I am whitening the specimens with Ammonium Chloride as shown to me by one of our local professional paleontologists. I have yet to do this myself. The basic method is to heat the powder in a glass tube where it forms a white vapor. A squeeze bulb at one end is used to “blow” the vapor out the other end of the tube where it condenses as a thin film on the fossil. The coating can be removed by the humidity of your breath.
I am surprised that you are able to use Magnesium Oxide as my experience with that is as a refractory and heat conductor used in the manufacture of heating elements for electric ranges and industrial equipment. I would not expect it to vaporize under heat as I described above. Are you using a different method?