The bow string spool is a reward from the Vale Totemsminigame, which can be obtained from rummaging through Vale offerings at a rate of 1/200, or bought directly from Vale Research Exchange for 250 research points. Spare spools can be combined to increase the capacity of the spool, or they can be traded in to Ranulph for 125 Vale research points.
The spool, when added with bow strings, can be used on an unfinished bow to string them, allowing the player to string 27 bows per inventory rather than 14. Additionally, it will automatically fill up with bow strings as they get spun from flax, providing the spool has not reached its maximum capacity.
The amount of string stored on the spool is tied to the player's account rather than the individual item, so even if the item is lost the charges are not.
The spool starts with a base capacity of 250 bow strings, which can be increased by using additional spools on itself. At six spools, it will reach the maximum capacity of 100,000 bow strings.
Bank notes can also be used on the spool to add bow strings, and bow strings can be withdrawn as an item or note.