Saturday 14 December 2019

Building a 13 Course Lute, Start to Finish, 11: Gluing the Bridge to the Belly

Welcome back. In today's instalment of the series, I'll be gluing the newly-made bridge to the belly of the 13 course lute that's under construction in my workshop. I do this job first thing in the morning, in a warm workshop that has a relative humidity of around 42-43%. I use fresh hide glue, which I've begun soaking the day before. I'm well-rested, alert, and ready to do my best work; and to ensure the best chance of creating a strong glued joint, I should have the work finished before lunch time. Here we go.

If you've been following the series so far, you will know that I've previously fitted the braced belly into the body, and located on the belly the exact position of the bridge. You'll recall, too, that I marked that position with two pin pricks in the belly (and blackened them lightly with the sharp tip of a pencil), one at each end of the bridge's string band.

All that's been wanting is the bridge, and now that that's ready to go, I can prep it, and the belly, and and stick the two together. I have a couple of things to do to prepare the belly.

First, with a couple of small pieces of double-sided tape I stick a caul to the back side of the belly, exactly opposite the bridge. As you can see, it covers a fairly large area, and I've cut recesses in it so it fits over the tips of the treble bars (that's where the treble end of the bridge will end up.)

The caul is the same one I used to glue the ebony cap to the bridge in my last episode. As I described there, the caul has a slight curve planed into it--about 1mm across its entire length--which meant that I was able to build that amount of curve into the bridge itself. Now, I'll use the same caul to ensure that the curvature is maintained when I stick the bridge on the belly.

If you go back even further in this series, to the episode where I glued the braces on the belly, you'll see that I also used shaped cauls under the belly to glue the first couple of bars above the bridge with a similar amount of curvature. The purpose of all of this curviness--this very slight doming of the belly at the bridge and above it--will, I hope, become clear in the next episode (so please stay tuned!)

The second thing I do to prep the belly is give it a light scraping with a very sharp scraper in the area where the bridge will be glued, not to remove much material, but simply to remove the oxidized and dusty layer and refresh the gluing surface. (The pin location marks I made on the belly will still be visible after this scraping.)

Now I can go on to prepping the bridge. First, I shorten the ends a little. The original of this bridge, on the Paris Schelle, has ends that are cut short, and perhaps at one point had separate tips made from another material. 

Mine will have ebony tips that I'll add after the bridge has been glued.  

Then, as I did with the belly, I refresh the gluing surface of the bridge. I've handled the bridge a fair bit throughout the process of carving it, and inevitably some oil and grime from my hands has transferred to the underside. In addition, though I was quite careful not to get any finishing oil on the bottom surface, I want to make sure that any residual bits are gone from the edges of the gluing surface. Just as with the belly, I need to remove a thin layer of material on the bottom of the bridge to expose fresh wood.

Here's how it's done: by running the bottom of the bridge across the #5 bench plane, upturned in the vice. The blade is freshly sharpened and very closely set, and I move the bridge over it slowly and steadily.  

In the previous post, I mentioned that I wanted to keep track of the best direction for planing the bottom surface of the bridge (I actually drew an arrow in pencil on it to remind me.) This is why.

If I've done my work well, I should be able to pick up a single curled shaving from the workshop floor, unfurl it, and see that I've removed a thin, even layer of material from the bottom of the bridge. Both gluing surfaces, the belly and the bridge, are now prepared, and I can begin the process of gluing up.

I lay out a double layer of masking tape, and cut it into triangle shapes.

I lift them and fold one corner over. I'll use these to locate the bridge for gluing.

I position the bridge very carefully over my pin marks. I can't possibly show in a photo what my eye sees when I'm doing this: front edge of the bridge is set exactly upon the pin mark, and the pin marks are directly under the single first course and bass-side 13th course string holes. 

Holding the bridge in place, I can then bring in my locator tabs. The double-thickness of masking tape will provide a positive stop when I want to get the glued bridge into position.

I can't really stop to photograph the stages of actual gluing, because it goes very quickly once I am ready. Instead, I'll show some shots of my dry-runs, which should give a pretty clear idea. Have I mentioned dry runs before? They're the best thing ever. For a process like the one I'm about to do, which has a number of discrete steps (none of which I want to omit), it's a great way to get the process out of my mind and into my hands. I might practice this gluing-up process a half-dozen times, making sure I know exactly how many clamps I'll need, where I can set them so that they're close at hand, where my glue pot should be, et cetera. All the while, I'm keeping an eye on my glue, which I am now bringing up to the optimum temperature and consistency. In a while, all of these different strands of the work converge, and my brain gives the order to go--and what has been a up to now a dry run, suddenly becomes the real thing.

