Subsidiary question: it was the first time I heard about salt being used, I only knew of natron. Did they mean natron, actually, in the .gif?

They almost certainly meant natron. But natron is a salt. When we think of salt in colloquial terms we tend to think of only NaCl (sodium chloride). But salts are an entire class of related chemical compounds. So technically both terms are correct. As a salt, natron was also used in food preservation such as drying fish. So it was used interchangeably in some cases for table salt.

Secondly ancient natron (sodium carbonate decahydrate Na2CO3·10H2O) was by no means pure. As used by the Egyptians it was only around 80% actual natron, the rest being sodium bicarbonate, sodium chloride, and sodium sulfate. So technically mummies thus were covered in a mix of salts of which natron was by far the largest component.

As to your primary question, unfortunately I don't have the information to answer that.