Soiling of building stones in urban environments

This paper studies the black soiling rate of building granite, marble and limestones in two urban environments with different climates. Specimens were placed horizontally, sheltered and unsheltered from rainfall. Colour changes were monitored. Changes in L* were utilised to determine the total darkening of the stones and their soiling rate and were compared by statistical analysis. Black soiling depended on the type of stone and exposure. Limestones, mainly, showed a soiling proportional to the square root of the time of exposure. In this case, soiling coefficients could indicate frequency of necessary cleaning. The rest of the selected stones did not show such an evident relationship. The soiling tendency of all sheltered limestones was quite similar and more severe than in marble and granite. Soiling was related to the stone colour, type of voids and surface characteristics. Soiling of unsheltered specimens also seemed to depend on the rainfall conditions (rain periods, intensity, amount, temperature, etc.), which could influence both particle deposition and removal from stone surfaces.