The best, most supple, intestines for making violin strings come from lambs that are no more than 4 months old. But it isn't the gut per se that is used. The string makers extract from the guts a fibrous tissue called collagen, which is cleaned and salted, killing off any bacteria which may be present. The excess salt is rinsed off leaving a stringy pile which looks like spaghetti. The collagen fibres are then chemically treated; hydrogen peroxide is used to bleach the fibres and fabric softener is used to help them retain their natural elasticity. After another rinse, the fibrous tissue is ready to be sorted and graded. It takes training and experience to identify the quality fibres that will produce the best sound in a concert hall.
When the best parts have been selected, a separating machine is used to split the fibres into smaller strands. Tiny cuts are made at the top of the fibrous tubes before they are fed over a piece of metal with a blade at the end. Bands of various thickness are then cut from the collagen strips. Thicker bands will be used for the larger instruments like the cello or harp. The thinner bands will be reserved for the more delicate instruments such as the violin. But the fibres aren't ready to be played just yet. They must first be combined. The deeper the note, the thicker the string needs to be. An E for example is made up of five fibres, an A needs seven and a D comprises ten. The string maker winds the fibres together and attaches them to a knotted piece of ordinary string. These string loops can then be attached to a ringer which spins the fibres together. This squeezes out any excess water and twists them tightly so that they bond.
The newly fashioned strings are then hung up and left to dry for four days. The string maker keeps a close eye on the drying process to ensure there are no kinks, warps or loose fibers. When they are done, he cuts the strings from their holders and takes them to be finished. Because each string is a combination of fibres, they are rough and this would make playing problematic. Therefore, the next step is to smooth each string down. Using a laser guide that finds the thinnest part of each string, a grinder then reduces the whole length to this same uniform width, smoothing any rough edges as it goes. The final stage of the production process is to cut the strings down to the correct length. No sophisticated equipment is needed here - a ruler and a pair of scissors are fine. The string maker will measure out sixty centimetres and cut each bundle down to size. All that remains is to pack individual strings into their respective packets ready for use by the most demanding musicians looking for that special acoustic quality that is only produced by natural gut.
Today most gut strings are wrapped with metal, usually steel and occasionally copper. Gut violin strings are the natural choice for many classical and baroque players, who find the inimitable tone produced by gut strings the only choice for their repertoire. Pure gut violin strings (such as Pirastro's Chorda) are often combined with a 'period' set up instrument, with the aim of creating an authentic and historically accurate aesthetic.