Basically, glass that keeps you safe from flying shards should you be nearby when it breaks. There are two types of safety glass, laminated and tempered. The laminated version has an interesting history that dates back to 1903 when French chemist Edouard Benedictus clumsily dropped a glass flask on the floor. The flask shattered but the fragments did not fly apart
When Benedictus examined the flask, he realized that a film had formed on the inside of the vessel. The vessel had contained an alcohol solution of collodion, a plastic made by treating cotton with a mixture of sulfuric and nitric acids. When the solvent evaporated, a film of plastic was left on the inside of the glass. An interesting observation but Benedictus thought no more of it until he read a story about a young girl being cut by glass in one of the first automobile accidents. He then spent the night trying to make a coating on glass and within a day had produced the first sheet of “safety glass” which he named “triplex” since it consisted of a sandwich of two sheets of glass with a film of cellulose nitrate in-between. Even if the glass broke, the pieces were held in place by the plastic layer. In 1909, Benedictus applied for a patent and triplex went into production. The first practical use turned out to be in the face shields of WW I gas masks, but by the 1920s triplex became a standard item in American automobiles.
One problem was that the cellulose nitrate yellowed with age. In 1933, triplex was replaced by cellulose acetate, which was not quite as strong but did not yellow. The synthetic resin, poly(vinylbutyral) eventually was found to be superior and has now been the standard in windshields since 1939. And it all began when a chemist couldn’t hold on to a flask.
Tempered glass has a somewhat less illustrious history. No accident involved this time. Tempered glass was the outcome of an experiment aimed at improving the properties of glass sometime in the mid 19th century by Francois Barthelemy Alfred Royer de la Bastie. How’s that for a name? De la Bastie heated glass until it melted and quickly quenched it. The heating and cooling made the glass strong and shatter-resistant. Essentially the same process is used today, although an alternate process exists in which glass is immersed in a bath of molten potassium nitrate. This leads to an exchange of sodium ions in the glass for potassium ions resulting in a really tough glass.
The windshields of automobiles are generally made of laminated glass and side windows of tempered glass. Hockey rinks are surrounded by tempered glass. If it does break, which is rare, it shatters into a zillion tiny pieces. Players may get a glass shower, but they will be “safe” from injury.