itch.io

You use them with the Q key.  Its shown along with the name of the vial you have on the right hand side of the screen, in little print.

I got 3 dead key pieces from Jean, went and picked up the first one from my wardrobe and as soon as I had 3 in my inventory the game crashed.  I rebooted the game and it happened again as soon as I had 3 pieces.