The truth about wood grain
R. Bruce Hoadley, the author of the classic text Understanding Wood, identified over 50 ways the word “grain” can be used to describe wood. In fact, some authors consider the word to be almost meaningless without including context or added description.
There is agreement that historically the word “grain” was used to describe the direction of the longitudinal cells that make up the wood of the tree. These cells, which we have discussed in previous columns, are arranged in the tree in a vertical direction. It doesn’t matter if you’re dealing with a softwood or a hardwood, the direction of these longitudinal cells determines the grain direction of a particular piece of wood.
The ribbon-striped figure in this piece of sapele is produced by interlocked grain. If you plane a board with this type of grain the result is often tear-out no matter which direction you plane.
Here is some rotary-sliced big leaf maple veneer with a broken curl figure. The curly figure is produced by wavy grain.
White ash, showing its characteristic cathedrals. The hand plane is pointed in the correct direction to minimize tear-out. Note the rising grain in front of the plane. This piece, as you can see on the edge, is also cross grained with a fairly high slope angle to the grain.
The perfect tree
The perfect tree grows straight and doesn’t lean or twist as it grows. It grows tall with a trunk that has a consistent diameter. The cells of this tree grow straight, parallel to the tree’s stem axis. It will produce what we call straight-grained lumber. If you enjoy chopping wood, this wood will cleave in a straight line. Similarly, it will plane and machine cleanly with minimal tear-out.
Unfortunately, this tree doesn’t exist. Trees taper and twist as they develop, and they often grow on uneven terrain which can impart a bow in the trunk of the tree. Also, lumber isn’t always sawn parallel to the tree’s axis. Consequently, most of the wood we work with will be cross grained, where the grain direction deviates from the stem axis of the tree or the longitudinal axis of a particular piece of wood.
It’s probably a good thing that this perfect tree doesn’t exist. Large old-growth softwoods, such as Sitka spruce, grow so tall and straight, they seem to approach perfection. The lumber from these trees is incredibly straight grained. Visually, however, they are boring to look at since the wood consists almost entirely of straight lines with little variation. It’s when the grain direction of a piece of wood starts to change that interesting figures like curl and blister develop.
When examining cross-grained woods, it’s important to consider the slope of the wood grain in the piece of lumber. Grain slope is the amount of deviation across a piece of wood or board along its length. Boards with little slope are the easiest to work. Increasing slope will make a board harder to work, decrease its strength and leave it prone to warping.
Two types of cross grain
The first type of cross grain is called spiral grain. Trees that exhibit spiral grain twist around the axis of the tree as they grow vertically. Trees often twist as they grow to maximize their access to sunlight. This twisting will impart a sloping grain to any lumber cut from these trees. It’s a common condition in urban trees, which is one of many reasons they don’t produce quality lumber.
There is an important derivative of spiral grain. This is interlocked grain. Trees with interlocked grain spiral in one direction for several years. Then for reasons not understood, the grain spirals in the opposite direction. This pattern repeats itself for the life of the tree and is common in tropical species, such as sapele and bubinga. When cut or sliced on the quarter, these woods produce a ribbon stripe figure. They are difficult to work since every adjacent stripe has grain running in the opposite direction that will tear out when machined.
The second type of cross grain is diagonal grain. Diagonal grain usually results from sawing boards that aren’t parallel to the bark of the tree. This is extremely common since trees tend to taper and some have subtle bends to their trunks. Flat sawn boards with diagonal grain produce what’s called cathedral grain. Most prominent in ring porous woods like oak and ash, cathedral grain is produced by the growth rings and show up as a series of upward-facing arcs that emphasize the long direction of a board.
Wavy grain is produced when the longitudinal cells don’t grow straight but weave in and out of parallel. The waves can be organized in different ways and produce different types of figures. Curly figure is probably the most recognized by woodworkers, but quilt, mottle and blister are some of the other figure types that result from a wavy grain.
Irregular grain occurs when the longitudinal cells twist and swirl rather than grow straight. This grain is found around knots and is also prominent in burls and crotch wood, the area where branches separate from the trunk. Irregular grain can produce some visually stunning pieces of wood, but the everchanging grain direction makes them difficult to work.
These different grain types don’t exist in isolation, either. For example, it’s possible to have diagonal grain and wavy grain together in one piece of wood. One of the attractions of woodworking is knowing that every tree is unique and so is the lumber that comes from it.
How to read the grain
Successful woodworking, therefore, requires an ability to read the grain direction. If you want to minimize tear-out when milling wood, or if you’re selecting a piece of wood for a certain structural application, knowing the grain direction is paramount. In every piece of wood, there are clues to a given wood’s grain direction, and they come from observing the longitudinal cells of the wood.
The first step is reading the growth rings. If the growth rings produce the cathedral pattern, planing towards the apex of the cathedrals often reveals that the grain is rising, which is the desired grain direction. However, growth rings can be deceiving, and grain direction should be confirmed by further observation of the longitudinal cells.
The next step is to look at the adjacent edge. If the face of the board is flat sawn, the edge will be somewhat close to quarter or rift cut. This will emphasize the linear direction of the longitudinal cell making it easier to read the slope of the grain. The basic premise is simple. Whether you’re working by hand or machine, you want a rising grain in front of the direction of the hand tool or cutter head. This is planing “with” the grain. If the grain direction in front of the hand plane or cutter head is falling away, you will have tear-out as you are now planing “against” the grain.
A closer look
Perhaps it’s easier to visualize what’s happening at the cellular level. When the wood grain is rising, the plane, chisel or cutter is neatly slicing the tops off the longitudinal cells as it passes over them. When the grain is falling in front of the plane or cutter, you are essentially bending and breaking the tops of the longitudinal cells off, leaving a rough surface on the wood.
That, in a nutshell, is the basic approach to reading the grain. Given that the perfect piece of wood doesn’t exist, we’ll have to come up with strategies to deal with imperfect wood grain in the real world. In the next column, we’ll start exploring the world of figured woods and dive a little deeper into grain direction, as well as talk about how to develop the approaches needed to work with the myriad figures encountered in woodworking.