History Behind the Pope's Method of Burial

A reader asks:

I am not sure if you can answer this or not. I have been reading all I can regarding our Pope and his burial. I am curious regarding the way he is to be buried. His face covered in a white vail, items placed in his casket, total of three caskets with the last being nailed shut with gold nails. I don't know if this is all true or not. If it is I am just wondering when and how this tradition started. I am curious if you know the answer.

As far as I can tell, the account is true. I am not certain how or when this this started though.

I will review the Order for Christian Burial this evening. I'm not certain what is and is not addressed here.

Here is an interactive guide to the Papal Burial Ritual

Can anyone guide us in the right direction to find the answers?