"The Sacrifice of Iphigenia"

The painting is François Perrier’s ‘The Sacrifice of Iphigenia’ (1632-33).  I had to do a little reading to understand what's going on, so I think I understand the significance of the deer in the clouds.

My question is - why is the knife shown being heated over the flames??  It's obviously going to be used to cut her throat (based on the positioning of her neck), but a sacrifice doesn't require a sterile instrument.  Or is instrument sterility not the point - is there some other reason (?religious, ritual?) for heating a knife before cutting someone's neck?

Found at Miss Folly.