The lost boys of London / Mary Lawrence.

Enhanced descriptions from Syndetics:

In this fifth Bianca Goddard mystery set during the final years of King Henry VIII's reign, the alchemist's daughter uses her skills to aid the living and helps seek justice for the dead...While her husband fights the Scots for King Henry VIII, Bianca Goddard earns her coin by concocting medicines that offer relief to London's sick. Some unfortunates, however, are beyond any remedies she can provide--like the young boy discovered hanging from a church dripstone. Examining the body, Bianca finds a rosary twined around the child's neck. A week later, another boy is found dead at a different church. When Fisk, Bianca's impish acquaintance, goes missing, she fears he may become the third victim...There are many villains who would prey on wayward, penniless boys. But Bianca suspects the killings are not brutal acts of impulse, but something far more calculated. In her room of Medicinals and Physickes, she examines the sole piece of evidence: a sweet-smelling dark cloth. If Bianca can unravel its secret, reputations and lives will be saved. The expected hour of the next murder is approaching, and a single misstep may mean another boy is lost forever...