I can vaguely remember reading that some bowyers put their stave into a bakers oven at the end of the day. Not sure if it was done before working the stave, into a bow, or whether it was done during tillering. I don't know if it was done in Medieval or Renaissance times, or if the reference was more modern, ie 18th/19th century. I can also remember other old sources saying not to expose it to heat. Toxophilus would be the first book I would check, if I was doing research.
As most experienced contemporary bowyers know yew doesn't like being too dry, or overly heated, it often leads to violent breaks. Whether the bowyers of the time did it with white woods???. I agree with Chumash. Fire hardening wood goes back to the stone age, so its very likely craftsmen of the medieval ages were aware and could have employed some such process to maximise performance, especially with white woods.