Deterioration of Grenadilla* Instruments - Part 3: Analyzing the Problem

Written by Larry R. Naylor

Part 3: Analyzing the Problem

The manufacturer of premium grenadilla instruments must make many design decisions to produce qual­ity instruments. Fundamental decisions include the cutting patterns and duration of aging of billets. Each manufacturer has developed his own method of drying and seasoning wood, but they all have the same end in mind: to produce stable billets that are as stable as possible. Let us remember that ideal, stan­dardized environment: 72 degrees Fahrenheit etc. However, most of us play in far different environ­ments.

In much of the country, temperature and relative humidity swing wildly from summer to winter and even from day to day. Some areas present less than ideal conditions most of the year. For example, the Rocky Mountain region is usually very dry, while the Southeast is usually very humid. These differing environments often affect grenadilla instruments in negative ways, including, as a worst case, the deterioration of the wood.

There is a definite cause and effect relationship underlying blown out instruments. The major causal factors include: manufacturing processes [especially sea­soning], drying caused by local playing environment, player care & maintenance [specifically, the lack thereof], and chemistry of a player’s saliva. In turn, these effects, in order of most to least dramatic as well as most to least frequent include: cracking, binding [keys], chipping and bleached wood.

The author points out that these causes and effects are very inter-related. Thus, the subdivision below is somewhat artificial and done just for the sake of dialogue.

Stress: The Manufacturing Process & Cracking

The most obvious, also the most visible and most dramatic, example of stress relief is cracking. Cracking indicates brittle, stressed wood often caused, regretfully, by a lack of good maintenance. Let us try to understand how and why a wooden bore cracks.

All instruments are under generalized stress: the greater the stress, the less elastic is the wood. Wood is most stable when it is still alive in a tree. After harvesting, the log is cut into billets, and drying begins. After wood looses free moisture (i.e., that contained within the tubes or cellulose), the moisture content is said to be at its fiber saturation point. This moisture interacts with the wood fibers’ cellulose walls, but it does not fill the voids within the fibers. Typically, wood at its fiber saturation point has approximately 25--30% moisture. A further loss of moisture will cause the wood fibers to change shape as the wood shrinks. This shrinkage increases internal stress within the wood. The more moisture the wood loses, the greater the internal stress — if you will, the more the wood “moves.”

The relief of internal stress in wood — during and after manufacture — is a major factor accounting for how and why wood “moves.” For example, one can join and plane a hard maple board so that it is very straight and even in all directions. As a single piece, the board is under stress from shrinking during seasoning, and it is only straight because the stresses are currently in equilibrium. If one then rips, for example, one-quarter inch thick slats from this board, each slat — as well as the original board — will usually warp and twist from the relief of internal stresses that are now out of equilibrium.

As billets dry, their fibers collapse or shrink into themselves. During drying, central heartwood billets will shrink regularly towards centers, while Figure 2.4 heartwood will shrink mostly in one specific direc­tion. This drying process is only the beginning of long-term dimensional changes in the billet.

Careful seasoning maximizes billet stability. However, each billet will “remember” its own internal stress from the aging process. Aging is an attempt to reduce internal stresses; but those stresses in fact reach equilibrium rather than disappear altogether. Like that maple board cut into slats, when machining a billet into an instrument, internal stresses may re-orient within the wood, causing it to change its shape until the piece reaches a new equilibrium. For example, binding keys on a new instrument indicates that the wood is continuing to stress-relieve itself as it adapts to its current, local environment. The wood is changing dimensionally even if no one is playing the instrument.

The presence of burls and uneven grain contributes to localized stress, as will a too tight key post or register insert. A burl near a tone hole may even cause that tone hole to change shape more than other tone holes in the same piece of wood. Uneven grain, burls, overly tight posts and register inserts represent possible origins for future cracks.

Cracks occur when stresses within the wood overwhelm the wood’s elasticity at any given point. We may liken this to cracks in the foundation of a house as it settles. Moreover, any rapid change in mois­ture content and temperature will further stress the wood and increase its chances of cracking. Let us now specifically analyze the phenomenon of cracking.

Cause and Effect: Stress & Cracking

Based on my repair experience, there are four types of cracks, listed in descending order of occurrence: [1] playing a cold instrument; [2] rapid cooling of the outer surface of an otherwise warm instrument; [3] local stresses compounded by rapid temperature and moisture changes; and [4] extreme saliva damage.

The most common kind of cracking occurs when one plays a cold instrument. The sudden rush of hot, moisture-saturated air causes the wood at the bore to expand more rapidly than the wood on the out­side of the instrument. If this expansion generates pressure great enough to overcome the elasticity of the wood, the wood will crack. In central heartwood, cracks will follow a path of least resistance through postholes, tone holes, register inserts, and areas of highly localized stress. In Figure 2.4 type heartwood, cracks will occur at the edge of the grain, also following a path of least resistance.

The second most frequent cracking occurs when a warm instrument quickly cools on its outside surfaces. The wood on the outside of the instrument simply contracts faster than the inside. For example, this can occur when one opens an outside door or window near a performance area. Cracking may also occur when one puts a very warm instrument in a non-insulated case and goes outdoors on a cold day.

