Except for rust, which is easily remedied, and possibly cracking from being heated too quickly over high heat, cast iron skillets are virtually indestructible. They are often passed down from one generation to the next as a favorite piece of cookware that can go from kitchen to campfire and back again with ease. So it isn't the skillet that needs protection but the smooth glass cooktop.
A Weighty Matter
Smooth cooktops are sturdy, tempered glass, but common sense should be the rule. The hefty weight of cast iron cookware, both heirloom and newer enameled brands, presents a concern. No cooktop warranty will cover the damage that may result from even the accidental dropping of a heavy skillet or carelessly laying it down too heavily. While scratches from sliding a heavy skillet across the glass may not affect the efficiency, they will be unattractive.
Manufacturers of glass cooktop ranges, such as Maytag, recommend smooth, flat-bottom cookware with no ridges, which is why enamelware may be the better choice. A couple of tests will help determine if the cookware is smooth enough. First, try the ruler test: Place the edge of a ruler across the bottom of the pan; if there is no space between the ruler edge and the bottom of the pan, it passes. If still in doubt, the bubble test will help: With the burner temperature on high, heat an inch of water in the pan and watch the bubbles forming as the water approaches boiling. If the bubbles are uniform across the pan, it is fine. Uneven bubbling means hot spots from poor contact between the pan and cooktop. The Garden Web offers another way of determining if your great-grandmother's skillet is O.K. to use: Run your hand across the bottom; if it feels smooth it will probably be fine. If you feel a few rough spots, they can probably be filed smooth, which won't hurt the skillet.
Metal Heat Diffuser
A heat diffuser, available at many kitchen supply stores, may be the best solution. Sometimes called a "flame tamer," a heat diffuser is most commonly used for glass cookware on open flame gas ranges. Because the diffuser distributes the heat evenly across the bottom of the pan, you should be able to remain faithful to your cast iron skillet even if it did not pass the bubble test for even heating.