Intermediate Stone Inlay for Woodturners
By Dwight Klaus
Revision 1.0
5/9/05
There are many ways to embellish woodturnings with inlaid materials. This article will discuss my experiences using stone inlay to enhance my woodturnings. Combining lapidary techniques and woodturning opens up new creative avenues for artistic expression. The variety of colors and patterns found in stone allow for a broad palette of possibilities. Let your imagination run wild!
Many turners use colored stone to enhance defects in the wood, rather than trying to mask the defects. This technique is very effective, and can rescue a piece of wood that seemed otherwise destine for the fireplace. The same technique can be extended to create decorative inlays in carved recesses. The carvings can render a pleasing image or simply be geometric patterns, or could emphasize a feature of the turning – like the rim, or shoulder.
The methods that I use for stone inlay have been published before – I am simply adding my two cents. My only hope is that you find this technique equally rewarding as I in your own creations.
Let’s start with some safety tips. Woodturning can be a dangerous occupation if proper safety measures are not followed. Read and follow all safety precautions on your power tools and other equipment and supplies.
The chemical compounds in a variety of minerals can be toxic, and grinding the stone will release these compounds into the air. Use appropriate dust control apparatus to protect yourself. Always wear safety glasses to protect your eyes from flying stone chips.
The curing of cyanoacrylate glue, particularly in large quantities, releases heat and very toxic fumes. Make sure there is adequate ventilation in your workspace to remove these dangers, and always wear eye protection when working with or turning a piece containing CA glue.
Many different effects can be achieved using different kinds of stone. The primary characteristics to look for are coloration and hardness.
Minerals used for inlay should not be too hard or too soft. Here are some comments on some of the minerals I have worked with:
Calcite (Calcium Carbonate - CaCO3) – crystals comes in a wide variety of colors, due to impurities present when the crystals formed. Clear, white, yellow, orange, blue, pink, red, brown, green and black are some of the common colors. Calcite is a very common mineral, and therefore reasonably inexpensive. Hardness is 3 on the Moh’s scale, making it relatively easy to grind. Onyx is a form of calcite, and can make a very attractive inlay. You can identify calcite using the acid test: calcite will bubble in the presence of an acid, such as vinegar (dilute acetic acid), releasing carbon dioxide gas.
Fluorite (Calcium Fluoride – CaF2) Also available in a wide variety of colors, from clear to intense purple, blues, greens, reds, and others. Some crystals contain several shades of color in the same piece. Fluorite is harder that calcite, measuring 4 on the Moh’s scale. Fluorite crystals form octahedrons (two pyramids back-to-back).
Turquoise (Hydrated Copper Aluminum Phosphate – CuAL6(PO4)4(OH)8-5(H20) ) – A semi-precious mineral commonly used in the jewelry trade. Comes in a variety of shades of blue, some with interesting matrix that adds character. Hardness is 5-6, which can make it a challenge to inlay.
Chrysocolla (Hydrated Copper Silicate – CuSiO3 – nH2O) – A pretty blue stone similar to Turquoise, but harder to work. It its pure form, it is too soft at around 2 on the Moh’s scale. It is often found as a mixture of other minerals such as quartz, which raises the combined hardness to 4 or more, which is more difficult to cut.
Living it the southwest, we are fortunate to have the worlds largest concentration of gem and mineral traders meeting in our neighborhood on an annual basis – the Tucson Gem and Mineral Show, held each year in early February. If you haven’t attended the show - it’s quite an experience, and a terrific source of materials for inlay.
The town of Quartzite turns into a rock and mineral trading Mecca every year in January and February, and there are many bargains to be had for the savvy mineral shopper.
You can buy turquoise directly from the Sleeping Beauty Mine in Globe Arizona. Just go to their website at www.sbturquoise.com to get pricing and availability. Note that the lowest grade of chips is perfectly fine, since you will be crushing the stone anyway. The price of premium turquoise nuggets is significantly higher, and doesn’t increase the quality of your inlay results.
Bill Baumbeck’s company called Arizona Silhouette located in Yuma supplies crushed stone ready for inlay in a variety of sizes and types. If you just want to try working with stone and only need a small quantity, Bill is your man. You will need to buy large, medium, and small chips, as well as fine dust in order to properly complete an inlay project.
