It Sifts From Leaden Sieves

This poem never explicitly states what "it" is but does provide a lot of valuable evidence to allow the reader to determine the unknown "it". In my reading of Emily Dickinson's piece, I determined the unknown to be time. The lines which support this hypothesis I believe are quite prevalent. Line 2 states "It powders all the Wood." which could be read to mean that all the wood as rotted. The passing of time is the one thing that rots all wood. Lines 5 and 6 declare, "It makes an Even Face Of Mountain, and of Plain--" This line refers to the weathering of the Earth through erosion of rock and the natural features of the land. A great amount of time is the one thing that is most necessary for this process to occur. One of the final pieces of evidence Dickinson leaves for us in this poem in order to determine that the unknown "it" is time, is found in the final two lines of the poem: "Then stills it's Artisans--like Ghosts--/Denying they have been--". Time is the one thing that erases the memory of people. No matter how famous one is, time will take away their remembrance. One's works may live on, but in the end, time will destroy it all. Time will always deny those who have been.
Perhaps I am misunderstanding Dickinson's intent but it is also possible that she left whatever the specific "it" was out of the poem in order to leave it open for interpretation. It is possible that the nouns "it" could define were too vast to allow her to choose only one to represent in the poem. She leaves the poem vague intentionally to leave the poem open for interpretation.