The paper presented by Vavro et al. (2018) is an interesting application of a relatively new technique, X-ray computed micro-tomography (MCT), for the determination of iron sulphides in roofing slates. Once the slate shingles are installed on the roof, these iron sulphides may oxidize, creating unaesthetic red stains running along the roof surface. This pathology is considered to be the main technical problem for the roofing slate industry (Cárdenes et al. 2016a). In their paper, Vavro et al. (2018) explain how the use of MCT allows quantification of the amount, size and spatial distribution of the iron sulphides on two samples of roofing slates. However, their work misses some important points that are worth mentioning.