Seasoning of Guitar Wood
Seasoning is a process used to reduce the natural moisture content of a wood after it is cut. It is also called as aging or drying. Seasoning gives any wood enough stability to resist shrinking, expanding, warping and cracking. Quality of seasoning is an important factor in determining tone quality and the life of the guitar.
Proper seasoning requires expert skill and sensitivity. Various methods of seasoning are
- Air drying: This process requires many years and is vastly superior to other methods.
- Slow Kiln Drying: This may take months. Faster the kiln drying, lesser is the stability of the seasoned wood.
- Very Quick Kiln Drying: This is used for low quality woods and essentially not suitable for guitar grade woods.
A lumber kiln is a shed that has logs or boards of wood stacked with adequate air space between them. Warm and dry air is then blown through the shed to season them.
It is possible to have similar hygrometer readings during air drying and slow kiln drying. Hygrometer readings do not account for more gradual evaporation of resins, natural glues that hold grains together and other volatile components during the air drying process.
Typical time for air drying is three years, while some manufacturers use even longer durations. Minimum duration for air drying should be at least one year so that one complete cycle of seasons is faced. Longer the duration for air drying, more prohibitive are the costs today. Hence majority of guitar are kiln dried.
Kiln drying depends a lot on the skills of the kiln operator. Success of the process heavily depends on the fine balance between
- Rate at which process draws the moisture from the depth of the wood to the surface and
- Rate at which moisture evaporates from the surface of the wood.
Imbalance between the two rates results in wood cracks or warps due to uneven stresses. About 3/32 inch thick body wood is subjected to many pounds of string pressure. Hence, it is essential for it to be free of faults.
As with air drying, better the kilning, more expensive it is, with much higher degree of consistency and predictability. Only a very small fraction of commercial lumber is adequately kilned to be of use in luthiery.
Seasoning even continues during the making of guitars. High quality manufacturers, allow wood to stabilize for many weeks or sometimes even months, between various stages of assembly. Even factories are properly conditioned to maintain relative humidity levels at about 40%, at which the wood was originally seasoned.
Milling the lumber
Logs are originally available in round shape. The process by which these are cut to make flat pieces of wood to make guitars is called milling.
Grain and Figure.
On every piece of wood there is a visual surface pattern. This pattern, called as grain, is formed by concentric circles, growing outward from the center of the log. Each wood species has unique appearance due to its characteristic cell structure. But we can draw many conclusions from the observed patterns of rings about the climatic conditions in which the tree had grown
- Evenness of ring size indicates consistent climatic conditions. These conditions allow the tree to grow by the same amount each year.
- Evenness of coloration demonstrates that the humidity levels have been consistent throughout the year.
Figure, on the other hand, is a term used by woodworkers to indicate overall visual pattern of a board. Figure depends on various factors such as
- Climate and environmental conditions
- Sawing method
- Grain structure of the wood.
Wooden logs have “Medullary rays” in addition to concentric growth rings. The rays are made of spokes radiating outward from the center. Their purpose is to store the nutrients over the winter months.
Maximum revenue per log is earned by deploying rotary cut method, used for making laminates. However, if the purpose is to make boards out of the log, the most economical way is slab or flat cutting, also known as plank sawing. The wastage in this method is also minimal.
Center or Radial Cut Board
Board cut from the absolute center of the log is called as the Center or Radial cut board. The growth rings are perpendicular to the saw cut in these boards. This is the best cut, as wood with this grain pattern, has the lowest tendency to warp or distort in adverse climatic conditions. This pattern also results in best vibration of the top.
Non Radial Cuts
Cuts away from the center of the log are called as non radial cuts. In these cuts, concentric ring patterns, become increasingly less perpendicular. Non radial slabs are less desirable for guitars, but are suitable for furniture making. These cuts show figure of the wood better than radial ones.
Through slab cutting, very few guitar quality boards, having near perpendicular grain are obtained. This small number of boards per tree makes the material costs of fine quality guitars prohibitive.
That's where another milling method, known as quarter sawing comes to the rescue. This method results in many more high boards out of each log. In this method, the log is cut into four identical quarters. Each quarter is then milled into boards, which are close to radial patterns.
- Very center cuts from each quarter are perfectly radially sawed with grains absolutely perpendicular.
- Rest of the cuts have ring patterns, close to being perpendicular and acceptable for guitar making.
- It is however difficult to radially saw each quarter. Hence, slabs are cut from edge of the quarter alternately, as shown in the video below.
Each Quarter sawed wood is more stable on aging than slab cut wood. This makes is structurally superior to slab cut version and first choice for tops, necks and sides of guitars. In real expensive guitars, even the bracing struts, bridge blanks and inside blocks may be quartersawed.
For guitars to qualify as good quality, tops must certainly be quartersawed. It is desirable to have back and side to be quarter sawn also. Some manufacturers prefer back and sides to be slab cut as they give more priority to beauty of figure over stability and acoustics, in addition to the economy.
Bookmatched Tops and Backs
Fine guitars have their tops and back made of two sections of bookmatched woods. To make a bookmatched section,
- Single piece of wood with 3/16 inch thickness is taken.
- The piece is then cut into half along its thickness.
- This results in two identical pieces, each about 3/32 inch thick
- The two pieces are then opened like a book.
Bookmatched sections mirror each other and have grain structure symmetry. Tops of acoustic guitars made out of bookmatched joint with have equal and symmetric expansion and shrinkage over the years and have acoustically equal sections.
Bookmatched sections on laminates are visually pretty, but do not have any acoustic or structural value.
Most big companies have their own wood supply division, responsible for procuring logs and milling. Others receive material for back and sides in what is called as a set.
Sets are unfinished pieces, cut to approximate size, ready for trimming and shaping to final requirements. Wood supply divisions of big companies also supply wood in the form of sets to manufacturing divisions.
For best guitars, it is desirable that the back and sides are from the same log of wood to ensure visual and acoustic compatibility. As the raw material and seasoning is identical for same log, these pieces expand and shrink at the same rate. These are, of course, very expensive in comparison to other choices.
Necks, bridges, heel and end blocks are milled approximately to final piece’s shape. This roughly shaped section of wood is called as Blank.
Blanks can be machine cut or carved by handwork to the final shape.