Ribs and Linings

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Over the years, I have seen hundreds of antique basses and ‘cellos and I have rarely if ever seen one without at least a few and usually many more than a few cracks in the ribs. Why is this?

Wood is composed essentially of a bundle of hollow cellulose tubes (wood fibers) glued together with a substance called lignin (I am greatly simplifying here but these are the essentials for understanding what’s going on). The great majority of these tubes are oriented along the length of the log from which the wood came (the longitudinal direction) and a few are oriented in the direction from the center of the log to the outside (the radial direction), these fibers are called the medullary rays . There are no fibers at all oriented in the direction of the circumference of the log (tangential direction). This explains what everyone who has split much firewood knows, that it is much easier if the axe or wedge is oriented radially than tangentially. In the radial orientation the wedge can slip between the wood fibers only having to overcome the lignin bond whereas in the tangential orientation the medullary rays must also be severed. The wood fibers shrink when the wood dries out and swell when it absorbs moisture but this shrinking and swelling is much greater in the diameter of the fibers than in their length. Because of this fact, wood shrinks and swells very little in the longitudinal direction but a great deal in the radial direction (maybe 100 times as much) and even more (200 times as much) in the tangential direction because the medullary rays reinforce the structure only in the radial direction. So what has this got to do with bass ribs?

In traditional construction, the ribs are made by soaking or at least dampening the thin pieces of wood then pressing them against a hot metal surface (bending iron). The moisture helps to conduct the heat into the wood where it softens the lignin allowing the fibers to slip a little in relation to each other as the wood is bent into shape. When the wood cools, the lignin re-hardens and the wood is now permanently bent to a new shape. Unfortunately it has probably also swelled in width due to the added moisture and unless the luthier waits for the wood to fully dry again before gluing the ribs to the corner and end blocks-which are always oriented with the wood fibers across the width of the ribs- the blocks will restrain the wood from shrinking. If the bass was made in a more humid environment than the one in which it lives today (and antiques were made before the advent of central heating), the drier environment will cause additional shrinking. The inevitable result is cracks that usually begin at the blocks. Some cracks are also caused by damage, but shrinkage is the much more common cause.

To prevent this from happening, I have developed a system for making my ribs out of two layers of Maple with a layer of silk in-between. This is done by first pre-bending the veneers on a hot pipe which can be done without water because they are so thin, then pressing the layers of wood, silk and glue together using vacuum. The result is perfectly shaped ribs which are reinforced against cracking by the silk. These ribs are only glued onto the blocks when they are very thoroughly dry. I also use vacuum to laminate the linings which reinforce the ribs at the edges and create a wide enough surface for gluing to the top and back. This results in more accurately shaped linings than is usual in the traditional method.

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In this photo you can see
an antique bass (Carcassi) with typical shrinkage cracks
radiating from the corner block.
 

In this photo you can see my vacuum pump with a set of bass ribs being laminated.
The lights are used for heat to thoroughly cure the glue
 

Gregg Alf learned this process from me and used it to make a ‘cello with silk reinforced ribs. He exhibited the rib garland and lamination set up at the 2006 Violin Society of America Festival of Innovation.

   
 

These 4 photos show the setup used for making the ribs for the ultra-light ‘cello. The method is the same except that Balsa wood was
used between very thin maple veneers
.

 

These three photos show the joint between rib-lining and blocks on the gamba and violin model basses.