A possible reason that some elements appear brighter than others is that the elements in your kiln are "tuned." This means that some elements are designed to produce more heat than others.
The lid and bottom of a top-loading kiln require more heat than the rest of the kiln. This is because the lid and bottom are massive areas that absorb energy. To give the lid and bottom extra heat, the top and bottom elements of some kilns are given extra power. The center elements may not even turn red until 1000°F.
Before changing heating elements, you must know if your kiln uses more than one type of element.
If a top/bottom element is reversed with a center element, the kiln will fire unevenly. So when you receive a new set of elements, check the element labels. Install elements marked TOP/BOTTOM and CENTER in the correct locations.
Your kiln’s wiring diagram and the element price list will show if your kiln uses top/bottom and center elements. (Some kilns use only one type of element throughout the firing chamber.)
When ordering elements, give the correct kiln model number. This is shown on the kiln’s electrical data plate—the small black plate riveted to the side of the switch box.