Cite

[Portrait of an unknown man] [art original].

MLA

[Portrait of an Unknown Man] [Art Original]. 0AD. https://collections.library.yale.edu/catalog/11138651.

APA, 6th edition

[Portrait of an unknown man] [art original]. (0 C.E.). [Graphite drawings British 18th century]. https://collections.library.yale.edu/catalog/11138651.