Brainstorm words that describe that piece. These do not need to be adjectives; write down anything that the sculpture conjures in your mind. Don't disregard any thoughts. Rather, write every word that you think of. Analyze the shape, style, construction and material of the sculpture as well as what it depicts, the meaning it carries and emotions it evokes.
Describe the sculpture using every sense possible. Even if you can't actually touch the piece, use descriptive words that make the reader feel as though she can. It may be rough, smooth, polished or worn. Sounds can be useful if the sculpture is situated in a garden filled with birdsong or at the center of a loudly splashing fountain. Include emotions associated with the piece to draw in the reader.
Review the results of your brainstorm and separate the words into categories. You may find that you have several words describing the type of stone the sculpture is made of, or several phrases about the movement depicted by the piece. Single these areas out for their own paragraphs and write a rough outline of the essay.
Structure your essay so that the object's description moves from the general to the specific. Don't begin with a close inspection of the detail on the piece. Rather, start by offering a broad image of the sculpture, sharing what one would first notice when coming upon the piece. Move on to more detailed aspects of the sculpture and end with an in-depth inspection.
Reread your finished essay and look for generic words. Avoid such bland terms as "beautiful," "unique," "big," "short" or "artistic." Hone in on what you are really trying to say and use a more descriptive phrase. Instead of "big," try "towering," "imposing" or "powerful." Don't force your reader to fill in the gaps because he is unsure of what you meant.