Some collectors have balls professionally cleaned by tumbling, which in essence abrades the glass to the point where the sickness is worn off. The SLR Co, Onion, JFG, Burgoon, and Ear of Corn pattern lightning rod balls were not designed for or use protective caps. Nor did the ceramic balls. However, the other patterns did use metal caps to protect the glass. These caps are found with some regularity, though they are not usually found in good condition.
Good examples have value and carry a premium.
Several lightning protection companies used embossed caps. That is, the company name was in raised letters on the cap. Electra King Ventilating Co. - on Flat Quilt balls. Kretzer - on Raised Quilt and sometimes Round balls. Thompson - on Flat Quilt and Thompson balls.