The following is an excerpt from the new book that I’ve been writing for the past three-plus years. It will be another while before it sees the light of day; but it’s copyrighted and if any of you wants to use it for anything you have to ask permission to do so, and then give me attribution for it.  Sorry about that: it’s how business works.

I had, in earlier writings, brought up the matter of top thickness and my refusal to reveal the magic numbers.  Well, the top’s thickness is, along with the layout of the bracing, the most debated and tinkered-with area of lutherie.  It is so for two absolutely important considerations.  The first is that the guitar top is the “soul of the guitar” (that is, its physical characteristics set the stage for tone) along with the corollary that “the lighter the construction of the top is, the better the sound”; in fact, there is an adage among Spanish guitar makers to the effect that “the best guitars are built on the cusp of disaster”.  And this brings us to the second consideration: there is a minimal top thickness/stiffness/strength that must be respected if the plate is not to cave in under string load.  If sheer durability were one’s principal consideration then the guitar could be made of 2 x 4s ; that will make any instrument very durable indeed. But if sound is one’s objective, then the luthier’s skill lies in finding the correct balance point between the imperatives of ‘not too light’ and ‘not too heavy’ construction; and that balance point will be where the guitar is just strong enough to hold together.

In my work, I take my tops to a target deflection under a standard weight rather than to a predetermined, formulaic thickness.  I’ve worked like this for a long time now and have written about my thinking and techniques at length.  Still, my method may not work for everyone.  There are a lot of guitar makers out there who swear by specific target measurements, and I’m not sure I have the right to say they’re wrong to do so.  My own preferred method is different; it just means that, because of the variations of specific physical characteristics of any individual guitar top, each of my tops is a little bit different in thickness. The question comes up, then, of what is the proper justification for focusing on one or another specific number for top thickness?  And, what would that number be?  Well, it seems to me that a good place to begin would be to have some idea of where the measurements that we do know about, read about, have heard about, and use come from.

Many of my generation of American luthiers got our start by reading Irving Sloane’s seminal book Classic Guitar Construction, which appeared in 1966.  This was, after A.P. Sharpe’s 32-modest-pages long Making the Spanish Guitar (published in 1957) the first available ‘real’ book on guitar making.  Sloane advised the reader to make his tops 3/32” thick, which is equivalent to .094”, or 2.34 mm.  Mind you, this instruction appeared before any of the two-dozen-plus books on lutherie that are now available, and before the plenitude of secondary sources of information that now exist.  How did Mr. Sloane — who was not only writing very early in the game but had, as far as I can ascertain, only built a few guitars on his own then — come up with this number?  Well, perhaps by reading Sharpe’s book (which recommends the same measurement), and very likely by measuring some guitar tops and by talking with some contemporary makers.

He doesn’t seem to have spoken with Vicente Tatay, one of the early Spanish luthier-transplants to the U.S., though.  Tatay came from a prominent Valencian family of guitar makers and presumably knew what he was doing, guitar-making-wise, even before he took his plunge into the New World.  Once here, he wound up working out of a store in Greenwich Village and became, by so doing, one of Mr. Sloane’s fellow New Yorkers.

There’s a wonderful article by Steve Newberry, published in American Lutherie (“Vicente Tatay and His Guitars”, issue #66, Summer 2001, pp. 47-49)about the state of lutherie and its lore in the U.S. many years ago.  It is told from the point of view of the author who, as a teenager, became fascinated by Mr. Tatay’s work and talked him into being allowed to hang out in Tatay’s shop after school hours and be of some help by sweeping, cleaning, etc.  In exchange he got to observe Mr. Tatay at work, of course. This turned out to be a mixed pleasure: Mr. Tatay is described as having been a gruff, cantankerous, cranky and closed-mouthed chain smoker who had an explosive temper and spoke only Spanish.  Still, one afternoon toward the end of the Summer, in an uncharacteristic moment of expansiveness and letting down his guard, Mr. Tatay motioned the young Newberry over to his workbench and, using hand gestures and some coins, indicated to him that the secret to his lutherie was to make the guitar top about the thickness of a nickel in the middle, and the thickness of a dime at the edges. (I should add that a lot of Spanish guitar making in those days was done just like that: by skilled feel and eye, and with amazing accuracy.)  Tatay might or might not have known the numerical values of his thicknesses but he certainly knew how to work to such tolerances at the workbench.  Incidentally, nickels and dimes are about .075” (1.9 mm) and .050” (1.34 mm) in thickness, respectively.  Give yourself a treat and look that article up; it’s as well written as anything Mark Twain ever wrote.

