This is natron, a salt that absorbs moisture and fat and was the key drying agent used in mummification. It's still mined just southwest of the Nile Delta and is usually sold as a washing soda.

CAIRO — Ancient Egyptian mummies don’t smell the way most people ... By the Old Kingdom (2700-2200 BCE), embalmers began using natron – a natural mixture of sodium carbonate and other ...

Mummies can be dried in the sun ... It was then covered and packed with a form of natural salt, called natron, and left to dry on the embalming table. Forty days later, it would be blackened ...