The third most frequent cracking involves a series of parallel cracks in one or more areas of the body. They are usually smaller than cracks caused by rapid temperature and moisture changes, and result from highly localized stress. Examples of this class of cracking occur in instruments that have been in continual use for many years — but have had little or inadequate maintenance.

Let us defer discussion of the fourth cause of cracking, saliva damage, until later in this article.

Cause and Effect: Stress & Binding Keys

Instrument dimensions will change once shipped outside of their manufacturing environments. The wood will move. This occurs very rapidly in dry environments as we have in the metro Denver area. Keys will typically bind, and joint and bell rings become loose. This contraction or expansion results from the interaction between the wood’s changing moisture content and internal stress.

Let us use premium Oboes to demonstrate how keys bind as a function of wood movement. Premium Oboes have keys fitted very precisely to hinges, pivot screws and key posts. Furthermore, hinge screws are usually fitted snugly in their respective postholes. Excess key movement is negligible; in fact, if keys were much tighter, they would not move.

Figure 3.1 represents a cross section of a premium Oboe body. Note the central, heartwood grain.

When the wood shrinks, its outside diameter gets smaller. Posts located perpendicular to the long axis will lean or move closer together as in the figure above. The associated key will no longer fit between the posts, and thus, binds. If shrinkage is great enough, the screw holes in the posts will no longer align. If the hinge screw is a snug fit in the post hole, and if the screw threads lock into the inner threaded post, the post misalignment often causes the screw to curve or flex down slightly, thus causing the key hinge tube to also bind on the warped hinge screw. Both causes for key binding typically occur on the D# and C# keys on an Oboe’s lower joint. If a key still binds after fitting the hinge tube between the posts, one knows that binding caused by post misalignment is occurring when, if you back the screw out slightly, the key then moves freely. Because springs are very light, these keys can tolerate little friction.

Cause and Effect: Brittle Wood and Chipping

In the case of brittle wood, the wood is not moving or cannot move easily. Brittle wood usually leads to chipping of tenons, sockets, tone holes, and cracks.

Wood’s lower moisture content, caused by dry climate for example, usually contributes to the brittleness of wood and, therefore, lowering its elasticity. The incidence of dimensional movement of the wood is also most noticeable during dry seasons in otherwise humid areas [e.g. winter in northern Texas, for example,]. When wood becomes more brittle, loose socket rings, and chipped tone holes increase.

Humid climates are much more instrument-friendly. However, extreme humidity can generate additional problems outside of the scope of this article. Dimensional changes and brittle wood are usually less severe in humid climates; and changes in dimensions are often so slight that they are not measurable.

Unless one damages the instrument, chipping is a manifestation of deteriorated wood. Its inelasticity relative to stress on the wood literally causes pieces to break off.

Cause and Effect: Saliva Damage and Stress

The fourth type of cracking occurs rarely, and almost exclusively in regular heartwood. This condi­tion results from extreme saliva damage or in dry areas where the swing of moisture content in the wood is extreme. Saliva damage typically is a compounding problem. That is, saliva damage usually accentuates already existent problems in the instrument’s bore.

As woodwind players, we routinely deal with saliva. Excluding physical damage, saliva is the major rea­son why reeds die. Left to their own in a relatively temperate environment, reeds will last for years However, subject that reed to acidic saliva or a foreign material like soda pop, and those substances interact chemically with cane fibers and the reed deteriorates.

In the case of the bores of grenadilla instruments, very acidic saliva usually causes grain in the bore to rise. Raised grain is especially common in dry areas, but it can be found in instruments in any climate. If strong saliva continues to erode the bore, the raised grain will eventually form deep “V” shaped recesses. Once the wood becomes thin enough from the bottom of the recess to the outside of the body, any slight stress can cause that “valley” to crack right through to the surface of the instrument.

Strong saliva will also bleach wood. Such bleaching is most noticeable, for example, on Clarinets, on the mouthpiece first, then the Barrel and top tenon of the upper joint. The bleaching may range in color from gray to white, denoting the depletion of natural oils and mineral deposits in the wood. As with brittle wood, saliva-affected wood is very inelastic. Not surprisingly, with wood in this condition, even the relatively gentle changes resulting from warming up may be enough to crack it!

Cause and Effect: Recap

Deteriorated, blown out instruments have lost resonance, compromised scale, and chronic intonation problems. The chronic degradation of pitch indicates that bore and tone hole dimensions have changed.

We have examined four major effects of deterioration: cracking, binding, brittleness, and saliva damage. In all cases, the movement of the wood [i.e. its lack of elasticity] is the cause. Stated differently, stresses on and in the wood have damaged it, or made it difficult to maintain within manufacturer-fabricated tolerances.

Must we relegate the deteriorated instrument to the back of the closet or discard it? For those inter­ested in restoring deteriorated grenadilla instruments, may I invite you to read Part 4: Immersion Resto­ration of Grenadilla and Rosewood Instruments?