The FTU walls have been coated with a silicon film in order to compare plasma characteristics with low-Z (Si) and medium-Z (Ni) first wall materials and to establish the role of oxygen for FTU operations. No variation in energy confinement time, temperature and total kinetic energy is observed when silicon covers limiter and walls, although the radiative properties of silicon and nickel are substantially different; at the same time the density limit is reduced by a factor of 1.5 for silicon and the MARFE instability also appears at lower densities, When the silicon coating is still present on the walls but no longer on the limiter, plasma performances do not change appreciably with respect to those obtained with stainless steel walls. A plasma core-edge coupled model can reproduce the experimental content of impurity, the total radiated power and also, qualitatively, the higher density limit in FTU as the atomic number of the limiter material is increased due to the change from Si to Ni, Mo, W. In addition this confirms that the impurity production is essentially due to physical sputtering and self-sputtering only and indicates that the mechanism responsible for both the density limit and the MARFE on FTU seems to be the power balance at the plasma edge between the conducted power and the locally radiated power.