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Visual or formal analysis is a basic but key element of art-historical writing. In a visual analysis, you should address and describe the formal elements of a work of art like the colors, lines, shapes, textures, and size. This sort of analysis should not interpret the details (iconography/iconology), nor should it include information about the artist, history, etc.
The best way to begin a visual analysis is to start by sitting in front of the piece for an extended period of time (in person is ideal but if you don't have access to the work then virtually will do) and write down any details you see, both big and small. A visual analysis should guide your reader's eyes around the work of art through a detailed (often lengthy) description, describing it as if your reader has never and cannot see it.
A majority of art-historical essays require the use of the Chicago Citation Style, but that is not to say that all will so be sure to check your professor's guidelines.
Footnotes, endnotes, and citations should all be properly formatted according to the chosen citation style.
Be sure to use your TA to your advantage, they are there to help you! More often than not, they will read a draft of your essay and give you helpful feedback on how to improve it for the final draft.