Basically, clay or any other condition/material that insulates the body from oxigen (ground burials or sealed tombs).
I wouldn't dare to say that weeks or month after death a corpse can still be "fresh", but in some places when/if you open a buried coffin after 5 years or so it is not rare that there has been not enough decomposition to allow the transfer of the body to an ossuary.
There are "aerated" tombs devised to accelerate the decomposition: