Answer:
The best inference of Dunbar's theme based on the iron throughout the poem is that he is exploring the theme of oppression and the use of masks to hide the pain and suffering caused by that oppression. The reference to iron, which is often associated with strength and durability, suggests that the oppression faced by the speaker and their community is severe and long-lasting. The "mask" referred to in the last line of the poem is a metaphor for the facade that people put on to hide their true feelings and emotions in the face of oppression. Overall, the poem suggests that even in the face of great suffering and oppression, people are able to put on a brave face and continue to smile and sing, but that the pain and suffering caused by the oppression is still very real and very present.