Fully tempered glass used in architectural applications may break spontaneously for a variety of reasons, including, but not limited to, unperceivable imperfections or inclusions in the glass substrate. Most inclusions are stable, but some inclusions, most notably nickel sulfide, may cause a spontaneous break if it is located in the tension region. The purpose of a heat soak test is to reduce the risk of a spontaneous break by influencing the inclusion to break the glass during the test. This document was first published in 2018.