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Crack Inlay
© Frank Ford, 2003; Photos by FF
Here's a classic little Hawaiian koa ukulele. Like so many Hawaiian ukuleles brought to the Mainland, this instrument has the classic syndrome of dryness shrinkage cracks. One crack runs right along under the treble edge of the bridge
A crack in this position threatens the integrity of the top because all the string tension is concentrated at the bridge. Left unattended, the crack will allow the top to deform under string tension and eventually make the instrument unplayable
There's a similar crack in the back, too. Both cracks were caused by the shrinkage of the wood as it acclimated to an environment with lower humidity than that in which the uke was originally constructed.
Looking inside, you can see where a store label had been pasted right over the original factory label. Both labels were torn down the middle by the shrinkage crack in the back.
Before repairing the cracks, I used a piece of wet paper towel to soak off the labels, which were originally affixed with hide glue.
The labels separated easily, with the store label coming right off as I lifted the wet paper towel out. The factory label came out easily, too, and I set both halves aside to dry.
Like most ukuleles, this one had a nicely arched back, but the arch was destroyed by the crack, and the back had the appearance of a slight "cave-in."
In order to reinforce the crack, I made a little mahogany patch to bridge over the cracked and sunken area.
I made the patch about half the thickness of the back, or about 1/32" thick, with the grain running perpendicular to the crack for support. It wouldn't have taken a heavy brace to rebuild the arch, and all I needed was a bit of new "skin" on the inside.
After calculating the approximate arch of the back, I made up a temporary work board consisting of a piece of 1/8" thick acrylic plastic supported by 1/8" stringers set on a piece of pine shelving.
For good even pressure on the inside, I selected a bit of closed cell plastic foam and a small hardwood block to distribute clamping pressure over the patch.
As I clamped the patch lightly, the back of the instrument pressed against the acrylic, causing it to sag and assume the appropriate arch for the back.
Supported by the new wood inside, the back resumed its original shape. If you look closely, you'll see that the crack widened a bit in the bargain, but that was to be expected since I was pressing it outward from the inside. While I could have tried to clamp the crack closed at the same time as I reinforced it, I'd have taken the likely chance that I'd have reintroduced the same stress that caused the crack in the first place. Later, after I had thought all my work was done, a new crack might well have appeared nearby.
I ran the thick tapered blade of my small knife down the crack, lightly crushing the walls of the crack. I was, in effect, burnishing the sides of the crack, smoothing out little shredded fibers and making the crack slightly V-shaped.
Then I took a thin strip of koa and scraped it to approximately the same V-shape cross section. I spent a fair bit of time locating a piece of koa that had about the same grain and color, so my new wood might match as well as possible.
As I pressed the new splint into the crack, it seated at different heights along the crack -- the width of the splint matching the crack as I pressed. The sides of the crack absorbed water from the clear hide glue, and swelled a tiny bit to capture the splint
After the glue had dried for a couple of days, I trimmed the splint flush to the surrounding wood
I gave the top pretty much the same treatment, adding flat reinforcements one ahead and one behind the bridge. This time I used a flat plate for clamping because the top had originally been made flat.
Same splint technique here, too.
I soaked the label halves in some diluted clear hide glue and carefully glued them back in place on top of my new back reinforcement patch
Now, when you look inside, you don't even see the patch, and the original label looks pretty good. You can't see it in this photo, but I stuck the Hanson Music store label farther down inside below the original label.
A bit of light brown stain and a touch of amber shellac completed the color touchup of my inlaid splints.
I did a bit of light "scuff sanding" to level the touched up area and to clean the original finish surface in preparation for a new coat of finish to match the original sheen.
This instrument was originally finished with shellac, and a coat of shellac rubbed in the traditional French polishing manner produced the typical soft glow we expect to see on a 80-year-old ukulele.
While the inlaid splints are still visible, the entire instrument has a good overall look, and it's stable, strong, and ready for years of playing.

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