The original threads of old paternosters and rosaries seldom survive. Having now owned and worn a number of medieval rosaries, I have a better appreciation of why that might be. Even silk thread -- which was probably the strongest available fine thread in the Middle Ages -- is neither unbreakable nor immortal. I don't wear my medieval-style reproductions all that often, and some of them are on their second or third stringing within ten years. Friends to whom I've given such beads have brought them back for re-stringing after as little as eighteen months, and discussions on some of the mailing lists I subscribe to include comments from people who wear necklaces of Anglo-Saxon or Viking-style beads, who also report that these need to be re-strung every couple of years (which provides an opportunity to re-arrange them, so it's not all bad).
(By the way, the photos in this post are just here as decoration. They are paternosters I've made at various times, mostly as gifts.)
Threaded rosaries are inherently more prone to breakage than the wire-chain construction so common in modern rosaries. My guess is that wire-chain construction began to gain in popularity at the end of the 16th century for precisely this reason. Wire chain has some disadvantages -- it does tangle and kink, sometimes quite badly, as the wire loops at the end of each link catch on each other. Putting each bead on a separate wire link also means you cannot slide the beads along the thread as you count them; instead, the entire rosary is passed through the hands, one bead at a time, which gives a different "feel" to the counting process.
Thread breakage in period must have been very common. I no longer wonder why so many of the threaded paternosters and rosaries mentioned in 15th- and 16th-century wills and inventories have strange numbers of beads, such as 114, 32 or 83 ;)
Why and how threads break depends on what kind of stresses the thread is subjected to. Tensile strength is one factor -- how hard it is to break a thread by pulling on the ends. Another factor is resistance to abrasion -- how much the thread is weakened by the friction of beads sliding back and forth. A third factor is "fatigue" -- how much something is weakened by being repeatedly bent in different directions. Silk thread is pretty good on the first, not good on the second, and the third is (as far as I know) no problem at all. Metal wire is very good on the first two, not so good on the third.
One thing to consider is that medieval silk thread may well have had more tensile strength than modern silk thread, so perhaps modern experience is not quite comparable. Most silk thread today is machine-spun, which requires that the very long silk fibers be chopped into short pieces. "Reeled" silk, which preserves the original fiber length, is likely to be stronger, and in fact a lot of modern synthetic fibers like nylon make strong thread at least partly for the same reason. Much of the silk thread available in the Middle Ages and Renaissance (if you could afford silk at all) was reeled, so this may have made a difference. It would be interesting to test the two types and see how different they are.
Another thing to consider is how much care is taken by the wearer of the beads to keep them out of situations where they might break. Modern rosaries are often rather carelessly knocked around, carried loose in the bottom of a pocket or purse, or perhaps hung from a car's rear-view mirror (however much this is frowned on by safety experts!). In the Middle Ages, rosaries seem to have been more often worn out in the open like jewelry, often pinned to clothing, wound around an arm or hanging from a belt. This puts them in situations where they are very likely to catch on something, and I've had several sets of beads do exactly that while I was wearing them -- a dangling loop catching on a doorknob, for instance, or the most recent instance where the cross at the end of my Saint Hedwig beads got caught in the crack of a wooden bench, and the string snapped when I stood up quickly.
I would guess, however, that silk thread's major weakness is from abrasion due to friction. It seems to have been the custom to slide each bead along the thread as it is counted while praying. The beads can also slide back and forth as they are picked up or put down, worn or carried. As far as I can tell, medieval rosaries did not have knots between each bead (as, for instance, pearl necklaces often do). That would have interfered with sliding them, and incidentally it also means that if the thread does break you are likely to lose several beads rather than just one or two. (So far I've mostly been lucky enough to find all the beads when mine have broken, though there are still three little red glass beads in my car somewhere.)
Friction between beads and thread also depends on craftsmanship. Virtually all modern beads are machine-made, and the holes are likewise drilled by machine. This guarantees a smooth hole inside, but it can leave fairly sharp edges where the hole enters or leaves the bead. I would guess that when beads were made by hand, the openings of the holes could have been smoothed with a file, and if so, abrasion would have been lessened.
Other threads available at the time probably shared some of these weaknesses. The most plausible thread for rosaries for those who could not afford silk is probably linen or hemp. Both of these are fairly strong in terms of tensile strength -- especially if the linen is "line" spun using the full length of the fibers -- but they are also not as smooth as silk, which would have increased the friction between thread and beads. Wool thread is not very likely as a stringing material because it also has a rough surface, and compared to the others, it is relatively short-fibered and has much less tensile strength. A single reeled silk fiber may be dozens of feet long in the uncut state. Linen and hemp fibers are often as long as the plant is tall, as much as three or four feet. The longest wool fibers I know of are six to eight inches, and they are far more commonly found in the one- to three-inch range.
Posts in this series:
String or Nothing
The thread thread
Threads of silk and gold
Of flexwire and time machines