Dropping an iron stiffened mesh into a sieve places it into the sieve. Once inside the sieve, the sieve can be used to sift items and obtain random loot more rare than the loot you can obtain from an flint stiffened mesh. An iron stiffened mesh can be crafted by dropping 5 string and 4 iron onto a crafting table. Crafting your first iron stiffened mesh earns the player the advancement "Re-Re-Reinventing The Web".