There is a laboratory test, Determination of sensitivity to accidental stains (UNE EN 16301) that specifies a method to assess the sensitivity of natural stone when exposed to certain types of stains.
With the results of this test you can know if your countertop is more or less sensitive to any of the most common staining agents, such as red wine, cooking oil, coffee, ketchup, cola, citric acid, or even cranberry soup. This allows knowing which attentions are needed for every specific countertop, although in a general way, they are 4 risk factors for a countertop against the stains:
1 Mineralogical composition and texture of the stone
The mineralogical composition influences when reacting with certain staining agents. For example, carbonated rocks (limestone and marble) are more sensitive to react with acids, so that exposures to citrus juices or Ketchup, can cause loss of brightness in a polished countertop or induce some roughness if not cleaned in a period short of time.
The texture of the natural stone, that is, the spatial distribution of its components, the size and shape of the grains and the empty spaces (pores and fissures) also influence, so that the more porous or cracked a material is, the easier to be penetrated by any staining agent.
2 Type of surface finish
This is another important factor, since the greater the roughness, the more easily stained. Although most kitchen countertops are polished, more and more new finishes are being used, such as the aged ones, which have a certain roughness but are finished off with a polishing or brushing process with carborundum or diamond.
Finishes that include any of these processes decrease the surface porosity resulting in a more protective finish.
In addition, protective coatings such as oleo repellents and/or water repellents are frequently applied at the factory, which are very protective against stains.
3 Staining agents
Although there is a wide range of products that could be susceptible to stain a countertop, the most common in a kitchen are those selected for the laboratory test:
– Agents of strong colours such as red wine or coffee.
– Agents with high penetration capacity such as cooking oil.
– Acid agents such as colas, citric acids and ketchup.
4 Exposure time of the stone to the stain
This is one of the most important factors. The damage that a polluting agent can cause on the stone depends directly on the time they are both in contact, so that, the shorter the exposure time is, the easier the stain produced is eliminated and even reversible.
For this reason, in the laboratory test designed to reproduce the accidental staining of the stone, exposures to stains are simulated for 15 minutes and 24 hours, being related to the average times in which cleaning is usually done. For example, a stain that occurs inside a private home is usually cleaned immediately or in a very short period of time; however, a stain that occurs in a public facility or a work centre may go unnoticed and be up to 24 hours without cleaning.
Few agents produce a permanent stain if it is cleaned immediately or after a short time.
The four factors are very interrelated with each other; for example, in the case of a granite countertop, which is a very low porosity material, the agents to which some varieties may be susceptible are red wine and cooking oil. If the finish of the countertop is polished a long exposure period is needed to produce a permanent stain, if an oleo repellent and water repellent product have also been applied then it will be perfectly protected against any stain, even if it is not cleaned in a short period of time.
Nuria Sánchez Delgado
Doctor in Geology
CENTRO TECNOLÓXICO DO GRANITO DE GALICIA