique
requires a bit of practice, but the results can be worth it.
 STEP
11. Finishing the back.
Once you are finsihed polishing, remove the stone from its dop stick. Use light
to medium pressure on the 220- or 280- grit wheel (or the 600-grit wheel for more
precious, more fragile, or smaller stones) to remove any remnant of the dopping
material. Keep the stone flat against the wheel, but rotate it slightly so that
there will not be any deep scratches on the back. At this point, decide what you
would like the final thickness of the back of the stone to be. I usually need
to remove a little ironstone to reduce the thickness. Once you have reached the
appropriate thickness, use the 220- or 280-grit (or 600) wheel to bevel the back
edge of the stone, which will reduce chipping when setting.
STEP 12.
Depending on the appearance of the back and your taste, you can either consider your opal finished, or achieve a more finished appearance by working the back through the remaining series of finer wheels. I usually work the backs of my stones through the 1200- or 3000-grit wheels, sometimes going so far as to polish the backs.
Eugene McDevitt is a Florida-based
importer, cutter, and wholesaler of Australian boulder opal and other stones.
He can be reached by e-mail at gene@koroit.com,
on the web at www.koroit.com, or phoned at
727-742-5471. Mailing address: P.O. Box 20288, St Petersburg, FL 33742.
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