Pearl Inlay Part I

by Sean J. Barry

This is a three-part discussion about the practice of stringed instrument inlay. Part I covers the principal materials and tools, Part II covers pearl cutting and layout techniques, and Part III covers inlaying technique. The usual disclaimers apply--I recommend specific brands only when either no other will work or I have no experience with others. As far as I know, no manufacturers mentioned here have ever heard of me.

Here is Part I, and the other parts will appear over the next few weeks as time permits. Your feedback is solicited and welcome. Feel free to download the text for personal use, but otherwise please do not crosspost, forward, or reproduce the text without permission.

Part I -- Materials and tools

Any number of flat or flattenable materials can be inlaid into the surfaces of instruments, furniture, jewelry boxes, etc., but the most popular for stringed instruments has always been mother of pearl from pearl oysters and a similarly-derived material from abalone shells. Mother of pearl (nacre) is the material which composes the pearl oyster (_Pinctada_ species) shell, although normally the term refers only to the interior lining of the shell. All shelled mollusks possess a shell lining that resembles mother of pearl, but those of the pearl oyster and abalone are especially attractive and the shells are large enough to yield reasonably large flat pearl blanks. Oyster mother of pearl is usually white or gold, with red, blue, and green iridescence and often with swirl, "eyes," a curly pattern, or other figure that results from proximity to the hinge or from imperfections or worm borings in the outer shell. Pearl oysters are native to the warmer parts of the Pacific and Indian Oceans, from the Gulf of California to the Red Sea, and are "farmed" in Asia for the cultured pearl industry. I don't know if they are also used for food. I also don't know what, if any, percentage of the pearl oyster shells that are imported for inlay pearl originate in cultured oyster beds, but I hope it's large. Abalone (several _Haliotis_ species, of which red and green abalone are the most popular for inlay) is confined to cold water parts of the Pacific Ocean, and doesn't occur in the Indian Ocean. Abalone for inlay originates completely from "wild" specimens harvested for their meat, which is considered an ultimate seafood delicacy. Some abalone is now being farmed, and perhaps in the future most of the commercial material for food and shell will originate from such sources. Other materials occasionally or commonly used for instrument inlay include bone, ivory, tortoise shell, silver, gold, brass, nickel silver, and various woods and plastics ("mother of toilet seat"). Each has its own peculiarities, but the process for cutting and inlaying all such materials is basically the same.

Several steps are entailed in converting an arched shell to flat pieces for inlay blanks. The first is to mark the shell (on the inside) to take best advantage of the figure and pattern, and to minimize the arch in any particular rough piece (the less arch, the larger and thicker the final blank). The resulting jigsaw puzzle is then bandsawn into arched individual pieces that are lined with mother of pearl on one side and with the shell exterior on the other. The rough exterior surface is then ground off to reveal the underlying mother of pearl. The resulting piece is anywhere from 1mm to 25mm thick (up to 1" for really thick shells at the lip), and it is still arched. chuck erikson, the man who taught me how to do inlay, had two large double-wheel enclosed grinders for flattening the blanks, and they did this job efficiently and well. i expect that other blank suppliers have similar equipment, because it is heavily tedious to flatten blanks against a normal grinding wheel, and the resulting product is very inconsistent. the amount of handwork that goes into planning, marking, bandsawing, and preliminary grinding renders the blanks rather expensive, especially for smaller, more difficult material, such as green abalone. also, at least one pearl supplier that i know of has been so overexposed to the fine dust that results from grinding the shell that he has been seriously ill with silicosis, a potentially fatal lung disease--if you grind shell or cut lots of inlay, wear an osha-approved respirator. even if you cut only a little inlay, wear at least a dust mask such as the fiber units sold in hardware stores, and it's a good idea to wear goggles too. the finished blanks are characteristically about 1.5-3mm thick. some suppliers furnish two thicknesses--the thin one for inlaying flat surfaces, and the thick one for arched fingerboards. blanks may be sold by the piece or by weight--thin blanks when sold by weight are usually more expensive because per unit weight more work is involved (there are more blanks).

