The application of the contact angle in monument protection: new materials and methods

The advancing angle is the commonest way to measure and use the contact angle. Sometimes its use is an acceptable approximation, whereas in other cases it is simply not useful or misleading. This is the case of a natural porous material, such as stone. The stone materials are heterogeneous and rough, and the application of protective agents makes their surfaces even more complex. This paper confirms that the measurement of the advancing angles can only indicate the presence of the protective, not its effective protecting ability. On the contrary, the receding angles correlate better with the chemical structure and protective ability of the protecting agents (e.g. with the water absorption). Some examples of evaluations of commercial and newly synthesised protective polymers show the advantages and limitations of the use of the contact angles and the difficulties of their interpretation. Moreover, new and originally chosen protective agents, i.e. some cationic surfactants already used as "rinsing aids" and polyvynilpyrrolidone waxes, have been tested as protective with interesting results. The time dependence of protection efficacy and its correlation with contact angle have also been tested. (C) 2004 Elsevier B.V. All rights reserved.