Pearl Inlay Part III


by Sean J. Barry

This is Part III of the three-part series on instrument inlay. Part I, a discussion of tools and materials, appeared in October, and Part II, an explanation of pearl-cutting technique, appeared at Thanksgiving. I will repost all three parts together in January.

As before, all standard disclaimers apply--to my knowledge, no company mentioned here has ever heard of me, and tools and materials are mentioned by brand name only when my experience indicates that few or no other alternatives exist, or I have insufficient experience with alternatives. Copyright rules remain in place: feel free to download the text for your own use, but please do not forward, crosspost, or otherwise distribute the text without permission.

Part III -- Routing and inlaying

By now you should be finished cutting your chosen pearl pattern, and you are probably tired of repeated trips to the safe deposit box. Undoubtedly you have been staring at the Dremel Moto-Tool and the router base that you purchased after you read Part I and wondering what they do. Now you shall find out.

Go to your safe deposit box and retrieve your entire inlay set, and arrange it on the table in the proper orientation. With a pencil, label each inlay, and draw a small arrow that points toward the end of the peghead. The arrow is only necessary for those pieces that are part of a radially or bilaterally symmetrical arrangement with several identical pieces, such as petals of a flower. Once you have scribed and begun to cut the mortises, you must avoid confusion as to the precise location of each piece. From this point forward, you must not change your assigned positions--to do so will result in confusion, broken inlays, and problems during the final inlaying process. This is your last opportunity to recut any inlays that are not on quality par with the others, and to rearrange and rematch pieces to best advantage.

The next two steps are really the most critical in the entire inlay process. Up to now, if you broke an inlay or your pattern was uneven in quality, the problems were fixed easily by cutting new pieces. After you (temporarily) glue your inlays in place and scribe the patterns into wood, it will be tricky at best to replace any, so take special care not to break any or to change your mind about placement or replacement. Glue your inlays in place on the surface to be inlaid. In my experience, this is best done with DUCO cement, because this material can be dissolved away with acetone (Fire and toxic hazard! Use only in well-ventilated areas away from sparks and flames! Store safely!). "Spot" the glue lightly in several places on the bottom of the inlay, and press the pearl firmly in place on the surface. Script inlays (written text) are especially tricky and obviously fragile, and should be glued thoroughly on the bottom. Endeavor to clean up as much of the glue squeeze-out as possible while it is still soft. Double-check that all inlays are properly positioned (remember: guitars are inlaid on the 9th fret, banjos and mandolins on the 10th), and set the object aside for at least 24 hours. I used to use white glue instead of DUCO, but had problems removing the inlays--the only options are to pry up the inlays or soften the glue with water. The former can result too easily in broken inlays, and the latter tends to obscure the scribe lines, so I went to DUCO. Again, once your patterns are inscribed, it is highly desirable to use the same inlay that was inscribed, because it is impossible to cut another piece exactly like the original.

The next step, the most critically important in the entire process, is to inscribe the inlay outline into the wood. Use the scribe that we discussed in Part I, and trace around the pattern as close to the edge of the pearl as possible (which should be flush with the edge), but avoid undercutting the pearl, and most of all avoid pushing on the inlay itself with the side of the scribe. At best you could dislodge the pearl (this only happens, according to Mr. Murphy, to complex inlays and then only after the outline is about 50% but less than 75% inscribed), and at worst you could break the inlay--again, this is a calamity if you have already inscribed much of the outline. If your scribe encounters a mound of glue, scribe carefully over it several times until it separates from the inlay, then scribe the wood. The wood grain will tend to divert the scribe point, so be aware of grain direction changes (relative to the inlay). Ebony is so hard that it is best inscribed by making repeated passes. Be slow, be cautious, be meticulous, be a perfectionist. This is your only chance to do this step correctly, and the quality of your final product depends on the scribed line (and your ability to follow it with the Dremel). I have tried to deepen the scribed lines later, after the inlay is removed, but with very limited success. Even though the pearl is not supposed to be a "fence," its presence offers a much better visual limit than does the scribed line alone. Inspect each scribed line carefully and make certain that all are complete and deeply inscribed. With a glass eyedropper, dribble some acetone carefully around each inlay, but just do one or two at a time.

