For what it’s worth here is a procedure I use to make Australian and Welo opal doublets. I previously had used black jade from the Australian Cowell deposit but haven’t been able to find another good inexpensive source. Color-Wright.com was my original source but they have sold out. I’d recommend using opaque black obsidian at this point….Steve
When making opal doublets I prefer black jade or obsidian for the opal backing, but there are a number of dark stones that may work just as well. Just be sure it’s relatively tough and takes a good polish. I slab the backing stone into 2mm-3mm slabs. I prefer to use opal pieces that are 2mm-3mm thick, but if you’re careful and the opal has few pits or flaws, slightly thinner pieces can be used. I’d like to experiment with dark blue lapis, but that can be relatively costly. After you have selected a backing the next step is to get both your opal and backing very flat so a tight bond will be created when glued. Flattening the opal using a flat lap works best, but 220 grit to 600 grit wet/dry sandpaper on a table or the flat sides of an old 220 grit silicon carbide wheel are alternatives. Be sure to remove any flaws from the glue-side of the opal as they will be very apparent after bonding. And the brightest color side should face the backing…I know this sounds kind of crazy but you will be working the opal down to the bright opal which the dark backing will intensify. My favorite (and the easiest) is to use a 360 grit diamond disk on the lap for flattening both stones. Some lapidaries suggest painting the back of the opal with black paint…I’m afraid the paint on the back of the opal will compromise the epoxy glue bond between the opal and the backing. But I do like to add a little lamp black dye powder, (lamp black or powdered graphite) into the epoxy mix for very deep black color. Hobby Lobby sells “Black Castin’ Craft Color Pigment Concentrate” for coloring epoxy…haven’t tried it as yet, but it’s an option. A tiny amount of pigment powder goes a long way when coloring the epoxy.
Before gluing, be sure to clean both the backing and the opal with acetone. This helps the epoxy to adhere better to the stones. I use Hughes Epoxy 330…it’s a crystal clear glue and doesn’t thicken for 15 minutes, giving you time to mix and carefully place your opals on the backing.
Mix the equal parts of the glue thoroughly and use plenty of glue. If you’d like, mix a small amount of black dye powder with the two parts of epoxy…mix VERY thoroughly. I place a little glue on the opal and a bit more on the backing. Carefully (I like to use tweezers) place the opal on the backing…moving the opal in a slight circular motion while applying a slight amount of pressure, will help remove any offending air bubbles. Setting the glued doublet on a 3” x 5” card 7-8 inches beneath a 60-watt spot lamp will speed up the curing process. But it will also make the epoxy thin and runny initially…thus the 3×5 card will catch any dripping epoxy and prevent the doublets from being glued to your work bench. Be sure to check the stone to be sure it’s not getting too hot…you don’t want to end up with a cracked stone…this is critical…I’ve cracked a couple of Welo opals this way!! The glue will set-up in about 15 minutes under the lamp. Then set the stones aside for about 24 hours before doing any work on the glued pieces…to give the bond time to set completely. If you’re not comfortable with the lamp you can certainly skip that part.
Trim off extra backing with your trim saw and attach to a dop stick. I prefer a thin low-dome opal top. This allows for greater contrast and brightness as the black backing intensifies the fire. Also, I prefer almost vertical sides of maybe 10-15 degree angle off of vertical. Makes for a tighter bezel fit in finished jewelry and helps hide the backing material. With a low dome cabochon watch carefully for a flat spot and scratches in the middle of the stone. And be sure to smooth and polish the black backing sides for a professional look. Any suggestions please post them…thanks Steve