A prison cellmate told the court Arnold had confessed she had done it because she was worried the boyfriend might discover that he was not the child's father.
The baby's DNA was found inside the microwave in Arnold's flat in Dayton which she shared with her four children and her then-boyfriend Terrell Talley, prosecutors said.
A forensic pathologist testified that the girl was likely to have died from a sharply raised body temperature after being in the microwave for more than two minutes.
Jon Paul Rion, defending, told the jury that Arnold couldn't remember what happened and was probably too drunk that night to have committed the crime.
He introduced testimony from an eight-year-old boy who said he saw another boy walk into the kitchen of a nearby apartment with the baby, heard the microwave go on, and then later saw the burned baby in the microwave.
Judge John Kessler declared a mistrial in February after he privately heard testimony from the juvenile, who had a history of putting cats and dolls into microwaves.
However, the eight-year-old's mother told the court on Wednesday that they lived some distance away and they were not at Arnold's apartment complex when the baby died. Her testimony was backed by her former boyfriend.
