Selection and Preparation of Spruce Wood for a Violin Top

Author: Juraj Vancik, Master violin maker, Topolcianky, Slovakia

Selection of Spruce Wood for Violin Making

The selection of spruce wood for the production of a violin top can be divided into three basic qualitative groups. We will discuss the selection of maple for violins in the next episode.

• Selection of wood according to the physical properties of spruce
• Visual properties and aesthetic quality of spruce
• Intuitive selection of spruce from the perspective of many years of experience associated with violin making

To begin with, I would like to touch on the most sensitive topic, which is which path of wood selection a violin maker wants to take from the perspective of the sound and maturation of the instrument. I am mainly thinking of the properties associated with the strength and color of the sound of the finished instrument. These are properties that are often contradictory. My personal path is to select spruce exclusively from the Italian Dolomites, just as Italian masters did in the golden age of violin making. Since these were all baroque instruments, the main quality was a pleasant sound. Age confirmed the correctness of this choice both in terms of sufficient strength and in terms of the boards drying out with age without evaporation cracks. From experience, we know that using, for example, extremely hard German spruce with a high speed of sound in wood allows violin makers to produce an upper violin board with under 2 mm thickness. In such a case, a higher specific density is not a problem either, since the radiation ratio-acoustic constant with such a high speed of sound still has a good value. The result is a very strong tone, but also with a negative impact on the warmth and color of the tone, on the formation of evaporation cracks due to the fragility of such spruce, deformation due to thin thicknesses, and the like.

With very hard spruce, the demand for thickness accuracy can also be a problem due to the formation of wolf notes when playing and other undesirable effects. These inaccuracies in the thickness of the board will be forgiven much more by the Dolomites spruce than by the fragile, extremely hard wood. Since I do not consider the tone strength to be the main quality of a violin, although it is such an important and frequent requirement nowadays, I will focus on spruce harvested in the Italian Dolomites in this topic. By choosing the place where this spruce was harvested, we have already touched on the physical properties of this wood. By many measurements of the speed of sound in wood using a Lucchi Meter, the measurements in different boards were very similar along the fibers, around 5800 m/s, so I focused mainly on the specific density, which in the case of spruce wood from Italian Dolomites is also very ideal on average from 0.37 to 0.4 grams per cm3. However, many top world violin makers already avoid spruce lighter than 0.38 g/cm3, mainly because of its inability to absorb energy and the strength of the tone.

Visual Qualities of Spruce Wood

When visually selecting spruce, we mainly observe visual properties such as the density of growth rings, the parallel alignment of growth rings, the evenness of the density of growth rings with a possible slight expansion towards the edge of the board, and whether the summer growth rings are sharply defined and thin. For me, the density of growth rings is also important in terms of spruce selection because of scraping off summer growth rings with a scraper to highlight the wood pattern of summer growth rings.

The colored varnish naturally flows down through gravity and surface tension into the scratched grooves, naturally accentuating the growth rings. Picture no. 1 and no. 2. I consider 8-15 growth rings per centimeter to be the ideal density. This number does not quite apply to bear claw spruce, where the pattern is more important than the number and regularity of growth rings. I think bear claw is really popular today, which is also very attractive from my point of view, even though the growth rings almost do not matter. And the fact that thinner spruce is acoustically better is, I think, just a myth, which rather stems from the topic of hard and brittle wood, which we have already mentioned. In general, there are more instruments from the golden era of violin making and even earlier ones that have a top made of dense spruce. Of course, visual selection also includes a general inspection of the wood, whether the spruce is free of visible defects, checking the perpendicularity of the growth rings, and an important check of the fiber alignment of the tree’s growth, especially in the case of sawn blanks, since splitting spruce wedges completely eliminates fiber runout. Then all that remains is an intuitive selection.

Intuitive Selection Based on Violin-Making Experience

Intuitive selection is basically a feeling as if we could already see and hear the wood being assessed as a finished instrument. These are immeasurable and scientifically unprovable properties, yet violin makers attribute great importance to them. Of course, the visual aspect of the wood goes beyond this, but the feeling in the hand is something that decides the selection itself. Tapping on the wood is also an irrelevant criterion due to the difference in wedge size, thickness, length, weight, etc., but the maker feeling and experience with tapping are for selection something similar to measuring the speed of sound in the examined wood with a device. Likewise, perhaps the tone of the wood contains a lot of information about the entire tree, but violin maker only feels it as intuition. Also, some wood is pleasant to our hands, others are not, since the surface also reflects the quality of the wood through the violin makers fingers. The violin makers hand subconsciously also functions as an accurate scale, and from these quantities, together with the speed of sound, we immediately have in our heads the radiation ratio-acoustic constant as an important physical quantity without measuring device. Furthermore, some wood comes to life under varnish, while others are unattractive under varnish. The growth rings also tell a lot about spruce.

