Responsa על בראשית 3:27

However, although the practical necessities would require burying the body in the receptacle in which it is delivered by the government to the family, the question is whether this is at all permissible according to Jewish law. There is, of course, the general legal objection to disturbing the remains unduly. On the other hand, there is some objection to a metal casket. The fact of the matter is that even a complete wooden casked has no firm legal basis, the law requiring that the body should be actually buried in the earth. See Nachmanides quoted by Karo to Tur Yore Deah 363, who says that the use of the coffin in Talmudic times was only for the bones after the flesh decayed. The only Scriptural warrant for a wooden casket seems to be the midrash in Genesis Rabba Chapter 19, Section 8, based upon the phrase in Genesis 3:8; "And Adam hid himself from before the Lord among the trees (etz) of the garden.— To this the Midrash comments: This is the hint that his descendants will be buried in coffins of wood. The law, Tur Yore Deah 363 and Shulchan Aruch, prefers burial directly in the earth. It seems to concede burial in a coffin provided that the coffin is open to the earth at the bottom. Then a concession is made to the completely closed coffin provided holes are bored in it to permit contact with the earth. However, Karo, Yore Deah 363 #5, speaks of stone caskets; and the Talmud (b. Sota 13a) says that Joseph's coffin was made of metal.