Stain test for Natural Stones (NT TR 595)

Abstract

  This report presents the results of Nordtest project No. 04019. The study focuses on the development of a standardised procedure for assessing the staining sensitivity of natural stones with and without surface treatment. The following different stone types with different mineralogy, chemistry and surface finishing were used: marble, mica-schist, ultrabasitee, granite and limestone. Various staining agents were used in order to find a suitable set of agents to be used as a standard set for general purposes. The following ones were chosen to be included in the method: Red wine, Cooking oil, Instant coffee, Ketchup and Coca-Cola. They represent a wide variety of stain types such as acid liquids (with high and low viscosity), extreme colour content (neutral and acid) and highly penetrating/migrating liquids. A matt finished surface (F 400 mesh) was chosen as a reference surface to compare the sensitivity of different stone types. Other surfaces can be tested for a specific application. It was decided to use the untreated surface as one reference and a surface treated with a soft soap, made from pine oil, as another reference for comparison of the sensitivity and also to relate the sensitivity to the every-day practice. Soft soap is the treatment recommended by most natural stones federations, especially for cleaning of carbonaceous stones. The procedure for applying the staining agents is very important, as is the time between staining and washing. These parts have therefore been given special attention and are detailed in the report and the method. The specific cleaning/washing machine used is not important, but the method of work and the cleaning cloth are, and have also been described in detail. Remaining traces of staining agents and potential etching effects are evaluated visually, by three persons, as the reference procedure. This has proven to give a satisfactory reproducibility. Alternative techniques can be used as a complement, e.g. spectrophotometry and, in cases, measurements of gloss. Complementary evaluation of cleaning procedures has been investigated but will be reported separately.