*BE EXTREMELY CAREFUL NOT TO ALLOW ACETONE TO CONTACT FINISH OR PLASTIC BINDINGS, AS IT WILL CORRODE THEM. ALSO, REMEMBER THE FIRE HAZARD! A NEARLY EMPTY CAN OF ACETONE IS A POTENTIAL BOMB! THE FUMES ARE TOXIC!*

After the acetone has contacted the inlay for a few minutes, gentle side pressure will usually dislodge it. Allow very delicate inlays to soak up the acetone for at least 30 minutes, and then use the gentlest side pressure distributed over the entire inlay to dislodge it. It will be almost the ultimate in disheartening feelings to break it now, exceeded only by breaking it later. After each inlay is dislodged, take a moment to clean the residual glue from the inlay bottom and crevices, and renew the label and arrow (the acetone may tend to disperse the pencil marks, and it will dissolve away virtually any ink). Arrange the pieces carefully because you don't want to make any fitting mistakes attributable to picking up the wrong piece during the routing process.

Now comes the three-step routing process, the most difficult part of inlay technique to do really well. You must wear goggles and a dust mask, you must keep a steady hand, you must STOP if you can't see clearly where you are cutting, you must cut very slowly, and you must keep the faith in your scribed lines, even though many times they don't seem to be correct. The first step is to cut the inlay outline deeply with the pointed dental bit (again, the pointed bits offered by Dremel tend to be too large, but they will work for many larger inlays, as long as there aren't tight corners). Use the Dremel freehand, not in the router base, and cut downward and sideways with the point of the bit from the line into the wood. Hold the drill like a pencil and use the lowest speed, but vary this to suit the hardness of the wood and the part of the outline you cutting. This is actually the most difficult of the three steps, and it must be done slowly. BE SURE TO CUT INSIDE THE LINE!! Only experience will help you improve, but this step will establish how "close" your inlays are, that is, how much filler space results. Try to cut 2-3 millimeters down into the wood. If you can't see your scribed line clearly, stop and rearrange the workpiece until you can. I use two or three 25-pound bags of #7 lead shot (available from shotgun reloading suppliers) to pad and support a typical neck. These give great flexibility on repositioning and do not dent or nick the wood. Cut completely around the inside of each scribed line, and examine each very critically to make sure they are of uniform depth and that the corners and tight curves are cut vertically and cleanly. When you are satisfied go back over the workpiece visually once again. I have never failed to find spots that needed work, even after two or three examinations.

Next, chuck the router bit (not the ball-end bit) into the Dremel, and mount the Dremel in the router base. Leave enough bit exposed so that it will cut a mortise to about 95% of the thickness of your inlays. To check, use scrap wood and adjust the bit depth so that one of your inlays protrudes just slightly above the mortise. If you are doing large inlays in a curved surface (D-45 hexagons in a guitar, for example), your mortises will be curved as well unless you shim the bottom of the router base with tape so that it rides perpendicular to the peak of the fingerboard. For these inlays, set the depth to 95-98% of the pearl thickness. Straight router bits function best at very high speed, so use the highest speed setting. Use the router bit to remove as much of the inlay mortise wood as possible, but be careful not to encroach the edge, because this bit will cut really fast and do terrible, irreparable damage if it is not controlled very carefully. As before, if you can't see very clearly, STOP and rearrange the workpiece so that you can. Cut out all of the mortises and then examine them critically. Do not yet attempt to fit the inlays, because the mortises are not quite ready, and you may break an inlay if it binds in a mortise.

