Exploring Sound Post Cracks
The humble length of spruce dowel we know as the sound post is more romantically named ‘anima’ (the soul) in Italian. Introduced into the instrument through an f-hole, it stands just behind the treble bridge foot and is held in place between the front and back plates by friction alone. As its Italian name suggests, the sound post is one of the violin family’s greatest strengths but it also happens to be one of its greatest weaknesses. The post causes the instrument to vibrate in asymmetric patterns which project sound very powerfully; in this way the sound post gives violin family instruments their exceptional loudness. The bad news, however, is that there is significant pressure where the sound post touches the internal surfaces of the front and back, so that the plates are susceptible to cracking at these points if an instrument is dropped or falls over.
When a sound post crack occurs, it is generally not adequate to repair it like a normal crack because as soon as an instrument with a sound post crack is back in use, the forces exerted by the sound post will tend to re-open a crack if it is only held together with hide glue and internal studs. In order to overcome this problem, luthiers traditionally fit an internal patch in the vicinity of the sound post where the crack has formed. The technique involves excavating wood from the inside of the plate, removing almost the entire thickness of the plate at its deepest point at the centre of the post and fitting a patch of fresh, un-cracked wood into the depression. This sounds rather radical, but it is a routine procedure and a very well-proven and tested technique, though it requires skill and experience to perform it well.
Fitting a sound post patch The first priority for the luthier is to close the crack as perfectly as possible and then to create a counter form which will support the instrument from the outside while the cracked wood is being excavated and also while the patch is being fitted and glued into place. Counter forms are usually made using a casting material such as Plaster of Paris – or a modern synthetic casting material ‒ so that the counter form fits perfectly to the outside surface of the plate in the area of the crack. The cello front or back is then carefully located onto the counter form and clamped into position. The luthier then carves out an oval hollow or depression in the plate, removing almost all the wood in the area of the crack. This is a sensitive process, requiring regular monitoring of the thickness of the remaining wood. The luthier finishes the hollow into a smooth ‘female’ shape to which it will be easy to fit a ‘male’ plug, and then prepares a plug made from carefully selected wood whose grain is in the same orientation as the wood of the plate. This plug will form the sound post patch when it is fitted, glued into the depression and carved down to the thickness of the surrounding plate.
When the plug has reached an approximate fit, the luthier glues a set of locating studs at each end of the hollow (see photo above) which enable the plug to be lowered into the hollow in a controlled and repeatable way. The luthier can then proceed to the fine-fitting stage by applying a fine coating of chalk onto the inside of the hollow, then lowering the plug down into the hole until it touches the chalk. The luthier then starts paring away excess chalk-marked wood to increase the area of contact in the hollow and repeats the process until the plug fits the hollow perfectly. In order to ensure a successful glue-bond between the patch and the plate, it is best to seal the pores of the wood in both the instrument and patch with a glue size. This minimises glue impregnation into the wood when the joint is finally glued, so that the hot hide glue can form a powerful bond between the plug and the plate. Once the plug is successfully glued into the depression, the luthier can carve away the excess wood until the patch is the same thickness as the surrounding plate.
The significance of sound post cracksAn open sound post crack in the front or back of an instrument causes a major loss of structural and tonal integrity, due to the sudden reduction in strength of an area of the plate which needs to be worked hard by the sound post. Repairing a sound post crack is a more complex and expensive procedure than repairing an ordinary crack, due to the complexity of the repair process. Cracks in the front are much more common than in the back because of the linear structure of spruce, compared to more homogeneous maple. Sound post cracks in the back are regarded as more serious than cracks in the front as the structural demands on a back sound post repair are more onerous than on a front repair. Sound post cracks in the back are also more time-consuming and expensive to patch than cracks in the front. Players worry that patches will change the sound of instruments and there have been cases historically when this has happened, usually when much larger patches such as ‘breast’ patches are fitted, but tonal problems can occur if a sound post patch is poorly designed or the luthier is over-conservative when finishing the patch to the correct thickness.
What is the effect of a sound post crack on the value of an instrument? A common misconception is that a cello with a repaired sound post crack will not make a good financial investment. However, the truth is that as long as the purchase price reflects the existence of the sound post crack, this cello will be just as good an investment as an instrument without a sound post crack. The crucial issue is that the cello should be priced appropriately. We have made a survey of our colleagues across the world, asking how they calculate the price of an instrument with a sound post crack. They all agreed that it is an important topic which is rarely discussed in public for fear of misinterpretation, on the basis that ‘a little knowledge can be a dangerous thing’. However, we will try to explain this complex process, hoping to create a fair picture of how prices are evaluated in the trade and how the presence of a sound post crack affects the value of an instrument.
First, it is necessary to understand how makers and dealers value instruments. This process is potentially very complex, but at its core, the procedure is as follows: 1. Identify the instrument. The value of an instrument is based primarily on its identity (i.e. maker or school of making) and not, for example, on how good the instrument sounds. 2. Ascertain the ‘normal price’ for a good example of this maker’s work. 3. Adjust this ‘normal price’ by calculating the impact of all the factors which apply to this instrument, for example the condition of the instrument, the quality of the materials used, its size ‒ string or stop length ‒ and which period in the maker’s output it represents. The ‘normal price’ of instruments is of great significance to experts because it is deduced from the existing historical price data available. The normal price therefore allows experts to base the pricing of an instrument on concrete data and gives a reliable reference point. The normal price for a 200 year old cello (or a 300 year old violin) is, in fact, based on an instrument which has a repaired sound post crack in the front but is otherwise in good condition. Because the vast majority of 200 year old cellos have sound post cracks in the front, this is the accepted norm for cellos. We would only penalise the price of a cello with a sound post crack in the front if the crack was complex or irreversibly badly repaired. However, as it is not the norm for a newly made instrument to have a sound post crack, it is appropriate to apply a price penalty if a sound post crack exists in a contemporary cello.
Conversely, if you come across a 200 year old cello or a 300 year old violin without a sound post crack, then you would expect such an instrument to have a price premium (above the ‘normal price’) for being in exceptionally good condition. So the first complicated truth is that, in practical terms, the price penalty for the presence of a sound post crack depends on the age of the instrument. Cellos are much more vulnerable to damage than violins, as they are fundamentally more fragile and are often left lying on the floor. Therefore, cellos tend to have many more repairs than violins of a similar age. For this reason, the average price penalty for a sound post crack in a cello is lower than that of a violin. For a violin, the price penalty for a sound post crack in the front is 20% and in the back, 50%. For a cello, the price penalty is more like 10% for a sound post crack in the front and 40% for a sound post crack in the back. The loss of value associated with sound post cracks is obviously quite substantial, but in our eyes a sound post crack does not make an instrument less desirable, as long as the price of the cello reflects its condition.
We would certainly regard an honestly priced but otherwise healthy cello with a well repaired sound post crack as being a much more desirable prospect than a cello with a large number of cracks in the front, none of which happens to be a sound post crack. Fine cellos with sound post cracks can be excellent and affordable instruments for players who otherwise would not be able to own an instrument of such quality.
© Robin Aitchison and Sarah Mnatzaganian 2012