Correct use of the contact angle in the evaluation of the protective action induced from polymer coating on the stone.

The control of the protective efficacy obtained on the stone by treatments with polymers is commonly performed through the measure of the static contact angle as it is described by the norm UNI 10921. However this approach does not allow an easy interpretation of the results, because of the porosity and of the heterogeneity of the stone surface, which represent an obstacle to the analysis. Moreover the commonest interpretation of this technique can often bring to important errors. The measure of the static contact angle substantially corresponds to the measure of the so-called "advancing" contact angle and it allows only to verify if on the surface a hydrophobic material is present; unfortunately it cannot determine if the stone is effectively protected. Vice versa, both, the measure of the "true" equilibrium contact angle obtained through a new technique called VIECA, and the measure of the receding contact angle give more coherent parameters which better correlate with the data of absorption of water by capillarity. The equilibrium contact angle corresponds to an "average" description of the surface, the receding angle corresponds, by excess, to the condition of maximum penetration of the liquid by capillarity. From the knowledge of the equilibrium angle of the protective polymer and from the measure of the advancing and receding angles of the protected stone, it is certainly possible to determine what is the minimum polymer quantity to obtain an almost homogeneous stone protection.