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Strings: How Natural Gut Strings Are Made

By Greg Raven

Man has made strings out of natural gut for bows and musical instruments for more than 3,300 years. Pierre Babolat manufactured the first set of natural gut string for the then-new game of tennis in 1875. Natural gut sports strings are currently used in tennis and badminton.

The raw material, animal intestine, is the most resilient material used to make racquet sports strings. It has better tension retention and greater softness than any other material. Compared to synthetic strings (“How Synthetic Strings Are Made,” RSI magazine, March 2011), making natural gut strings is labor-intensive: It takes 60 days for Babolat to make each set of natural gut.

The manufacturing process can be broken down into five key steps:

1. Harvesting and selecting the raw materials
2. Chemical treatments
3. Winding and drying
4. Finishing and coating
5. Packaging

The characteristics of the final string depend on the quality of the raw materials, the amount of damage sustained by the threads during processing, the number of threads in the string, the twist applied to the string, the quality of the bonds among threads, and the type of coating.

Let’s take a closer look at the manufacturing process. The basic process is simple, but there are many variations that determine the characteristics of the final product. Not all manufacturers follow each of these steps, and some may do the steps in a different order than outlined here. Manufacturers need to combine the sensibility of the artisan with the attention to detail of the scientist.

Raw Materials

Modern racquet strings need high-tensile strength and long lengths. Although strings have been made of sheep and lamb, bovine or beef intestines best meet both of these basic criteria. Without the beef industry, there would be no natural gut strings, as the intestine used in the manufacture of string is a byproduct of processing beef for food.

The outer layer of the intestine is the serous membrane, or serosa, that surrounds the muscle layer. Serosa is comprised of a fibrous protein called collagen. The serous membrane has both highly oriented and random collagen fibers: The highly oriented fibers contribute tensile strength; the random fibers contribute elasticity. The collagen fibers have tremendous strength and elasticity, allowing the underlying muscle sheath to expand to accommodate the passage of food, while preventing the muscle from becoming constricted during contractions. Because of its role in the complex process of digestion, beef intestine has evolved into a very sophisticated structure. Designed to last for the animal’s entire life, this structure has to be strong, flexible, and durable, just like a premium racquet string.

The actual collagen molecule is a “triple helix” or three strands nestled together. This triple helix is directly responsible for natural gut’s elasticity and ability to act as a shock absorber. Huge numbers of molecules combine together to form collagen fibrils. The “lay up” and the secondary chemical bonding form a very strong elastic building block. Natural glues bond the microscopic fibrils together into collagen fibers. These are the little hairs you see coming loose from the string as it wears. These fibers are what make serosa special.

Each animal species and to some extent each breed within a species has different intestinal compositions. For this reason, manufacturers have rigid standards regarding the breed, sex, and age of the beef, and even where the beef is raised. In a sense, this is the first of many quality-control checks that occur throughout the manufacturing process.

One big enemy of natural gut at this stage is bacteria, so the slaughterhouse cleans and splits the intestines immediately after removal. After surface cleaning, the 160-foot-long intestines are sliced lengthwise, with roughly 40 to 60 percent of the circumference of the original intestine going to string production. This section can be a single “split” composed of roughly 160 to 180 degrees of the original intestine, or it can be a double split (two threads of between 80 and 90 degrees), or a triple split (three threads of between 50 and 60 degrees). Because the intestine can break during splitting, the threads can come out with different lengths.

Groups of 80 to 100 individual threads are gathered in “knots” (so-called because there is a knot at one end holding the group together to facilitate handling), and coiled into barrels with salt as a preservative.

Chemical treatments

Once string manufacturers receive the barrels of “knots,” the intestines are inspected and sorted by length, with any that are fatty or discolored being discarded. At this point, the manufacturer has the choice of leaving the threads to cure in dry salt (re-salting and re-barreling every couple of days) that is cooled or refrigerated, or rinsing the threads in salt brine and freezing them until needed.