If you decide to crush our own stone, there are several ways to do it. The simplest technique is to use a cold chisel and hammer, and a coffee can. Be sure to wear eye protection, and cover the top of the can with your hand, because the chips will fly everywhere otherwise.
I quickly tired of this technique because it doen’t take many strokes to punch through the bottom of the can and spill your stone, so I built my own dedicated low volume rock crusher. It is constructed of an 8” section of 2” diameter steel pipe welded upright to a heavy steel base. The crushing action is delivered via a 24” x 1.25” cylindrical steel bar. I drop a few rocks into the pipe, and then slide the cylinder downwards. A few strokes are usually plenty – don’t pulverize all the stone. You want a mixture of sizes, from large chips to fine dust.
I use a series of coffee cans covered with different size screening to sift the crushed stone into similar sizes. For the smallest chips, I use a hand-held kitchen strainer (don’t tell my wife).
If you discover you like doing stone inlay, you will use a lot of CA glue. Buying those little bottles of glue at retail prices will cost you a fortune. I have had very good luck buying the Starbond CA adhesives in quart bottles. They even give you plenty of small empty plastic dispensing bottles when you buy a quart.
You’ll need two different viscosities. Use the very thin viscosity glue for most of the stone setting. The medium viscosity is useful to fill those last few blemishes to make a perfectly smooth inlay.
There is also black medium viscosity CA glue from Starbond. I have not had much success using this because it always ends up where I don’t want it, and it’s hard to fix. If you want to set stone in a black matrix, I would recommend using epoxy mixed with black pigment rather than black colored CA glue. Note that using contrasting color glue is a very effective technique to set off the stone.
I keep the bulk of my glue in the original quart container inside another plastic container in the back of my refrigerator. I also stuff silica bead packets in the outer bottle to dry the air (more on this below). It is guaranteed to last 20 months this way by Starbond.
For my working supply, I dispense a few ounces at a time into the bottles supplied by Starbond.
CA glue sets in the presence of moisture, so the trick to making it last is to remove the moisture from your storage area. When not in use, I place my working bottles in a large plastic jar with tight lid. Also in this container I place a collection of silica bead packets that I have saved from other shipments. These packets will dry the air inside the container, and create a low humidity storage chamber. This really works! I don’t even close the bottles of CA glue any more, and they stay fresh inside this container.
CPH International – Starbond CA glue (www.starbond.com) 1-800-900-GLUE
You cannot cut stone with steel turning tools – the cutting edge of your tools will dull instantaneously when it contacts the stone. Then the tool will knock your carefully set stone chips loose, and centrifugal force will launch them all over your shop.
The only effective way I have found to work the stone is with abrasives. Normal sandpaper is made of aluminum oxide, which has a hardness of 9 on the Moh’s scale, and it will cut any stone you reasonably would want to use for inlay (unless you are Warren Buffet and like doing diamond inlay). Silicon carbide sandpaper (the black stuff) is harder yet, and works well on stone, but tends to clog quickly if used for sanding wood.
After a few minutes of hand sanding, you will quickly decide that there must be a better way to reduce your stone to a nice smooth polished surface. Here are the power tools I have found to be effective:
This is the fastest way to get the stone close to level with the wood. Don’t forget to wear eye, ear and lung protection. Be careful not to get carried away and grind the stone too far. If you do, you will have to glue more stone chips into the indentation, and start again. Also, be careful not to gouge the surrounding wood.
This will take the stone down and even out any irregularities created by the grinder. Work through a course grit sequence until the stone is shaped properly, and the surface is smooth. Be careful not to over sand wood next to the stone, and cause an indentation, particularly if inlaying in soft wood. I like to use an air sander because it is lighter and quieter than electric, but either one will do the job.
I use the Velcro backed sanding discs in my hand drill to finish sand, and smooth the stone and wood. Run the lathe at fairly low RPM to avoid dislodging the stone. I typically use 120, 150, 180, 220, 320 grits.