Now chuck the tiny ball-end bit into the Dremel (leave it in the router base) and set the bit depth so that the ball cuts flush with the bottom of the routed mortise. However, if with this setup the top of the ball is flush or within 1mm with/of the wood surface, the bit must be set deeper because otherwise it will greatly enlarge your mortises. You are going to undercut the edges of the mortises, and if the ball is too close to or at the surface it will overcut them too. Ideally, the ball diameter should exceed the shank diameter somewhat, and should never be smaller than the shank--the shank rubs against the wood "fence" (the edge that you established with the pointed bit) and thus keeps the ball from undercutting too deeply, but it must undercut a little or the inlay may bind when it is inserted. Use a low or medium-low speed and go around the edges of your mortises very slowly and carefully with the ball. Be very aware of the mortise limits, and do not exert any pressure against the wall of the mortise with the bit shank. Otherwise the spinning shank will tend to erode the wood and enlarge the mortise, and unfortunately you will be unable to see this because your scribed lines are gone. Tight corners and narrow curves may not admit the bit shank, and so these will be unreachable with the ball. Use the pointed bit freehand again to undercut these. When this job is complete, inspect each mortise carefully and recut any questionable spots. It is not unusual to spend hours with each large mortise (and sometimes with small ones as well)--be prepared to devote lots of time to a large inlay project.

Now remove the Dremel from the router base and chuck the pointed bit, and begin fitting the pearl inlays in their mortises. Once again examine each mortise for any rough edges or uneven lines, and smooth them carefully with the pointed bit. Gently press the inlay into place. If it won't go in easily, stop and find out why. Some inlays go a short distance and bind, and then yield to slightly greater pressure and slip in to full depth. This is undesirable-- remove the inlay, find out where it is binding, and smooth the edge. If you don't, even slight expansion of the wood during seasonal changes may crack the inlay. Tight spots are usually visible after the inlay is removed because the pearl leaves a white mark. Examine such spots carefully and decide how deeply to cut-- this is the place where all of your careful early work can be compromised by impatience, so use good judgement about enlarging the mortise. Remember that you followed the scribed line accurately (you did, didn't you?), so any binding has to be the result of little ridges and bumps on the mortise edge between the top of the ball-cut and the surface of the wood. Look carefully for these and smooth them a little, then try to fit the inlay again. I have modified a discarded dental "elevator" into a tiny chisel for cleaning out areas that will not admit even the tiniest of dental bits. I don't need it very often, but when I do nothing else seems to work. It is relatively rare for any inlay to fit perfectly on the first try, but it will become more common as your skill and experience accumulates. Now and again a delicate inlay (especially script) will become wedged in the mortise so that it is very difficult to remove. You must resist the temptation to 1) pry it out other than extremely gently; 2) leave it in place and attempt to pack filler around it. Work very carefully with toothpicks around the edges, and lift it out. It will come, but if you don't work carefully it will break. Then find out where it is binding and smooth the edge.

Once all of the inlays are fitted they can be glued into place, and any gaps between the pearl and the mortise edge filled at the same time. The process is simple: fill the mortise with a glue/filler, press the inlay into place, level it, allow the filler to set up, and file, scrape, and sand the inlay flush. The standard glue/filler has been epoxy with dust from the same type of wood mixed in for color and texture. This works well with ebony, but less so with rosewood--finely-divided rosewood dust mixed with epoxy is usually much darker and greener than solid rosewood. Some artisans use tinting colors, such as are used to tint house paint, with some success for rosewood, but I have yet to see a perfect match for rosewood with any coloring system. I just use rosewood dust and try to keep the mortises as close as possible.

My pearl inlay mentor experimented during the 1960's with various epoxy brands (five-minute epoxies were unavailable at the time, so he worked with overnight-cure material) to find those that set up hard (without a tacky surface) and that crept minimally--epoxies are technically fluids even when set, and tend to flow just like water, except much slower. Really creepy epoxies soon leave gaps and pits in the fill space, and a superior inlay job can end up looking very inferior. My mentor selected Wilhold epoxy (standard disclaimers, remember?), and I followed his advice with good results. Unfortunately, lately I have been unable to find the correct material--a long-setting yellowish two-part adhesive. I know that many are using five-minute epoxy, and I guess this is all right except that one must mix several batches in the course of an inlay job, and many of these preparations never lose a slight tack. I think the best material is the light-colored long-set material, and recommend that you find and use it if at all possible. Whatever epoxy you use, be sure to mix dead-equal quantities of resin and catalyst, as a mixture of unequal quantities (particularly an excess of resin) tends to creep. I make up the mixture, stir carefully to ensure uniformity, and mix with just enough wood dust to yield a fluid mixture of the correct color. In my experience, the best dust is produced by filing wood with a metal file, as other dust may be too coarse or may be mixed with "impurities" (sandpaper abrasives, etc).

