if i was to put anything copper any where outside the slugs would weigh it in or the locas would complete waste of time, also my i point out that slugs will travel over crushed glass, nails, razor blades and carpet gripper rod, and you think egg shells are sharp and pointy , dont belive the tv gardeners, just like bee books they all speak the same old dribble with no new ideas for themsevles, one of the oldest ways to kill slugs is salt water, either spray it one several hours before it rains so it wash away or at any time as a weeker solution, anmd before some twit says salt water kills every thing, may i just point out i am on about spraying the soil not the plant unless its a cabbage or brassicas they love salt water because they are a sea shore plant