Here are some hints on making a piece that is visually pleasing. Disclaimer: this section contains my opinions, which may not be consistent with yours, so feel free to do your own thing and ignore what I have to say. At the end of the day, make something that pleases you, and don’t worry about what anybody else says. There are always some that will love your work, and others that will hate it – you can never please everybody, so stop trying.
It’s hard to fix a bad shape by plastering stone to the surface and grinding it into a good one. So, the better approach is to turn a shape that is already pleasing, and then improve the visual character with stone inlay.
There are many other articles on what makes a shape “work”, so I’m not going to go into great depth here. I agree with those that say “A good shape is equally appealing upside down as it is right side up”.
Inlaying a channel with colorful stone is a very effective way to add interest to the rim of a platter, the neck of a hollow form, the base of a vase, the center of a platter, etc. You can cut single channels, or multiple channels, and inlay them with a variety of different stone color schemes and patterns.
Use a Dremel or Foredom power carving tool to carve a recess in your woodturning to a depth of at least 1/8th of an inch. Then inlay stone in the recess in whatever pattern pleases you. Note that you can paint a picture by using different sizes and shades of stone. If the stone is opaque, then don’t worry about what the bottom of the carved recess looks like, since you can’t see it after the inlay is complete. If the stone is transparent, then be more careful about the appearance of the carving. You may also want to add color to the recess before inlaying transparent stone, so you can see something other than bare wood through the inlay.
If there are natural openings in the wood, then stone inlay can be used to accentuate these openings. It’s a very popular technique to fill cracks and voids with a contrasting color of stone. Be aware that the location and distribution of these openings can make a big difference in how the final product appears – not every checked piece of timber is appropriate for stone inlay.
Another technique that can create an exciting effect is to use a different color material as the filler between larger chips of stone. Brass filings can be sprinkled into small openings in place of stone dust to create the appearance of veins of gold in the stone inlay. Soft metals such as brass, copper and aluminum have about the same hardness as the stone, and therefore will polish level with the stone surface.
You can use whatever carving method appeals to you in order to cut a recess for inlay – hand tools, Dremel or other power carving tool, or my favorite – a dental drill.
The carved recess must be at least 1/8th inch deep, and preferably more. This is to enable the stone inlay to be reasonably thick. A thicker stone inlay will be less likely to chip or tear out as it is polished. Also, some stones loose their color if too thin – calcite and fluorite are prime examples. These stones will look white or clear if too thin.
The edges of the carved recess should be abrupt – don’t carve a rounded indentation and try to inlay into it – the stone will chip out at the edges.
CA glue will be absorbed into the surrounding wood, and stain the surface, or discolor the final wood finish. To avoid this, I seal the surrounding wood with a light coat of spray lacquer. The lacquer dries quickly and seals the wood, but is easy to remove with abrasives. I am not particularly careful about keeping lacquer away from the recess to be inlaid – it doesn’t seem to prevent the CA glue from binding to the wood.
Another approach is to use a light coat of wax to accomplish the same glue resist effect. Apply the wax carefully, keeping it away from the inlay recess, because it will definitely prevent the CA glue from attaching to the wood. You may want to use a small artists brush for fine details. If you get wax in the wrong place, you can remove it with mineral spirits on a rag or Q-tip. One nice feature of using wax is that it’s easy to touch up if you sand it away while grinding the stone. You will need to apply CA glue several times as you repair tear out or pinholes in the stone inlay, so you must reapply wax on any bare wood that may absorb glue during these repairs.
I cannot over-emphasize the importance of sealing the wood to guard against glue discoloration – it makes a big difference in the final appearance of the piece. If CA glue is absorbed into porous wood, it can be nearly impossible to remove, and will surely be visible under the finish (unless you finish the whole piece with CA glue).
For the following steps, I will assume you have your piece horizontal, so that you can place stone chips in a recess without them falling out. If you are inlaying on the perimeter of a bowl, then you must perform these procedures section by section, while rotating the bowl to bring the next section to the horizontal.
The biggest design mistake that you can make is dumping a pile of rock chips into a recess and adding glue. If you do this, your piece will look like a pile of rocks and glue when complete – not the effect we are after! A good inlay job requires careful placement of each stone to form a dense, but random matrix. This is the difference between artistic results and a concrete mess.
