I was watching a home improvement program the other day and the presenter mentioned that a copper bath kept the water warmer for longer. She didn't say what it was being compared with but I have assumed plastic/acrylic. I subsequently did a Google search and found sales literature that claimed the same – conveniently without stating what the comparison was with.
Surely copper being a better conductor of heat than plastic, water in a copper bath will cool more quickly than water in a plastic bath?
The only other material I can think that baths are made of is steel, but isn't copper a better conductor of heat than steel too, in which case the whole basis of the presenter's and the manufacturers claim is false?
Best Answer
I strongly suspect that if you want your bath water to cool slowly, you want the least thermally conductive material you can get. Acrylic or any plastic will be better than steel will be better than copper. Let's assume you fill the tub over a time span of a few minutes and regulate the temperature to be perfect just as the tub gets as full as you like it. You have set up a thermal gradient in the tub material from the water temperature at the inside to room temperature at the outside. The more conductive the material is, the deeper this gradient will penetrate into the skin material. As the bath progresses, the more conductive material will continue to draw more heat into the tub wall, cooling the water.
What could give copper the advantage? If you get the part of the tub that contains water all the way up to bath temperature while you are filling the tub, you won't have the heat leak of heating up that copper any more. You will still be conducting heat up the tub wall, but that is probably slower. Against this, you now have the back side of the tub losing heat by convection to the air, more than doubling the surface area of your convective heat leak.
To really calculate it, we would need to know the thickness of the material in steel and copper tubs. It could also be that the copper tub is really a steel tub with copper plating the inside. In that case, there should be negligible difference between copper and steel.