Sieve pans are necessary to collect the fine material passing through the bottom sieve in a sieve stack. Sieve covers, or lids, on the top sieve, prevent loss of material during sieving operations. Most Gilson brand Sieve Shakers, except for our Rotary Sifters, feature built-in covers. Covers are required for use with many other brands and are useful for manual sieving operations as well.