Posted in comments, Anonymous asked what, in my opinion, were the themes in The First Vial.
The principal theme in The First Vial is one of resilience - the resilience of the human spirit - accompanied at several junctures by a motif of fire.
Katherine's resilience is tested in connection with fire at three points - the burning bed, the burning field and the burning stake. At any one of those points Katherine's spirit could have been crushed, but each time although discouraged, she regained inner strength and did not give in to hopelessness or fear.
It also seemed fitting to me that the one attempting to use fire as a tool against her should himself be overcome by fire.