If you're remodeling a house and you want to know if your tub is cast iron, you can figure it out with a few telltale signs.
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Touch the tub with your fingertips. If the material is cold to the touch and looks like the finish on a ceramic coffee cup, then you probably have an iron or steel tub. Fiberglass tubs are warmer to the touch and generally have a plastic-looking finish.
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Touch a magnet to the tub if you are still not sure whether you have a fiberglass tub or some kind of metal tub. A magnet will stick to steel or cast iron tubs, but not to fiberglass.
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Knock on the tub a few times. If you have a cast iron tub, the sound will dampen quickly and will be a lower pitch. These are very dense tubs, so knocking on cast iron should result in a quiet sound. If you knock on a steel tub the sound will com back much louder and higher pitched since the steel underneath is thinner than cast iron.
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Lift the tub. If the tub is not connected to the plumbing, try picking it up. If you can easily lift it by yourself and possibly with one arm, then you have a steel tub. If you have a cast iron tub, it will require two to four people to lift and possibly more support underneath the floor where it will rest.