Sparks from electrical welding or from electrical grinding damage surfaces of glass or tiles while they do not damage the leather apron and even plastics. For example protecting glass in the old welding helmets has to be replaced frequently while the rest of the helmet, even plastic parts of it are not damaged at all. The recent helmets even do not have glass protection but use a plastic cover which does not show any damage after use.
There is probably something fundamentally different between materials which bounce back hot iron pieces (droplet) and which melt and hold hot droplets, which results in permanent damage. Crystal lattice, temperature of melting, chemical reaction, surface tension?