In chapter 6, Douglass describes this change of character that is directly attributable to Sophia Auld's exposure to ideals of slavery. When he first arrives, he describes his new mistress as having a "kind heart," and notes that he is "utterly astonished at her goodness." She has had to learn to depend on herself, making her living as a weaver. She has thus never owned slaves and is unlike any white woman Douglass has ever met. Mrs. Auld is kind to her slaves, allowing them to look her in the face and bestowing "heavenly smiles" upon them. When she learns that Douglass cannot read, she begins instructing him herself, which is likely the greatest kindness a white person ever bestowed upon him.
Yet it doesn't take long for her nature to transform. She soon learns the more "accepted" demeanor of owning slaves, and her new sense of power overtakes her cheerful disposition. When her husband learns of her literacy instruction toward Douglass, he scolds her for the efforts. She thus begins watching Douglass's every move, determined to be an even "better" slave owner than her husband.
Douglass notes that "slavery proved as injurious to her as it did to me." When he first arrives, Douglass notes Mrs. Auld's generosity toward the poor and suffering. The longer she touches slavery, the more her heart turns to stone and her character to "tiger-like fierceness."