To set the scene, in chapter...
In chapter 7 of The Scarlet Letter Nathaniel Hawthorne uses the breastplate to create imagery and symbolism that represents the idea that Hester Prynne and her illegitimate daughter Pearl are perceived to be sinful and deserving of their separation from the rest of society.
To set the scene, in chapter 7 Hester and Pearl have gone to Governor Bellingham’s home, ostensibly to deliver a pair of gloves that Hester has embroidered for the governor. However, Hester also has an underlying motive. She has somehow heard that a number of the townspeople feel that she is an unfit mother and are trying to have Pearl removed from her care. In this chapter Hawthorne describes the beauty of Governor Bellingham’s home, and then focuses on a suit of armor on display in the hallway.
There was a steel headpiece, a cuirass, a gorget, and greaves, with a pair of guantlets and a sword hanging beneath; all, and especially the helmet and breastplate, so highly burnished as to glow with white radiance and scatter an illumination everywhere about upon the floor.
The imagery here is meant to communicate the impressive nature of this suit of armor. In a sense, it represents what is considered gallant and courageous by society. Hawthorne characterizes Bellingham by telling the reader that he actually used this armor in battle in the Peqoud War. The fact that the breastplate is able to “glow with white radiance” imbues the reader with the feeling that this armor, and by extension Bellingham, is an upstanding member of society.
Then Hawthorne takes it a step farther when little Pearl becomes excited about seeing her mother’s reflection in the breastplate. But Hawthorne doesn’t just draw for the reader an image of a simple, straightforward, proportional reflection. Instead, he takes this opportunity to present Hester symbolically.
. . . owing to the peculiar effect of this convex mirror [the breastplate], the scarlet letter was represented in exaggerated and gigantic proportions, so as to be greatly the most prominent feature of her appearance. In truth, she seemed absolutely hidden behind it.
With this passage, Hawthorne has created for the reader an image that symbolically represents society’s perception of Hester. She is defined by her earlier deed, adultery. For now at least, this deed, like the overblown reflection of the “A” on her breast in the breastplate’s reflection, is all anybody sees when they see Hester. It completely defines her role in the community. The fact that it is represented this way in the governor's armor hints that Hester is certainly in danger of losing her daughter due to his disapproval. Hester and Pearl are saved separation by Reverend Dimmesdale, who is at the governor's home at the time and makes an effective appeal on her behalf.
At this point, the reader would probably be surprised to find out that the community’s view of Hester is going to change in a positive way over the coming years. Hester’s quiet kindness and sympathetic nature is eventually going to reform her image in the eyes of the townspeople.