Cookies, Slabs, and Timbers

Why cookies crack

A cookie is a crosscut slice of a log, with the pith in the middle and the bark around the edge. Almost every cookie cracks as it dries, and the reason is pure geometry. As the wood dries, every growth ring wants to shrink around its circumference by the tangential amount, while the cookie’s radius shrinks only by the smaller radial amount. Using white oak’s numbers, a ring tries to shorten by 10.5 percent but the radius only allows 5.6 percent ¹. A circle whose circumference shrinks twice as much as its radius can’t stay a closed circle: something has to open, and it does, as a V-shaped radial crack from the bark toward the pith.

End grain also loses water fast, so a thin cookie dries quickly and violently, which makes the cracking worse.

What works, more or less:

• Accept the crack. Many cookies are dried deliberately to one large check, then filled or left as a feature.
• Choose low-shrinkage, low-ratio woods. Northern white-cedar (4.9 percent tangential) and white pine crack less than oak or maple.
• Keep them thin and dry them slowly, wrapped or in a cool, humid space, so stress builds gradually.
• Stabilize with polyethylene glycol (PEG), which replaces water in the cell walls and bulks them so they don’t shrink. It works but changes the feel and finish of the wood.

The same geometry is why a log or post with the pith in it develops a long radial check, and why round wooden things like turned legs and tool handles dry oval.

Slabs

Live-edge slabs, long boards sawn through and through, often 2 to 4 inches thick, are big, heavy pieces of mixed grain: flatsawn in the middle, near-quartersawn at the edges, often with the pith running down the center. They dry slowly and move a lot.

• Saw them thick. A 2 1/2- to 3-inch green slab flattens to perhaps 2 inches after drying and machining.
• Plan for time. Drying time rises much faster than thickness; 8/4 already takes about three times as long as 4/4 ². Thick slabs typically need a year or more of careful air drying, often followed by kiln time, and the commercial practice of kiln-drying slabs to 6 to 8 percent is the reliable way to get them dry through.
• Deal with the pith. A slab with the pith in it will usually check down the center. Many slab sawyers rip the center slab along the pith and book-match the halves later.
• Sticker and weight heavily, with stickers every 12 to 16 inches, end coat immediately, and keep slabs out of sun and wind.
• Expect movement after drying. A 36-inch flatsawn-center slab moves several tenths of an inch across its width over a Wisconsin year, which the earlier movement lesson can quantify. Attach slab tops with fasteners that allow movement.

Timbers: framed green

Timber frames are usually built from green timbers. A large oak beam takes many years to dry through, and its outside would check and twist long before the core reached equilibrium. So framers cut joinery in green wood, which is easier to work, and let the frame dry in place.

North House Folk School’s frames use mostly locally milled eastern white pine, which they note is easy to work and excellent to use green because it stays fairly stable as it dries ³. Its low tangential shrinkage (6.1 percent) and very low radial shrinkage (2.1 percent) help. Green framing depends on:

• Boxed-heart timbers (with the pith near the center), which check but remain straight and strong.
• Joinery designed for shrinkage: pegged mortise and tenons, often drawbored, that tighten as the timbers dry; housings that hide gaps.
• Accepting checks. Seasoning checks in big timbers are normal and generally don’t affect strength in properly graded timber.
• Moisture content of the green timbers well above 30 percent at framing time, settling toward the building’s EMC over years.

The Timber Framing course goes into the joinery in detail.

What to take from this

Cookies crack because each ring shrinks around its circumference about twice as much as along the radius; accept, minimize with low-shrinkage woods and slow drying, or stabilize. Slabs are thick, mixed-grain pieces that take a year or more to dry, usually with kiln finishing, and keep moving afterward. Timbers are framed green, using species like white pine that stay stable and joinery that tightens as the frame dries.