I warm the belly and the bridge with a hair dryer.

I brush hot glue on the bottom of the bridge.

I stick the bridge down, and squadge it about slightly to ensure good contact and to bring it into position against the stops. Then I put cam clamps at each end of the central part of the bridge (i.e., not on the finials), and partially tighten them. (To tighten them fully at this point might risk pulling the bridge out of position.)

I move the rest of my cam clamps into position, tightening each one partially. All these partly-tightened clamps give enough pressure so that the bridge will not move, so now it is safe for me to go ahead and tighten them all up.

And there it is. I can't imagine that the whole gluing process takes much more than a minute--maybe closer to 30 seconds. If I'm well-prepared, it happens very quickly.

Now I can set this aside, and go on to other work for the rest of my day.

Next morning, I remove the clamps, and remove the locator tapes and glue squeeze-out.

As I do with other hide-glue cleanup jobs, I wet the glue with a brush, and then apply wetted paper towel on top of the glue. I like this method better than continually brushing on water: the towel holds the water in contact with the glue, and keeps it from evaporating or soaking away. 

In about 10 or 15 minutes, the glue is softened enough for me to remove it with a sharpened stick.

I'll let this dry for another day, before fitting and gluing the ebony tips.

And here's how I do that: put a locator tape to the side, and shape the end of an ebony stick to fit and match the profile.

Like this. (I'm not quite there yet.) When the piece is shaped, I cut it to length, and stick it on with hide glue.

Moderate pressure from a cam clamp will keep them in place. They are not structural in any way, only decorative, so the routine for gluing them in place is a bit less... regimented than for the bridge itself.

And that is all for this job, and for this week on the blog. Next time out, I'll talk about gluing and shaping the particular piece of sitka spruce that's responsible for all of these curves and arches. Here's a sneak peek:

Until we meet again!

Saturday 7 December 2019

Building a 13 Course Lute, Start to Finish, 10: Making the Bridge

Hi there. Today, a small, crucial, highly-detailed and finicky item, the bridge.

My favourite material for making bridges is plum. I search high and low for it, and good quality stuff is perhaps surprisingly not easy to find. The tree grows everywhere, but most of what I see for sale has such twist in the log that it's impossible to use for anything but the smallest items--tuning pegs (if I'm lucky), or more likely end buttons. Most of what I've seen could, sadly, be best used as firewood.

Yet every once in a while a log or a board will come my way that has enough straight growth in it that I can use it for bridges. For a bridge blank, I need a width of about 20mm (that's the dimension that's quartered), a height or thickness of about 12mm, and a length of around 200mm (depending on the number of courses and length of the bridge.) If I find such a log or board, I covet it, and cut it very carefully to yield as many bridge blanks as possible. Only the best, straightest sections of it go for bridges; smaller pieces will be used for pegs. (For this purpose not only is it beautiful to the eye, it also turns well, and is stable over time and wears well in the peg box.)

The main reason I like it for bridges is that it is one of the most beautiful sounding woods I have ever heard. If you get the chance, tap a couple of pieces together, and listen. Plum makes a magical, bell-like sound that is both pure and rich, and I want that sound to be part of every lute I build. Every piece of wood in the instrument has its own beauty of tone to bring, but they are all different, like different beautiful voices in a choir. The best choirs blend those sounds into a single voice, and the result is greater than the sum of the parts. That's what I hope, and want, for my instruments.

And so to work.

(By the way, the model I'm using for this bridge is from the 13 course lute of 1727 by Sebastian Schelle, in the Cité de la Musique in Paris. You might see a photo of it in one or two of the photos in this blog post. This lute also serves as the model for many of the features of the peg box, as we shall eventually see.)

Here's my blank. I've flattened the bottom first, with my low-angle block plane. If you look carefully at the photo, you will see on the right end of the blank that I've drawn an arrow in pencil--this marks the best planing direction for this piece. As you'll see if you tune into the next episode, I will want to know that direction later, after the bridge is finished, and I'm ready to glue the bridge on the belly.

Here I'm shooting the front edge of the bridge. I want it straight and perpendicular to the bottom surface.

When I've got those two surfaces, I can begin marking out the length and width(s) of the bridge. Like most if not all lute bridges from the period, this one tapers from bass to treble end... you see here. I've now got bottom surface (facing up), and front and back sides.

Now the top. I set the piece in this jig, which allows me to plane a slope into the top surface with my low-angle block plane. (Again, the back-to-front slope is characteristic of most authentic lute bridges.)

Here's a look at the jig. There's a slot or edge on each side of it, allowing me to plane the sloped top surface (on the left side) and the sloped back edge (on the right side, as you will see in a minute.)

