KNOW-HOW

Tubs are manufactured from many different materials. The most common are: 

Acrylic

It can be molded into just about any shape and size. The color runs through the entire substance rather that just in the surface coat, making it less likely to show scratches or to fade. 

Enameled steel

Enameled steel tubes are pressed from sheets of steel and coated with a baked-on enamel. The enamel layer is usually thin, though susceptible to chipping. In addition, enameled steel doesn't retain heat. 

Cast iron

Is the most durable material available for tubs. Iron is cast into a tub shape and then coated with a baked-on enamel. The enamel layer is thick, strong, durable is scratches, resistant to chips and stains and has a rich, colored finish. Cast iron is just about indestructible but it's also heavy. A standard tub weights between 300-400 pounds. In some cases the floor must be reinforced to support the additional weight. 

Cast iron is used most commonly for claw-foot and other stand alone bathtubs. The enamel finish of old cast-iron tubes can be reglazed but it is not a do-it-yourself process and the new finish will be not as durable as the original. 

Fiberglass

It is an inexpensive and lightweight material that can be finished in a variety of colors. It is easily molded, so fiberglass tubs can have seats, soap dishes, shampoo shelves, molded into the sides. The major disadvantage is that the surface can be scratched  easily and that the color will gradually fade.