Brashness or Brittle Heart

The following is excerpted from “Cut & Dried: A Woodworker’s Guide to Timber Technology,” by Richard Jones.

Jones has spent his entire life as a professional woodworker and has dedicated himself to researching the technical details of wood in great depth, this material being the woodworker’s most important resource. The result is “Cut & Dried: A Woodworker’s Guide to Timber Technology.” In this book, Jones explores every aspect of the tree and its wood, from how it grows to how it is then cut, dried and delivered to your workshop.

Jones also explores many of the things that can go right or wrong in the delicate process of felling trees, converting them into boards, and drying those boards ready to make fine furniture and other wooden structures. He helps you identify problems you might be having with your lumber and – when possible – the ways to fix the problem or avoid it in the future.

“Cut & Dried” is a massive text that covers the big picture (is forestry good?) and the tiniest details (what is that fungus attacking my stock?). And Jones offers precise descriptions throughout that demanding woodworkers need to know in order to do demanding work.

For the first year or two of working wood in the 1970s, I didn’t come across the term brash wood because the craftsmen I worked with called the condition “carroty” or “carrot wood” and I assumed, being young and naïve, this was the normal name. The woodworkers around me, on finding some particularly weak stick would say things like, “It’s rubbish; the stuff just carrots off in your hands.” It was an apt description because a brash break in wood is visually slightly similar to a carrot broken into two half-lengths.

Brash wood has a variety of related names including brashy, brashness and brashiness. Other names for this condition are brittle heart, carrot heart, spongy heart, brash heart and soft heart. Natural brashness or brittleness develops in the living tree caused by the way a tree grows and the stresses it experiences in life. In every case brash wood is weak wood and it unexpectedly snaps across the grain under a load normal wood of the same species would carry with ease.

Brashness often develops in association with cross shakes discussed in section 13.3.3. In another instance, it develops in exceptionally slow-grown ring-porous species where the tree lays down a high proportion of soft spongy and weak spring growth, and a low proportion of denser stronger summer-growth wood. Ring-porous species with unusually narrow year-on-year growth rings are one possible feature to look for to identify brashness; the result of this growth pattern is the wood is also likely to be exceptionally light for its species, and this may indicate potential brashness. Fast-grown conifers tend to lay down a much greater proportion than normal of weaker, lighter spring wood than they lay down in denser and stronger summer wood, and this, too, is brashy. Juvenile wood is frequently brashy, especially if it has grown fast with widely spaced growth rings. Unusually dense reaction wood in coniferous trees, known as compression wood, is often brash, and this type of wood should not be used in furniture, but carvers and turners may find uses for it (Hoadley1, 2000, p 99-100). Shield (2005, p 133) discusses brittle heart or brashness being the result of growing stresses within plantation-grown Eucalypts. He notes that growth increments develop tensile stresses in their length with each successive new growth increment developing slightly more tensile stress than the previous year’s growth. To compensate for this the tree develops longitudinal compression stresses toward the tree’s core. Finally, an artificial cause of brashness is induced when wooden artefacts are subjected over time to high heat “such as wood ladders used in boiler rooms.” (Rossnagel, Higgins and MacDonald, 1988, pp, 43-44.)

The lesson for woodworkers is brash or brittle wood is not appropriate for load-bearing structures, e.g., floor joists, floorboards, table or chair legs and rails etc. The safest thing is to not use it at all except perhaps for purely decorative items such as small carvings or other non-critical parts. Secondly, materials other than wood might be better choices for shelving, steps, ladders and so on in high-heat environments including forges, boiler rooms, certain areas within commercial kitchens, glass-blowing workshops etc.