## 4 Discussion  
In the last two decades, there has been a major shift in thinking with regard to the herbicidal treatments applied to cultural heritage. The treatments have changed from invasive procedures such as controlled burning and flooding [22] and the use of chemical herbicides containing harmful compounds to the present use of natural, eco-innovative compounds with a low risk to human health, e.g., essential oils and quorum sensing inhibitors [36; 37]. The efficacy of herbicides or other protective measures has been tested numerous times against target plants, but as already noted, the question regarding how the treatments affect masonry materials has rarely been examined. In this respect, although herbicides are commonly used to prevent the growth of weeds, very few products have been tested or developed specifically for use on historic masonry materials. Indeed, most products are borrowed from agricultural and landscape applications. The first published investigation into whether chemical treatments affect masonry is from the late 1970s [25] and the next major study of the effects of a herbicide on historic masonry was carried out 1989 [26]. In the former, brick and mortar samples (clay brick, clay tile, teracotta, cinder block, concrete block, adobe) were soaked in two different herbicides (Roundup(r) and Garlon(r)) for one week. In the latter, Roundup(r), Weed-B-Gon(r), and borax were applied to brick, limestone, concrete and granite. Both studies concluded that the cumulative use of herbicides could cause mechanical damage and staining on stone and mortars. In a later comprehensive study, in 1999, in which a glyphosate-based herbicide (Round-up(r)) was tested [22], it was concluded that glyphosate causes three types of damage to masonry. It first attacks calcareous stone by acid dissolution. Secondly, the compound and the solvent (in this case water) introduce or redeposit soluble salts. Thirdly, in the presence of calcium, the compound forms insoluble salts. In acid rocks, such as schist, glyphosate also caused mineralogical changes, such as the disappearance of vermiculite, as confirmed in the present study. However, the most important problem with glyphosate is the doubt regarding the danger it poses to human health. According to several studies, the potentially carcinogenic effects of glyphosate are complex in nature [38]. Although considered a probable carcinogen by the IARC since 2015, many regulatory agencies, including the European Food Safety Authority and European Chemicals Agency, continue to hold that glyphosate is unlikely to pose a carcinogenic risk [38]. In Galicia (NW Spain), during the 1990s, chemicals such as Neo Desogen, Paragon Invisible, Hyvar-X, Santi-S and the cleaning agent AB-57 (a calcium dissolving solvent) were used to remove lichens and algae from granite monuments. The deleterious effects of these compounds are similar to those caused by Goal Supreme(r) in the present study. Hence, although there was no mineralogical alteration, there was an appreciable colour change, in addition to saline efflorescence after treatment [39]. Important colour changes of more than 30 CIELAB units were reported by [PERSON] et al. [24] after the application of biocide (Koretrel(r)) to grey granite. However, the change in colour was reversible because after washing the sample in running tap water for 48 h, the colour change measured was less than one CIELAB unit. In the present study, the colour change induced can be considered more stable over time, as it was observed up to three months after application. The use of chemicals will be increasingly questioned in terms of both human safety and environmentally friendliness. However, this attention should also be extended in the field of the conservation of stone monuments and buildings, to determine the short-term and long-term potential damage to historic masonry materials caused by herbicides or other protective measures.