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. The Technical Services Division of the National Glass Association (NGA) has produced this Glass Technical Paper solely for informational purposes. This Paper was developed by dedicated member volunteers and subject matter experts. The original version of this document was published in 2018. This version of the document was updated and published in January 2024.