Hi Bigal, Its possible that being a grease, it will deter molluscs from crossing but as far as I'm aware, its the fact that copper (in its metallic form) holds a very mild electrical charge and its this that deters the slugs and snails. This effect only lasts whilst the copper is very clean and bright and as soon as the surface of the metal oxidises, it loses its effect. So the fact that its copper grease wont deter them from the metallic element, just the physical thing of being grease but no doubt others will have their own views ???
Regards, Lannerman.