It’s quite common for organic materials to yellow with age. The molecular chains become shorter and that’s what causes the yellowing. It will be possible to wash the robe by hand using an additive-free cleaning agent. But if the robe isn’t dirty, don’t wash it.

Get some thin white cotton fabric that’s gone through at least a 60°C cycle in the washing machine and use it to pack up the robe, making as few folds as possible. Light is very damaging for both textiles and paper, and it will be best to store the robe in a dark place with minimal fluctuations in temperature.

Kind regards from the textile conservators