Well, at least that's what we call them. The guitar top and back are often refered to by luthiers as "plates". Quarter-sawn boards are prefered, cut radially from pith to bark. Very rarely is a plate made from a solid piece of wood -- this would have to come from a tree whose diameter is at least twice as big as the width of the guitar, as it's essential to avoid the pith (a very weak area of a board). Such boards are sometimes available from big, west coast North American conifers, like Douglas-fir and western redcedar, but more usually plates are made by joining at least two pieces together, to make up the width required.
For Andrew's OM, the material for the top and back was supplied as book-matched pairs. The pair was made by resawing a single board into two thinner boards. The first step in joining the plates is to figure out which edges will be joined, the edges closest to the tree's pith, or those closest to the bark. For the Malaysian blackwood back, it was an obvious choice to orient the bark-side edges together, so that the light coloured "sapwood" formed a decorative figure. The choice for the German spruce top was more subtle. Even in a "master-grade" piece, one can usually find some tiny defect that is exposed when the piece was resawn. The pieces will be joined so that any defects are outside the guitar outline, or perhaps oriented out-of-sight on the inside surface. For Andrew's OM, the piece of spruce I started with was very high quality and large enough for a jumbo-sized guitar, so I had lots of latitude as to how to orient the joint.
The traditional way to make the centre joint in a top or back plate is to use a long plane and a "shooting board", which helps to keep the plane at 90°. The maker must check the joint carefully to ensure that no light is visible when he holds the two pieces together. I take a non-traditional approach and dress the edge with a router running against a VERY straight steel guide.
After jointing, glue is applied and the two pieces clamped. The photos in the album show the jig I use, made from some straight pieces of wood, some wedge-shaped pieces, and rope.
With the plates joined, a few passes through the thickness sander cleans up the surface and brings them to uniform thickness. At this point, we make a final choice of which surface will be outside, and use a sanding plane to ensure that this surface is very flat and free of any defects -- essentially ready for finish.
The top will have a rosette inlaid around the sound hole. This is an interesting decorative touch, but is also an important structural element, discouraging the formation of cracks that might form at the edge of the soundhole. For Andrew's OM, we've opted to make a 3-ring rosette with strips of black and white "purfling", and a centre ring of paua shell -- a type of abalone native to New Zealand. A small router is used to cut out the rosette channels, the purfling glued in place with a Teflon spacer in the centre ring. After the glue has set, the Teflon strip is pulled out and replaced with the strips of paua which are then flooded with "super glue". When everything is set, the rings are scraped and sanded flush with the soundboard and, voilà, we have an inlaid rosette!
Thicknessing the plates is part of the luthier's art. We don't want the back too heavy, so not too thick, but we want it to remain very stiff, so not too thin. The guitar top is a critical part of the instrument and has huge effect on the sound volume and quality -- the stiffness profile of the top will ultimately determine how it drives soundwaves into the box when the vibrations of the plucked strings are transmitted through the bridge into the top plate.
You'll note the growth rings in the top are oriented perpendicular to the surface -- quartersawn, as it's called in the timber trade.
Each ring represents growth over a single year. Wood scientists refer to the light and dark portions of the growth ring as "earlywood" and "latewood", respectively (also known as "spring-wood" and "summer-wood"). The earlywood is made up primarily of large-diameter, thin-walled fibres (tracheids), while the latewood fibres are small in diameter, thickwalled, and rich in "lignin". This makes the latewood much more dense and stronger than the earlywood. When a board is quartersawn, the latewood portions are oriented like floorjoists and give the plate great stiffness even though it is quite thin. How thin? Well, that depends on the luthier's assessment of each top as more material is removed. By flexing the top and tapping it, the guitar maker can feel and hear when the top really begins to sing -- ya kinda have to get a feel for it.
(Note: see the photo album for additional pics!)