The present day practice of making Pongal and offering to Family Deity or Kula Deivam is through stainless steel vessel or aluminium also. Even pots are also still used. Pieces of dried sticks are burnt with three stone or rock arrangement and gap in between the stones are left for oxygen supply for the wooden sticks to burn. The practice of offering Pongal to God has started from very old days and a part of Tamil Custom.
Also refer http://en.wikipedia.org/wiki/Kuladevata