A solder leaded spool! That sounds very helpful since it’s considered the basic spool leader solder, the fact that it contains 63/37 being sorted as water soluble resin core, 2% Flux, 0.031" gauge and 100 grams makes it such a good spool for advanced users, and the more soldering you do, the more it affects the range of how long it takes till it lasts but usually between 6 and 12 months.

Mix: 63% Sn / 37% Pb