If you are using long-set epoxy, you can fill all of the inlay mortises to about 3/4 depth with the epoxy filler. If you are using five-minute material, only fill one or two large mortises at a time. I emphasize this--if your epoxy sets up before you embed the inlay, you will have to re-rout the mortise. After the requisite number of mortises is filled, press each inlay into place, and level it by rocking gently with a couple of toothpicks or thin dowels. Another advantage of long-set epoxy is that there is time to self-level prior to embedding the inlay. Embed all of the inlays and double-check each to make certain that they are seated to full depth and that filler has oozed all around. Be especially careful that none are tilted. In the past I have applied heat from a high-intensity reading lamp to each inlay-filler to increase fluidity and allow bubbles to escape, but this practice is now discouraged because it has been shown that most epoxies liberate toxic gases such as phosgene when heated. This practice also accelerates the cure, so that long-set epoxy when heated may harden in just a few minutes. In any case, allow the epoxy to harden completely before proceeding.

The final steps are to clean up the excess filler and level the inlays with the surrounding wood. I use a cabinet scraper (sorry I forgot to mention this in Part I), a double-cut mill file (ditto), and a hard rubber sanding block with various grits from 80 (pretty coarse) through 600 (pretty fine). Start with the (dull) scraper and shave away the epoxy from around and on top of the inlay. Be very careful in this and the following steps not to gouge or otherwise damage the surrounding wood. Continue to shave until most of the epoxy is gone. A coarse file can take the process a step further and begin to level the inlays with the wood, but again please be careful not to dig into the wood. Finally, use the sanding block alternately with the scraper on each inlay (avoid the surrounding wood, because it is much softer than the inlay and will erode at a much higher rate--this will result in high and low spots). Change to 120 grit after the inlays are completely leveled and flush with the wood, and sand carefully to remove the 80-grit scratches. I emphasize--use a sanding block, or at least use cork- backed sandpaper... ...do not use your fingers as a sanding pad for this or any other operation in lutherie. If large bubble holes show up in the filler, take an extra day to fill them and to cure the new epoxy, then level with the sanding block and scraper. After the 80-grit scratches are gone, move on in turn through 220, 320, 400, and 600 grits (all used dry). Sand the entire fingerboard with all of the grits from 220 and finer. By the time you get to 600, the inlays should be free of visible scratches and they should look pretty good against the dark wood. You should be beaming with pride....

Carefully clean out the fret slots with an X-Acto knife and #11 blades, and with a vacuum cleaner hose before you attempt to install frets.

I don't oil fingerboards, other than to allow skin oils to put the characteristic patina in the board with time and playing. I have found that a final vigorous polish with a cloth diaper or dish towel does at least as nice a job as oil on the board and inlays, and doesn't add any chemicals to the wood, so I recommend that over any oil, plant-derived or not. If your pattern involves engraving, now is the time--this is beyond the scope of this discussion, as it is one of those processes that is technically easy but takes vast experience to even begin to master. Buy some gravers, draw some lines on the inlays, follow the lines with the graver(s), use jeweler's wax or the epoxy-ebony dust mixture to darken the lines. Just keep doing that until you're good at it.

Dot inlays: Purchase the correct dots, purchase a matching (preferably brad-point) drill bit, purchase a drill press. Do not attempt this with a hand drill. Drill the holes to nearly full depth, press each dot into place, put a drop of cyanoacrylate glue (Crazy-Glue or similar) around the edge of the inlay, let it set up, sand off the glue and polish as above. And while on the subject, some of the old Gibson Mastertone patterns use small dots in floral array--buy these, don't try to cut them with the jeweler's saw.

I'd be interested in hearing what others are using for inlay filler these days, as the good epoxies are ever scarcer. Your input on or off the newsgroup is most welcome. Again, my thanks for the positive feedback from earlier parts, and I hope you have found this segment worthwhile as well. If interest abounds, I'll write about other lutherie operations as the time and my experience allows.

End of Part III.

End of Article.

Back: [Beginning of Inlaying] [End of Pearl Cutting]


Sean J. Barry
Davis, California
e-mail: sjbarry@ucdavis.edu