Four other books on guitar making followed Irving Sloane’s pioneering work on guitar building.  Classic Guitar Making y Arthur Overholtzer, published in 1974, immediately doubled the available information on this subject.  The other three were Donald Brosnac’s The Steel String Guitar; Its Construction, Origin, and Design (1973), David Russell Young’s The Steel String Guitar; Construction and Repair (1975), and Irving Sloane’s follow-up book Steel String Guitar Construction (1975).  These last three were the first sources of published information on the steel string guitar and their recommended guitar top measurements were 3/32” (.094”) . . . 3/32” (.094”) . . . and 7/64” (.109”), respectively.  Overholtzer’s top measurements took into account wood density and hence presumably stiffness: for classic guitars his recommendations are 3/32” (0.094”) for soft spruce and 1/16” (.062”) for hard, dense spruce.  For steel string guitar tops he recommends 3/32” to 1/8” (.094” to .125”).

With the exception of Mr. Overholtzer, who had been a violin maker for some years previously, the others were pretty much acting as novice discoverers, craftsmen, and pioneers — as I myself was, except that I hadn’t written a book yet.  I think it’s safe to assume that these young makers/authors were following each others’ and the Martin Company’s leads; and I was certainly following theirs.  The Martin Guitar Company comes into this discussion because it was the best known steel string guitar producer of that time and would have been everyone’s main point of reference for making a steel string instrument.  Certainly its most recognized and popular model, the Dreadnought, was that.  (The Gibson company’s guitars were almost as well known as the Martins; however, all considerations of quality and tone aside, that company simply put fewer of its resources into advertising and its level of public recognition/popularity would have been consistent with that. The Guild company would have come in third in name-recognition; but it copied the Martin dreadnought shape, thus further reinforcing that model’s dominance.)

Irving Sloane, whose second book Guitar Repair (1973) focused on steel string guitar repair procedures, was surely in the Martin camp: the photos were taken on the Martin Guitar factory premises, and the repair procedures that are described were carried out on the Martin company’s workbenches — on Dreadnought guitars.  That guitar model was David Russell Young’s and Don Brosnac’s primary focus as well, in their books.  I asked Mr. Brosnac where he got his book’s recommended measurements from; he told me that he got them from Jon Lundberg, the legendary Berkeley-based Martin guitar retro-voicing pioneer, who was in those days possibly the world’s leading expert in that guitar — at least away from the Martin factory premises.On the other hand, both Overholtzer and Sloane seemed to take a lot of cues toward their classic guitar making from the work of Robert Bouchet (1898-1986), a noted and innovative French builder.  While information in general seems to have been scarce in those days, Bouchet appears to have been relatively open with information.  The established Spanish makers weren’t talking or writing anything about their approach to guitar making.

In 1987, twelve years after the last of the above books was published, the bibliography of guitar making took a major step forward when William Cumpiano and Jon Natelson published Guitarmaking: Tradition and Technology.  This was the first book to address making both classic and steel string guitars; its recommended top thicknesses were the most comprehensive yet because it recognized that not only does (1) size of guitar and species of wood used make a difference, but that (2) different makers have significantly different building designs and ways of using their materials because (3) a guitar’s intended target sound might not be the same in every instance.  A maker might use a thinner top with increased bracing, or a thicker top with minimal bracing, or a different bracing pattern entirely, or use different strings with different string pull and torque, etc.  Accordingly, top thicknesses are suggested rather than instructed.

Top thickness targets for classic guitars are given as around .100” (2.5 mm) for spruce and .110” (2.8 mm) for softer wood such as cedar.  For steel string guitar the recommendation is 1/8” (.125”, or 3.17 mm) for a first-time project, but otherwise ranging from .095” up to .130” (2.4 mm to 3.30 mm) depending on size and shape of instrument as well as species of wood used.  One can see that thinking about top thickness was getting more sophisticated — although, given that these are all method-books, the suggested measurements must still all be considered to be Rules of Thumb.

In 2009 I published my two-volume book.  In it, I didn’t give any specific measurements for optimal top thickness; instead, I dwelt on the practice of thinning to deflection rather than to dimension; this has always seemed to me a better guide than the thickness of the wood is, in order for one to most meaningfully track their progress over time.  I also wrote at length about the balancing act that one is called on to make in matching a guitar’s bracing to its top’s stiffness.  I furthermore said that I worked to thinner target dimensions than the average luthier does.  It may have been self-serving of me to not mention a specific thickness for tops and keep that information to myself; but I don’t really believe that simply telling a young luthier to make his tops, say, two millimeters thick — and without mentioning the multiple other factors that have to be met correctly — is a high-quality communication.  And my book does dwell at length on all the dynamic factors that I consider important.

