Can a string develop fifth impurity?
In principle, this is possible. There are two reasons for this.
1. Due to the strong loading on a small subsection of the string by the left hand or sweat, the string’s winding is only partially worn at this point.
This means that the whole vibrating string length is not worn evenly. Instead, it is only worn in sections. This causes a string to develop fifth impurity. Multi-layer E-strings, e.g. Peter Infeld® PI01SN (in Set PI101), Vision® VI01 (in Set VI100) and Vision Solo® VIS01 (in Set VIS100), have a thinner tin layer, making them less prone to developing fifth impurity. They are also more resistant to corrosion. The surface of these strings still becomes rough in the short term due to sweaty hands, but by playing continually over a number of days, it normally becomes smooth again without suffering any loss of quality. In general, plain chromium steel strings or chromium steel strings plated with gold, platinum or titanium, as well as strings wound with chromium steel or titanium flat wire are very resistant to corrosion.
2. For gut or synthetic strings, the core can also be stretched unevenly for the same reasons (sweat, uneven loading of a subsection), leading to fifth impurity.
Contrary to popular assumption, air humidity plays virtually no role here because this effects the entire vibrating string length. If the string develops fifth impurity, it must be replaced.