Creating Imagery with Stone Inlay
by Stephen Hatcher
I attended the 1998 Utah Woodturning Symposium just several months after starting to turn wood. There I saw many incredible demonstrations, but one truly inspired me: Kip Christensen and Lane Philips were inlaying bowls and platter rims with stone. I was impressed with the effect and, although my materials and design ideas have evolved since then, I have pursued this design enhancement technique since. These days I inlay scenes inspired by Mother Nature. The forests, wetlands, and oceans here in the Pacific Northwest suggest endless design motifs as the different seasons progress through their four separate stages. Nonetheless, I owe Kip and Lane my sincere thanks for introducing me to this idea.
Stone (or more specifically, minerals) can add breathtaking imagery to already beautiful wood. I prefer to use translucent crystals inlaid into big leaf maple.
My preference for maple is founded in its exceptional figure, color variation,
and abundance here in western Washington State. The piece made for this article literally grew in my neighborhood before dropping in a winter storm.
This particular species of maple is about the same hardness as the minerals I favor, so the sanding process results in a surface that is level between the wood and minerals rather than domed. I have used redwood and spalted woods, but have found them to be a little too soft; however, if they are well saturated with wood hardener or thin cyanoacrylate glue (CA or superglue), they will work well too.
SELECTING STONE
The Moh’s hardness scale is used by rock hounds and geologists to describe a mineral’s hardness on a scale of 1 to 10. A hardness of 1 is talc (chalk), while a hardness of 10 is diamond. Aluminum oxide (corundum) that is used in abrasive paper has a hardness of 9.
The stone selected for inlay needs to be soft enough to finish with normal power sanding tools, yet be durable enough to provide lasting beauty. There are several minerals that are readily available, brightly colored, and/or translucent with high reflectivity that range from 3 to 4 on the hardness scale. This range of hardness (about equivalent to a copper penny) is fairly easy to work while still remaining durable. My favorites in this range include azurite, malachite, calcite, and fluorite. Turquoise (with a hardness of 5 to 6) is popular, but not as intensely colorful as the other minerals, and usually more expensive. All these minerals are quite common and can be found in most local rock shops or online (see Materials Recommendations).
Options certainly exist to use other types of stone, but just beware. Soapstone (with a hardness of 1 to 2) is fun to turn by itself and pretty, but too soft for durable inlay. Minerals harder than 6 are very difficult to finish, but that doesn’t necessarily preclude their use. You can layer these beneath a softer translucent material (like calcite) and the color of the harder mineral will show through, while the surface mineral is readily finished.
Minerals can be mixed randomly, uniformly, or in patterns. The addition of brass or aluminum filings can enhance the overall effect as well. However, when adding metals, I have found that “less is more.” A small amount of metal adds a beautiful gold or silver glint whereas too much metal tends to wash out the color of the minerals.
THINGS TO CONSIDER BEFORE STARTING
The inlay results in various woods (ranging from soft spalted alder to hard maple) are always great as long as the wood is relatively dry (about 12% moisture content or less). At this range, any movement of the finished piece through the different seasons is small enough that I’ve never had the stone inlay disfigure.
When turning or carving a notch for an inlay, the notch must be very well defined because the inlay sides will look sloppy if the notch has tearout along its sides. I suggest using a wood hardener on softer woods to ensure cleaner cuts.
It is advisable to cover the surface immediately around the area where an inlay will be cut with a finishing wax regardless of what wood is used. Leaving this barrier in place on the wood surface after cutting the inlay will keep CA adhesives from staining the wood as you build up the minerals for the inlay. The carved inlay recess must remain free of wax, however, so if any gets in there, re-trim the notch to produce a clean surface.
Like wood, sanding stone creates very fine dust. Be sure to use a dust collection system and wear a well-fitting mask. The fine dust that comes off the minerals will adhere to the insides of dust collection bags, so give the bags a gentle shake after you have been working with the stone to keep the filter pores open.