the tools necessary for cutting and inlaying pearl include good lighting, a jeweler's saw, a homemade cutting jig, a scribe (with a sharp metal point that is hard and stiff enough to scribe very hard wood), a few needle and small mill files, a dremel moto-tool (or similar high-speed drill) with a dremel router base, various bits and appropriate collets for the dremel, a jig or vise to hold the object to be inlayed, and a 2.5" x 5" or similar rubber sanding block. for lighting, use a ledu or similar swing-arm lamp (some prefer fluorescents). the jeweler's saw resembles a coping saw with a very slender blade, and they and the blades are available from lutherie and lapidary suppliers. blades are typically graded as "fine," "medium," and "coarse," but the actual thickness varies among suppliers, because dozens of thicknesses are available from the manufacturers. i use "medium" blades for most of my work because they are less subject to breakage than fine blades and less likely to bind and break the inlay sheet than coarse blades. fine blades are usually recommended for scrollwork and other intricate inlays, but as your skill increases you will have less occasion to use them. beginners should purchase at least 2-3 dozen blades. many inlay artisans use a jeweler's saw with an adjustable throat to accommodate variations in supplied blade length. such an adjustment feature also permits the use of broken blades, but in my experience this is a waste of time unless the blade was broken before it was ever used. the homemade cutting jig consists of a long piece of wood (preferably hardwood, such as maple or birch) about 12" x 3" x 3/4" with a narrow slot through the middle and a 1/2" or 3/4" hole at the end of the slot:

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the jig is c-clamped flat to a table so that the slot and hole extend beyond the edge, the pearl sheet is positioned over the hole, and the wood supports the sheet while the saw cuts downward. the scribe is used to inscribe the exact shape of the inlay into the wood that will be routed for the inlay. many hardware stores sell utility scribes--the one i use consists of knurled steel shaft with a fairly fine hardened steel point that is removable with pliers--spare points are stored at the opposite end of the scribe, which is sealed with a hexagonal plastic cap. a small mill or needle file may be handy for removing the small spur that sometimes remains at the end of the blade path, but otherwise its use is rather limited. do not rely on a file to clean up roughly-sawn inlay, as the pearl is quite hard and is not easily filed. just holding an inlay for filing invites breakage.

the dremel moto-tool, which is one of the most useful of all tools for general lutherie, is used to delineate and rout the inlay pockets in the wood. economy on the dremel is false: buy a variable speed, ball-bearing model, and if you can afford it, buy two. three is not too many. numerous bits, sanders, cutting wheels, buffers, etc. are available, but i have found that just 2-3 bits are necessary for inlay. these include a fine-pointed bit, a bit with a plain shaft that ends in a tiny cutting ball that is slightly larger in diameter than the shaft, and a fairly large (1/16" is good) router or downcut bit. the pointed bit is used in the tool freehand to cut down the edge of the inlay pocket, the ball-end bit is used on the tool in a router base to undercut the edge after the pocket is delineated deeply, and the router bit is used (also in the router base) to hog out waste wood in the middle of the pocket and to even up the pocket depth. all three bits are offered by dremel, but i have found that their pointed bits and ball-end bits are generally too large for inlay work. i use dental bits that i obtained for free from my dentist--used bits are entirely sharp enough for inlay, and will remain sharp for a long time. quite a variety of dental bits is available, from exceptionally finely-pointed carbide bits to tiny ball- and cone-shaped carbides and various straight and pointed diamond bits. one request to my dentist and a 30-day wait yielded a lifetime supply, even if i live a really long time. you will still need the router bit(s), and you will need one or more dremel collets to match the dental bits. dremel's router bits (as far as i know) are not carbide--they do produce some carbide-coated general cutting bits, but i have not been satisfied with way they cut ebony and especially rosewood. however, various lutherie suppliers stock very satisfactory carbide downcut and router bits with 1/8" shanks to fit the standard dremel collet, and i happily use these for the rough work. stewart-macdonald and perhaps others (standard disclaimers) have various plexiglas router bases available for dremels, but it is pretty easy to make your own from 1/8" plexiglas or (as i recommend) 1/8" polycarbonate (lexan). i don't use clear bases for inlay, but i do for some other lutherie operations. you can use a padded wood or metal-working vise to hold the object that you are inlaying (usually a fingerboard or peghead already attached to a neck), or you can build a jig to hold it on a tabletop--either way is fine. purchase a standard rubber sanding block from the hardware store, along with lots of 80, 100-120, 220, 320, 400, and 600 grit open-coat and wet or dry sandpaper--a large mill file is handy also. other tools you might need include a very small chisel for cleaning inlay pocket corners, various gravers if you intend to engrave the pearl, and other bits for the dremel as the need arises.

end of part i.

next: cutting pearl

back: beginning of tools

sean j. barry
davis, california