Cast Iron Tubs
It is very difficult to distinguish between steel & cast Iron tubs. While both are metallic (i.e. magnets will stick to them) and both can have porcelain-like finishes, a good rule of thumb is that bathtubs installed prior to the mid 1960’s are usually cast iron. Here are some examples.
The "Tapering Rail" - another classic shown to the right; the tub rail starts out wide (usually at the facet end of the tub) and gets thinner as it goes towards the opposite end.
The “Colorful Fifties” - from blue, to pink to green, cast iron bathtubs in 1950-60’s era joined the retro décor. Most non-white tubs, like the to the right, tend to be cast iron.
Raised Embossing on the front apron of the tub shown in Figure 1 is a distinguishing characteristic that the tub is steel.
Next, the “muffin top” feature seen in Figure 2 is another style of steel tub. Muffin tops can come with either a bowed (Figure 3), or a straight (Figure 4) apron, but the walls are always parallel until they curve at the ends of the tub.
A third, but not always a defining characteristic of steel tubs, is that their inside and outside walls are generally parallel; that is, until they reach the ends of the tub. This is shown by the red lines in Figures 3 & 4 birds-eye sketches.
And due to their lighter weight, the bathtubs found in mobile and manufactured homes are generally constructed of either steel or fiberglass.