If you are going to invest time and effort in making a piece of knitwear, you want to be able to enjoy the finished item for a long time, however much pleasure you gain from the actual knitting. What you don’t want is a sweater that felts in places or pills everywhere the second time you wear it or socks that hole quickly.
Having knits that stand the test of time comes down in a large part to the yarn you choose. Of course patterns can become dated, as can colour choices – hands up everyone who has been seduced by a distinctive variegated or textured yarn and never worn the garment. But interesting fashion choices aside, the key to a long life for a knit comes down to the fibre content of your yarn and how it is spun.

Fibre Content
What your yarn is made from will influence how your finished item will wear. For animal protein yarns such as wool, cashmere and alpaca, there are a number of factors to consider. The staple length is the length of an individual fibre from the animal. Longer staples (more than 10cm) such as in fleeces from Bluefaced Leicester, Romney or Wensleydale sheep, hold together more than the short fibres of say cashmere, which means they don’t have to be twisted so tightly for the yarn to be strong.
The reason that animal fibres helpfully stick together in yarn is because their surface is covered in tiny scales that “hook” the fibres together. This is also what allows us to felt fibres like wool. Finer fibres have more scales than coarser ones, making them more likely to felt when rubbed or agitated in wet conditions. This is why cashmere is not necessarily the best choice of socks.
Finer fabrics such as alpaca or cashmere and some wool breeds give us softer yarns, but there is a trade-off with durability. The finest fibres are the most likely to pill as a garment is worn and parts rub together. To test how quickly a yarn might pill, wash a swatch and rub it on various surfaces to stimulate wear.
The natural crimp of animal fibres also help knits keep their shape because the fibres want to spring back to their original state. On the other hand, lustrous non-animal fibres such as silk and viscose bamboo do not spring back in the same way, and as much smoother fibres they don’t hold together much. This means that while these yarns run less risk of felting, they are more likely to stretch because the fibres hold together less. However, these fibres can bring other qualities such as sheen, strength in the spinning process and softnes, which is why blended yarns can be a great choice.
Blending various fibres together could be seen in a similar way to blending a good whisky. It is about balancing various attributes to bring out the best of the ingredients. So the crimp in wool or alpaca can be balanced with the sheen and strength of silk, or wool with nylon to create a hardier sock yarn that is still comfortable to wear.
Man-made fibres also have a place when we are talking about the longevity of yarns. Check your own wardrobe for your longer lasting knits: there is probably a garment with a high acrylic content from when you first started knitting that is still wearable.

Yarn Construction
The other balancing act when it comes to durable yarns is the “twist”, or how the fibres are spun together. Yarns that have a low twist, including the roving-type yarns where all the fibres seem to be lying side by side, won’t hold together well under abrasion, so they will wear, pill or felt. By contrast yarns that are plied and twisted are likely to be more robust. In these yarns the fibres are spun together into a number of finer yarn threads – plies – that are then twisted together to form the final yarn. The twists in the plies and final yarn hold the fibres together, reducing the risk of pilling and creating a stronger yarn. But beware of tightly twisted yarns, which may be brittle and break easily.
You will also see yarns referred to as worsted. The worsting process removes the short fibres before the yarn is spun. This means they are likely to be more durable with less risk of pilling.
One final thought on helping your knits stand the test of time: if we treat them well, they will repay us. So store your investment knits carefully and wash and dry them gently to reduce the risk of pilling, felting and stretching.