Traditionally you wpuld use banana leaves. They simply seal the pig up a little bit an add a small layer to seal in the steam and protect the pig from scorching too badly. U can't get banana leaves easily so cabbage works pretty well as a substitute. The burlap is washed in warm soapy water to get all of the pil out of it and then rinsed in warm water. That is simply to hold it all together.
I went with a different method this year and had a huge roasting pan fabricated. It was a mistake for the first try. I didnt have any holes in the pan, it filled with pig fat, and deep fat fried 300 lbs of meat.
So next year... I'm drilling some holes