Start by adding the biggest pieces of stone that will fit in your opening. You can grind or chip a piece of stone if necessary to make it fit snugly into a specific location. Make sure the stone is high enough that it extends above the inlay region, so that you can grind it level with the top of the wood.
Occasionally, after you have a nice mosaic of large chips covering a section, add a few drops of thin CA glue to tack these stones in place. Then continue with large chip placement, until the whole inlay is complete. Tacking the stone in place will prevent an inadvertent bump from ruining your careful placement job.
Let the glue dry naturally, without accelerator, to insure that it doesn’t foam or bubble.
Once the large chips are in place and the glue is dry, its time to place small chips around and between the big chips to fill in any gaps. I use tweezers to place each chip in exactly the right place, and in the right orientation. Try to get enough of each small chip below the surface of the surrounding wood, so that it will not be jarred loose as the stone is leveled.
Take your time, and carefully keep adding small chips until all the gaps are filled. Then add enough CA glue to tack these small chips in place, and let the glue dry before continuing.
Now we will fill all remaining gaps with fine stone dust. Sprinkle the dust into any crevices, and level the powder with your fingers. Tap the wood lightly to encourage the powder into any deep fissures, and add more stone to bring the surface back to level. The surface of the stone dust must be above the top of the wood.
Now slowly saturate the stone dust with CA glue – the powder will run into the nooks and crannies between the stone chips. Do not attempt to move the damp stone dust – you can always add more dust and glue later.
If the inlay recess is on the exterior of a bowl, then you may need to do this step in stages, letting the glue dry in one section, then rotating the next section to the horizontal and adding more dust and glue.
I do not recommend using accelerator during this stage to speed up setting of the CA glue – it is too easy to ruin the results.
Once all the glue and stone has hardened, you are ready to cut the inlay back to level with the wood surface. Some artists claim to be able to turn stone inlay with sharp carbide tipped tools – I have not found this to be effective. I use abrasives to cut the stone with minimal tear out.
The fastest way to remove the bulk of the waste material is by using the 4” angle grinder with an abrasive wheel (not a cutoff wheel). The stone dust and glue mixture forms a concrete bonding material that supports the larger stone chips during grinding, but I am still gentle with this tool. It makes short work of getting the stone nearly level with the wood – be careful not to grind below the wood surface.
Now that the bulk of the stone has been removed, switch to course abrasives and the random orbit sander. You can bring the stone level with the wood quickly with 80 grit. Continue with finer abrasives until you can clearly see any surface defects in the stone inlay.
There will always be a few spots that didn’t get completely filled with stone, or places where the stone was knocked loose during the leveling process – this is normal and unavoidable. Before attempting to patch these defects, protect any bare wood near the repair site with wax to prevent CA glue from discoloring the wood, but keep the wax out of the void.
If the defect is large, clean out the void with compressed air, and place new stone chips into the opening and tack them in place with a drop of CA glue. Repeat this for all the large voids.
Now sprinkle stone dust into any remaining small voids and pinholes, and around the newly set stone chips, and add CA glue to saturate the dust. Once the glue has dried, sand with 80 grit to bring the patches level with the surrounding stone.
Any remaining defects can usually be patched with medium CA glue – just place a drop in the defect and let dry.
Continue with the random orbit sander through the complete grit sequence. I typically sand to 800 grit to produce a nice smooth stone surface, then buff with a little rouge to polish the stone perfectly smooth.
You can use most any finish at this point. I like to use lacquer as a finish, and it is applied to the stone the same as to wood – I spray it on in many thin coats. Then, once it has hardened, I buff the final piece to a satin finish.
I hope that the information above helps to inspire you creatively, and gives you some new ideas that will enhance your woodturnings. Stone inlay is a fun technique, and a great way to make your woodturnings stand out from the crowd.
One additional note: don’t be discouraged if your first attempt at inlay doesn’t meet your expectations – just learn from the experience and continue on to your next piece. I have many pieces lining the walls of my shop that didn’t come up to my standards – the key to learning is doing. If you don’t keep an artists journal, then start one today – take notes about what went well, and what didn’t go well, and you will quickly correct the errors and put them behind you.
“Stone Inlay” by Stephen Hatcher
American Woodturner magazine, Winter 2002