Wood stability discussion and is a 1/2” bow tie inlay thick enough for a 2” slab?

Many people think of a piece of wood as a static object.  Those of us that work with wood, have seen it split, bow, cup, curl and twist.  If you work with live edge slabs that have pith or include large branch take offs, you’ve seen a flat kiln or air dried slab cup or curl when you sand or level plane it.  I was working with a large kiln dried walnut slab that cupped 3/4” when I level planed the top and then moved back the other direction 3/4” when I did the bottom.  An award wining fine furniture maker gave me the advice to always stand slabs upright when done for the day to allow the humidity to evenly equalize.  If time permits, allow a slab to rest for 2 weeks after initial sanding or planing to allow it to equilibrium.  I like to think of wood as collection of springs of various sizes and tensions in equilibrium.  Anything you do to the wood, will reset the equilibrium.  We also had a discussion about bow tie inlays.

There are several ways to stabilize cracks in live edge boards that can be used individually or in combination.  Techniques include filling with epoxy, loose tenons on the ends, flat iron or wood underlay cross pieces, and bow tie inlays.  The wood cracked as a result of the dried wood not being able to resist the force of the tension.  Once it cracks, the tension remaining in the piece is minimal.  The goal with any of these techniques is to prevent the split from progressing as the piece ages and it contracts and swells as the humidity changes.  Of all the techniques, filling with epoxy provides the most stability.  

Bow tie inlays serve a functional purpose stabilizing cracks while providing a decorative detail that displays your craftsmanship.  As mentioned, the crack released the pent up tension and the goal is to prevent progression.  So how thick does a bow tie inlay need to be to withstand the remaining tension to prevent further splitting?  The stability provided by the bow tie comes more from the glued surfaces than the mechanical advantage of the shape.  A bow tie inlay provides transverse stability but will not prevent cupping.  Understanding that the goal is transverse stability, bow tie thickness is more a function of workablity.  

I cut my bowties 3/8 to 1/2 inch thick for slabs which are usually 1 1/2 to 2 inches thick.  For my charcuterie boards which are thinner, I’ll use 1/4 inch bow ties.  For me, the aesthetics and workability of the thickness is what’s important.  I personally think 1/2 inch bow tie spanning a crack just looks right, not too thin and not too thick.  I also chose the length based upon what looks right.  Workability is also a big factor.  My router has 1/8 inch depth stops.  So every quarter inch deeper adds two passes.  That creates a lot more potential to ruin an edge.  It is also hard to perfectly vertically finish paring the edges after hogging out with router.  Any mistake will affect the visible fit of the inlay.  

My preference is to fill all the cracks with epoxy and then do my inlays.  I glue my inlays with epoxy and this is especially important when working with rosewoods that have a high oil content.  For my customers that don’t want epoxy, I will put bowties on the bottom cracks.  Why would you only stabilize the cracks on the top?  For large cracks on very thick slabs that you don’t fill with epoxy, you could bow tie both sides.  The crack always travels further on one side.  

I hope this discussion provided a rational explanation as to choosing bow tie size.

Stephan

The Wood Surgeon