I wonder if the internal copper lining oxidizes and ends up looking like a gross old penny?
I know this is a week late, but I’ll answer anyways. Could not recommend this thing more. The internal material is stainless steel. I think the copper is inside “the vacuum.” I ordered this in July and have used it daily since. I’ve started filling it at night and leaving it without the cap on in the fridge. The water easily stays cold throughout the next day. It has stayed perfectly clean with no buildup or color change occurring anywhere.
This thing is excellent keeping your ice water with ice still in it for dayssss. This goes with us everywhere or we keep it in house always full of ice water for us and our small dog’s water dish. Dog’s love the ice cold water. I is very well made. You won’t go wrong with this but!