The next step is a heavy-duty wash to remove the preservatives, soluble proteins, fats, carotenoids, and other miscellaneous impurities from the threads, leaving clean, undamaged collagen. This is done in a series of painstaking, labor-intensive water-based baths involving caustic soda, soda ash, enzymes, and softening detergents. The knots holding together the threads have to be loosened and moved to allow the chemical processes to penetrate the entire length of the threads.

Cows are not picky eaters, so cleaning goes on throughout the wet-processing phase to remove bits of sand, pebbles, small rocks, fencing wire, and other foreign objects, along with other non-collagenous material.

Between baths, workers squeeze the threads by hand to remove as much liquid as possible. Depending on the manufacturer, chemical treatments can take up to 10 days. When finished, the wet threads feel silky rather than slimy.

Typically, all chemical processes are completed before assembling the threads into strings, so the threads in the bundles are left fully exposed to the chemicals. This includes any bleaching to mitigate or remove the natural orange color of the threads, and coloring if desired. Just like with human hair, bleaching weakens the strands. Every step of the process must be finely tuned and balanced to get the correct final properties.

Winding and Drying

The “knots” of threads have now been cleaned, sorted, bleached, colored, inspected for quality, and otherwise readied to be made into string. Workers loosen the knots to permit the removal of however many threads are needed for the desired gauge, based on the width of each thread. The average animal provides between three and 10 threads, so it takes two to four animals to produce one string.

The thread bundles are looped at each end for mounting on the measuring bench. For tennis strings, the starting length must be 42 feet, as 2 feet are lost in processing. It is critical that all threads in the bundle have the same tension to ensure that each thread carries its share of the load when under
tension.

The thread bundles are then mounted on the spinning bench, where they are wound into something that has begun to resemble a string. The gauge and desired elasticity determine the number of turns.

The string then goes to a climate-controlled drying room, where it is held under tension for up to three weeks. During this time, workers apply additional twists to the strings. This is critical to the final product.

Next the strings go to the holding room for up to two weeks — still under tension — until the gut is totally dry. It is then moved to a normal, uncontrolled environment.

Finishing and Coating

The dried string is rough at this point, more like a rope than a finished string, so it is passed through a sander or centerless grinder to give it a smooth finish and uniform dimension.

Any string that fails the quality-control check at this point can be used for other, non-sports purposes. Strings that pass final inspection are ready for coating and packaging.

Despite all the processing of the serosa to this point, it’s still a natural material that is vulnerable to environmental conditions, in addition to the normal wear and tear to which all tennis strings are
subjected.

In order to improve abrasion and water resistance (including reducing absorption in humid settings), a special polyurethane (PU) coating is normally applied to the string. However, some players forgo the protection of a PU coating for the extra performance of an uncoated string; in this case the string is finished with a light coating of coconut oil.

To make it easier to weave the crosses, some manufacturers apply a thin film of lubricant.

Packaging

To further protect the finished string from humidity and give it a longer shelf life, natural gut strings come in airtight packages.

Seeing all the steps that go into creating a set of natural gut, it’s easy to see why it’s more expensive than synthetic string. For some players, though, the higher initial price is more than made up by the feel, playability, and longevity of natural gut.

Thanks to Babolat, Bow Brand, Pacific, and Fred Schwacke of Performaxx Strings for supplying background information for this article.

Babolat’s New BT7 String

Babolat recently introduced its BT7 line of natural gut, the “7” designating that it uses seven 42mm strands of raw material instead of the thirteen 21mm strands previously found in Babolat natural gut.

This change was made possible by using a different layering technique that results in a more homogeneous structure in the final string. Each of the wider strands has more surface area for bonding and cohesion with adjacent strands, and the 42mm strands have more resistance to stretch and robustness than the 21mm strands.

This change also allowed Babolat to optimize comfort, power, and tension maintenance, while increasing durability by 15 percent.

See all articles by Greg Raven

About the Author

Greg Raven  is an associate editor for Tennis Industry magazine and technical writer. He is certified as a Master Racquet Technician by the U.S. Racquet Stringers Association. He can be reached via e-mail at greg@usrsa.com, or through Facebook, LinkedIn, and Twitter. He plays tennis three to five days a week, and is turning into an avid cyclist.

From the August 2013 issue
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