it seems probable that mud is used for culture because

There is one fascinating question that arises out of the contemplation of mud sculpture. Why should anybody use unbaked mud, the most perishable of materials? Is it because no other material is readily available? The question is not easy to answer definitely. Mud, is, of course, the cheapest and most readily available material. Yet there is ample proof that mud is not used merely because it is easy to get hold of and cheap. Many Igbo Mbari houses are the only buildings in the village that have an imported corrugated iron roof – which prove that the people who built them shun no cost to make them look important. In all the areas where I have seen mud sculpture, wood carving and brass casting are also known and practiced. In Yoruba country, stone is also used as a medium for sculpture.
One important thing to realize is that different materials are not necessarily used because they have lasting, durable qualities. In Yoruba country today, brass can only be used by Oshun or Ogboni worshippers. Ivory can only be used by Obatala worshippers, copper by Sonponna, iron by Ogun and so on.
Materials are used for their mystic properties of absorbing or repelling human radiation. The Obatala worshippers used Ivory as protection, in the sense that it is protecting him from the destructive psychic influences of a man whose mentality is basically different or opposed to his. Similarly Oshun worshippers uses brass figure in their shrines – not because brass last longer than wood, but because brass possesses certain magical qualities that are sacred to Oshun.
It is not difficult to understand why mud is considered the appropriate medium for Ala (the Igbo earth goddess). Olokun (the Bini god of the ocean), or Legba (originally an earthgod of the Fon). The fact that the material is perishable and sometimes does not even last five years does not enter into the consideration. One does not interfere with the natural life of a carving. When it perishes, a new one simply has to be made.