While the kettlebell's origins stretch all the way back to ancient Greece, they were more recently popularized in the 1800s by Russian athletes, who recognized their potential as tools for muscular development. The design, resembling a cannonball with a handle, has not changed over time. While cast-iron and vinyl kettlebells serve similar functions, their differences come into play if you're thinking of purchasing or using one.
Cast-iron kettlebells are made of just that -- cast iron. Vinyl kettlebells usually look similar in shape and design, with a plastic coating layered over the metal. Unfortunately, the vinyl does not protect your floor from accidents. Sarah Lurie, in her book "Kettlebells for Dummies," warns that the vinyl coating also prevents you from being able to see if the kettlebell is filled with a material other than iron, such as sand. Although these materials are meant to weigh the same as the conventional kettlebell's iron, you may not be able to verify that unless you put it on a scale yourself to check the weight.
Both types of kettlebells can be used at home or in the gym. The vinyl coating is thin and will do little to change the standard size or design of the kettlebell, so the differences in performance between cast iron and vinyl are minimal. However, the vinyl coating has a tendency to crack and peel, unlike the very durable cast iron kettlebell.
The vinyl coating's primary purpose is to give the kettlebell a pleasing and bright appearance, making it an attractive option for those who might be new to weight lifting or who enjoy expressing themselves in every aspect. The cast iron kettlebell has a traditional appearance; in Russia, they have come to symbolize strength and, by extension, national pride.
Because a well-made vinyl kettlebell and a traditional cast iron kettlebell are virtually identical, inappropriate use of either can lead to serious injury. Make sure your kettlebell is one solid piece, instead of having a separately attached handle; kettlebell workouts often call for a hard swinging motion, and you want to minimize the risk of the weight flying off the handle. An alternative to both is a rubber kettlebell, which possesses a thick rubber shell that provides more protection for the floor in case of accidents.