John Calvin Commentary Genesis 50:3

"And forty days were fulfilled for him; for so are fulfilled the days of embalming: and the Egyptians wept for him three-score and ten days." — Genesis 50:3 (ASV)

And forty days were fulfilled for him. We have already shown that Moses is speaking of a ceremonial mourning; therefore, he does not prescribe it as a law or present it as an example that we should follow. For, by the laws, certain days were appointed so that time could be given for moderating grief to some degree; yet something was also conceded to ambition.

Another rule, however, for restraining grief is given to us by the Lord. And Joseph stooped, more than he should have, to the corrupt customs of the Egyptians; for the world affects to believe that whatever is customary is lawful, so that what generally prevails sweeps along everything it meets, like a violent flood.

The seventy days that Moses sets apart for solemn mourning, Herodotus, in his second book, assigns to the embalming. But Diodorus writes that the preservation of the body was completed in thirty days. Both authors diligently describe the method of embalming. And though I will not deny that, over time, the skill and diligence in practicing this art increased, yet it seems probable to me that this method was handed down from their ancestors.