Do not use wet sanding to try to keep the dust down–at least not with calcite. Calcite can be etched with water and you can damage the surface or edge of the inlay if it stays wet very long.
Stone inlay requires the use of relatively large amounts of superglue. I used about one ounce for the platter described in this article. Superglue fumes are irritating to your eyes and respiratory system, so be sure to keep the area well ventilated.
PREPARING THE STONE
Larger chunks of stone must be crushed to fit the notch width. I use a small coffee can and a concrete chisel held backwards so the flat end strikes the stone to smash my inlay material. Cover the top of the can with one hand while pounding the stone with the chisel, otherwise pieces of stone will fly all over the shop! A piece the size of a quarter yields a lot of crushed material.
PLACING THE INLAY
The following step-by-step process summarizes the procedure that I use to create the inlay on my work:
1. Crush the stone only enough to allow the largest pieces to just fit into the notch. The remaining pieces will be progressively smaller, down to a fine powder.
2. Place the largest pieces in the notch in whatever pattern you desire; usually a deliberately “random” arrangement is a good start. The largest pieces don’t necessarily need to be centered in the notch; rather, position the pieces so they are uniformly distributed across the entire notch width.
3. Add enough superglue to hold these pieces in place and use an accelerant sparingly to prevent clouding. I sometimes mist a small amount from about two feet above the piece just to hurry it along. The superglue will cure to a clear polymer, but too much accelerant will result in bubbles or clouding of the CA polymer.
4. Fill voids with smaller pieces of the crushed minerals, but don’t use the fine powder yet. Add super-thin superglue, but use very little additional accelerant.
5. Add brass or another complementary material to the voids at this time. The result will appear like veins running through the inlay.
6. Add the fine powder to fill the remaining voids and repeat the application of superglue. It is important to build the superglue up in layers using progressively finer material to avoid pockets of liquid superglue within the inlay. These pockets will foul your abrasive paper when finishing and the pockets of unglued inlay material will tear out during finishing on the lathe.
FINISHING THE INLAY
Aluminum oxide (AlO) abrasive paper has a hardness rating of 9 so it will cut through any of the minerals I have recommended. If the inlay is thick, I use silicon carbide (SiC) abrasive paper for the initial sanding and then switch to AlO to finish. SiC (often referred to as stearated paper) is used most often because of its high friability.
The following step-by-step description summarizes the steps I use to finish most of my work:
1. Take the stone inlay down to where it is about level with the wood surface by power sanding. At this time, try to get the inlay almost flush with the wood surface just enough to see any voids in the initial inlay setting.
2. Remove the chuck and woodturning from the lathe as a unit and blow away any dust in the inlay voids. Apply more wax to the wood only as needed to limit staining. Then add finer mineral pieces and mineral dust on top of the inlay to get the surface level. Reapply super-thin CA glue, this time dripping it onto the fine material and letting it spread. Return the assembly to the lathe and repeat the sanding process with 120-grit abrasive. Sand into the wood slightly to remove stains created by the superglue or mineral dust. If necessary, repeat this step.
3. When the remaining voids are small enough that no wood is visible within the inlay and the largest void is less than about 1/8", use gap-filling superglue to fill these voids. This can be done while the piece is on the lathe.
4. Progress through finer abrasive paper grades just as you would in finishing any turning. The stone inlay may be a little tougher to sand when it’s coarse, but when the inlay is essentially completed, go to 180, 220, 320, and 400 grits for the final sanding. Run the lathe at a slow speed during this phase and keep the abrasive cool. In soft woods, use care not to erode the wood around the inlay, creating a stone “dome.”
5. Occasionally a small piece of inlay will tear out during sanding and leave a void. If it’s small enough, just refill with gap-filling superglue or otherwise add a little mineral dust and use super-thin superglue. For these patches, I just continue sanding with whatever grade of abrasive I was up to when the tearout occurred.
6. I polish the inlaid surface with a buffer and recheck for defects. The inlay will polish to a nearly perfect luster. Apply your favorite finish and rebuff. |