Like the original, this bridge is going to have an ebony cap, that's slightly more than a millimetre thick. If you take a close look at the photo, you see a sloped, cork-lined caul on the top (the slope matches the slope I've planed into the top surface); below that, a piece of plexiglas as a non-stick liner for the caul; below that, the ebony veneer; below that, the bridge itself; below that, a caul with a gentle curve of about 1mm planed into its length. At each end there are two aluminum pins, which allow me to locate the plexiglas, veneer, and bridge.

It occurs to me that interested readers might have one or two questions about what I've just described. Please feel free to leave them in the comments section at the bottom. I will anticipate one here: why do I place a caul underneath everything that has a 1mm curve planed into its length? Answer: I want this bridge to have a slight curve or crown to it, and the best way to ensure that it does is to incorporate it at this stage, when I'm gluing the veneer to to the top. When I remove the bridge from the cauls and clamps, that curve will be there, built-in. As you'll see when I describe gluing the bridge to the belly in the next episode, I will incorporate the curve into that gluing procedure (and if you remember my description of gluing the braces on the belly in the 7th episode of this series, I did the same thing then.) All will become clear in the fullness of time, I pledge it!

Here's the result, with ebony edges trimmed close to the sides of the bridge.

Here you see clearly the taper from bass to treble end. You can also see where I've marked out the final width of the top of the bridge; I will now plane the back angle into the right side of the bridge.

The plane, in action. As when I planed the top slope, all I need do is hold the piece lightly against the jig, and let the pressure of the plane do the rest. 

My result: the top to its final width, the sloping back surface to its final dimension, and--if you look carefully--about 1mm along the bottom of the back side that I haven't planed the slope into. I don't want where the back slope meets the bottom to become a knife-edge, for two reasons: I don't think it will look good, and it won't be well supported when it's glued to the belly. 

Time now to drill the string holes. I have pretty specific sizes of bits that I use for each of the courses, and the strings within the courses (with 24 holes to drill altogether, I need to keep track of them carefully.) The bar of wood that the bridge is clamped to is sloped slightly--3 degrees, as you see written on it. I will drill through the front face of the bridge through to the back, which means that the hole will tilt slightly upward from front to back. (Again, this is a feature of the original bridge, and most originals from the period.)

The bass side of the course gets a larger diameter hole. In addition, the octave on all the bass courses is drilled slightly lower than the fundamental. (By the way, the big brown waxy-looking block you see in this photo and the one above is a block of lead, which I inherited when I moved into my first shop space about 10 years ago. I use it to weigh down the drilling jig to keep it steady on the drill press platform.)

Back to the vice for another jig. This one allows me to plane a slight slope (an undercut) into the front surface of the bridge.

Then I can begin to cut recesses into the front and back of the bridge. Here's the front side: I make a knife cut, with a straight edge, down the line of string holes. Then I cut into that line, across the grain, with a small chisel.

Here's what the first round of cutting with the chisel looks like--now I need to do the same from the opposite side.

There's the result, tidied up with knives, files and small scrapers.

Here's my sloping jig for cutting the recess into the back side.

I use the same procedure here: knife cut on the string hole line, then cut in toward it with the chisel.

Here's the rough result, before the final tidying up with files, knives, and scrapers.

I'm finished with the shaping of the piece--now I can carve the finials on the ends. I mark them out with a pattern on the bottom of the bridge, then cut them out on the band saw.

In this photo and the next three that show me carving the bridge ends, you can see that I'm working against a block of wood, which I sometimes cover with a soft piece of leather. If I have to make some chisel or knife cuts across the grain, as in the photo below, I might hold the piece against the bare wood to back up the cut--so I don't chip out material. If I'm making some free cuts with the knife along the grain, I will usually hold the piece against leather, so as not to mar it.

I need to be very conscious of grain direction when I'm carving these things. One wrong move, and a chunk of this bridge-end could break off--and I might be left with no choice but to start the whole process over, and make a new bridge from scratch.

As it is, things worked out quite well with these bridge ends, and with the bridge as a whole. Here's the bass end, which I finished up with knives, files and scrapers.

Here's the whole thing. I've left the bridge ends a little long--they will be cut back a little before gluing on the belly, and I'll fit them with ebony tips (be sure to come back for the next episode for all the exciting details.)
To finish the bridge, I give it a couple of coats of a drying oil (T&T Oil, to be specific), and a vigorous polish with a soft bristle brush. (It should go without saying that I'm very careful to not get any oil on the bottom of the bridge!) Here it is with the dyed-black bridges for the two other lutes I'm building, a 7 and 8 course. 
And that's my story for today. I wish you all a fine week, and I'll see you next time for gluing the bridge to the belly. Cheers!