I've had a number of requests for advice on how to make a mold for building a lute back. Well, here goes. This is a vast subject, and my own method takes a lot of explaining (I've found), so I think the best way to deal with it is in a short series of posts. And let me just say at the outset, what I write here won't be a step-by-step, how-to explanation. There will necessarily be a lot of details left out that you will have to work out on your own (just as I once did, and am still doing.)
There are plenty of ways to go about making a mold. The first one I ever made was a very simple cross-shape, using a single piece for the longitudinal section and a single piece for the cross section, following instructions by Ian Harwood from a book on making musical instruments. I also tried making a solid mold using the method described by Robert Lundberg in his book Historical Lute Construction, which is basically building up a big block of wood, then carving away everything that doesn't look like a lute back! Lundberg uses external templates to give shape to the block, but I found this to be a very difficult method to use. It was a great relief to me when I finally met Grant Tomlinson, and he set me on the path to mold-making righteousness.
My method is based on Grant's, and it differs from his in a number of details, but the fundamental principle is the same: to built a lute mold with the plan section, longitudinal section and cross-sections built into the mold itself, rather than to shape it using external templates. It takes a bit of doing to put the pieces together, but once they're glued up, the basic shape of the mold is already in place.
To get started, you'll need two things: a good stock of wood, and a good working drawing of your lute. The wood first: jelutong works very well for making lute molds. It's relatively light and stable, and of a uniform density throughout, so it carves easily. It's also readily available in dimensions that suit this job (2" stock that's 6-8" wide).
Next, a word about your drawing. Whether you're using a museum drawing or a commercially available plan, you'll probably want to make your own accurate working drawing. If you have a technical drawing of an historical lute, you'll want to rectify or modify the long, cross and plan sections to make a pleasing (and viable) shape; if you're using a commercial plan, you'll at least want to the check the accuracy of the sections you're given. Before you even think about working with wood, make sure that all the sections fit together--you don't want to get halfway through the process only to find that your dimensions don't match.
Here we go.
The Base Plate
The first piece you need is a base plate, onto which the body outline will be traced. The plate is made up of two planks edge-glued together (the glued joint then becomes the centerline). The piece should be planed very flat and smooth on both sides, and should be a very even thickness throughout. What the specific thickness is doesn't really matter, but later on you will need to know exactly what that thickness is. I've found that somewhere around 20mm works well.
Lay your outline drawing down on the base plate, aligning the centerlines carefully. Tape or pin the drawing down securely. Then with a sharp pin, prick through the locations of all your cross section lines through the drawing down onto the base plate.
You are now ready to transfer your body outline from the drawing to the base plate. However, because your working drawing is a representation of the full-sized, finished lute body, you need to subtract the thickness of the ribs from your outline as you transfer it. To do this, use a sharp pair of dividers set 1.5mm apart (the approximate rib thickness), and prick through the drawing. When you prick through the body outline with one point, the pin line you create with the other point (1.5mm inside) will be the outline of your mold. (Around the capping strip area there's a double thickness of wood, so this width will be around 3mm, blended in at the ends of the capping strip.)
On the bandsaw, cut close to the line, except at the very bottom and very top of the outline. At the bottom, leave 2-3mm extra length--this is to account for the fact that many lutes have a longitudinal section that protrudes farther than the bottom of the belly (if you've ever seen the Vienna KHM's drawing of the Gerle lute, you'll know what I'm talking about.) Leave about an extra mm at the top end too. Shape to the line with a disk sander, or on the bench with a sanding block.
With me so far?
The Mold Blocks
Now we need to make two long blocks, which make up the remaining wood of the mold. Because these blocks will sit on each side of the mold's centerline, I think of them as treble and bass blocks, and I mark them as such to keep track of which is which as I go along.
The rough dimensions of these blocks are as follows: the width of each block is a little wider than half the base plate, while the depth will be a little deeper than the depth of the mold, minus the thickness of your base plate. The length should be a fair bit longer than the base plate, maybe 20 or 30mm (I'll explain why this extra length is needed in a minute.) Depending on the dimensions of the mold you're making, three layers of 2" jelutong should be more than enough thickness; cut the pieces to length, joint them flat, and glue them up. When the glue is dry, true up the bottom and inside (centerline) surfaces with the jointer. It's important that these surfaces be very flat and perpendicular to each other.
To ensure that they are, I finally clamp the blocks together on the workbench, then flip the whole thing upside down so the bottom surface is facing up. Using a jackplane, I flatten the whole bottom surface.
Do not unclamp these blocks. The surfaces of the blocks are now very flat and close to perpendicular, and are ready to accept the base plate. Align the base plate centerline with the centerline created by the blocks. The ends of the blocks should overhang the bottom edge of the base plate by 3-5mm; the rest of the excess length of the blocks should extend beyond the top end of the base plate. Clamp the base plate securely to the blocks.
I think I'll stop here, for now--this seems like a good spot for a cliff-hanger ending. I hate long goodbyes. Take care, and I'll post again in a little while.