Each piece of wood is unique, even in how it resists the chisel and the tools. It is very interesting that the speed of sound in the board, the bending strength and the height of the tapping tone are linearly the same quantity. But for all this we need many years of experience to choose the best wood by intuition alone.

For makers looking for individually selected spruce pieces, the Tonewood Maestro Collection includes violin spruce and other tonewood options that can be reviewed before purchase.

Cutting a Spruce Wedge

After careful selection of wood, we can proceed to cutting a spruce wedge in half. We assume that we have chosen wood with a sufficient material reserve, perpendicularity of the growth rings and that we have selected wood according to all the quality parameters that we have mentioned. Manufactures have various mechanical devices for cutting blanks, but a violin maker will only need a band saw with sufficient clearance. Figure 3. On the upper surface of the wedge, we draw a longitudinal line of the future cut and also a control perpendicular line to the front edge of the wedge.

Personally, I do not use any auxiliary guiding tool, I just let the band saw guide me along the drawn line on the upper surface of the wedge. Figure 4.

Gluing a Spruce Board from Two Wedges

In this part, from my point of view, we will touch on perhaps the most important and little-known trick of how to glue a cut spruce wedge to a board. The secret of a high-quality glued joint lies in the quality of the glue, its preparation and the procedure for preparing and planning the surfaces of the future joint.

For gluing the spruce board, we use the highest quality glue that we can buy. We will not discuss different types of glue, such as whether rabbit glue, animal nerve glue, shark glue, fish glue, etc. is better. I personally use Italian glue for artists made from rabbit skins. The most important thing in the preparation itself is to heat the glue, not boil it since when heating the glue, we must not exceed the temperature of 60 degrees C. The best glue, in my opinion, is the glue poured with water in the evening, the next day we heat it to 60 degrees C, let it cool off, and the next day we heat it again and use it to glue the boards. We also test the glue between our fingers for the right density, so that the boards do not slip too much, but so that the glue is not too watery. We can dilute it at any time, we just have to heat it evenly again. This again touches on the violinist’s intuition and the hard-to-describe practice. Glue molecules are very unstable, they break down into shorter chains, so we weaken the polarity between the molecules and the strength of the dried joint. This is mainly caused due to overheating. Furthermore, glue is susceptible to contamination by bacteria, mold, etc. The glue must not smell, so we should always prepare fresh glue.

After preparing the glue, we can proceed to preparing and planning the surfaces for gluing the boards with glue. Here we come to the trick I mentioned. For some time after school I worked as a designer, and the statics of pressing two halves of a wedge, which are two rectangular trapezoids, during gluing tells us that if we plan either manually or on a planning machine, the two glued surfaces at a right angle, we have zero moment of force at the right angle during pressing and therefore zero force when pressing at the opposite angle, right at the location of the future highest arching. This is the place where the joint can open up during gluing. However, if we plane the board at a small angle, a very small one is enough, Figure 5., we already obtain the necessary force to press and squeeze out excess glue even from the joint point that we mentioned.

As a safety precaution, we can press the wedge already pressed with clamps with our fingers at the location of the future highest arching, to make sure that all excess glue has been squeezed out. I use a smaller, one-handed microscope to check the joint. We can also use a planning machine to plan the surfaces, if we can adjust it perfectly and sharpen it. Some violin makers, before gluing, gently sand the joint lengthwise in the middle with 400-600 sand paper and wipe it with a clean cloth, so that it is not as smooth as glass. Next, we glue it, for example, as in picture no. 6., we can use gluing clamps, or not use them at all and just use the polarity of attraction of glue molecules and so-called rub two wedges which we put down vertically without clamps and lean against a wall or other perpendicular support. Before gluing, it is necessary to heat the joint, for example with a hairdryer, so that the glue does not cool down too quickly and I prefer to have a higher temperature in the workshop.