Capped steels have  characteristics similar to those of rimmed steels, but to a degree  intermediate between those of rimmed and semi-killed steels. A deoxidizer may  be added to affect a controlled rimming action when the ingot is cast. The  gas entrapped during solidification is in excess of that needed to counteract  normal shrinkage, resulting in a tendency for the steel to rise in the mold.  The capping operation limits the time of gas evolution and prevents the  formation of an excessive number of gas voids within the ingot. Capped steel  is generally cast in bottle-top molds using a heavy metal cap.