In 2011 Australians Trevor Gore and Gerard Gilet published another ambitious two-volume book, Contemporary Acoustic Guitar: Design and Build. They do not seem to have fallen into the trap of advising a fixed target number for top thicknesses.  They seem to have very intelligently advised working in an optimal range of thickness.  That’s really the way to go, I think. Besides Gore’s and Gilet’s books, a number of other volumes on guitar making have also appeared, each of which suggests certain target numbers for top thickness.

Finally, I want to call one other book to your attention: Don Brosnac’s An Introduction to Scientific Guitar Design, published in 1978.  It didn’t offer specific measurements for anything, but I want to mention it because it was the first book that looked beyond the how-to-do-it and who-did-it-first level to explore the guitar’s wider accessibility from a discipline other than that of basic woodworking.  Up until then, everything else written had been (and would for years to come) be mechanical-level-instructional, archival, historical, and otherwise full of declarative sentences.  But Brosnac’s was the first book to take a step back and address the more general topic of what else besides woodworking the guitar might be about— which made it interesting to me personally.  That publication has since been followed by a fleet of books, articles, and essays that have examined the guitar as art, science, physics, wood technology, disciplined efficiency in production, engineering, acoustics, a collectible object, a genre/cultural icon, zen, an artifact of musical and/or economic history, etc.  With the entry of many talented Born-Again-Christians into the field, I expect that there will sooner or later be a book about The Modern Christian Guitar too.  Mr. Brosnac did pioneering work and the field is still wide open to new ways of understanding, and approaches to, this interesting instrument.

PUBLISHED RULES-OF-THUMB FOR TOP THICKNESSES

Anyway, getting back to top thicknesses: according to the published instructions that I’ve cited in the three decades between 1957 to 1987, top-measurement for classic guitars are:

1/10”  (.100”)  to  7/64” (.110”),
or   2.5 mm to 2.8 mm;

3/32”  (.094”),   or   2.34 mm;

1/16”  (.0625”),   or   1.59 mm;

  0.050”  to 0.075”,   or  1.34 mm to 1.9 mm
(i.e., the thicknesses of a dime and a nickel)

. . . . . and for steel string guitars they are:

3/32” (0.094”  or   2.38 mm)  to  7/64” (0.109”
or   2.77 mm);

and from 1/8” (.125” or  3.17 mm )  to a fat 1/8” (.130”  or   3.30 mm),

Does this get us anywhere?  Well, sort of.  It tells us that, at least in the classic guitar, one can go as thin as 1/16” (about 1.50 mm) and still have the instrument hold together.  That’s useful to know — as is the fact that Overholtzer is in a minority in promoting such thinness; he and contemporary luthier Greg Smallman go remarkably thin, but very few others follow suit.  As for steel string guitars, we have no published accounts of whether there is a top-thickness limit that’s below 3/32”; if anyone one (other than me) has tried to push that envelope they haven’t written about it, to my knowledge.

You should know two things.  First, that stiffness/thickness numbers are just that: they are not very meaningful in the absence of information about doming and brace layout/treatment.  And second, that Tatay’s previously mentioned top-shaping approach is the traditional one used by Spanish classical and flamenco guitar makers: the top is made to a target dimension in the middle but it is thinned in the outermost inch and a half or two of the lower bout, from the waist down, to another target dimension.  We know this because work of this type is found in the instruments of established classical guitar makers whose guitars have been studied and carefully measured.  Experts can even date certain classic guitars through specific variations in their measurements, which will have been documented from the various periods of their makers’ careers.

Flamenco guitars lack the social and academic respectability of their rosewood-built sisters and have not received such formal attention; they get played a lot but are not studied or otherwise paid serious attention to.  Ditto steel string guitars.  And speaking of these, Sloane’s and Overholtzer’s recommendations of uniformly-thicknessed classic-guitar-top measurements, previously cited, actually come out of the steel string guitar making tradition in which the top is the same thickness throughout, without any selective tapering or thinning.  This itself follows from an efficient manufacturing methodology of putting the wood through a sanding machine and then using the wood that comes out the other end, without any further refinement– as opposed to the pre-Industrial European traditions of using hand tools in tapering, controlling, and achieving variable dimensions of the parts. Go team.