Glass shelves allow you to view your display items from all angles, but overloading them can result in damage to both the shelves and the items. Glass shelving's maximum weight load depends several factors, including the type of glass, its thickness and the distance between the shelves' supporting brackets.
The amount of weight your shelf can safely hold depends on the type of glass. Typically, manufacturers use only toughened glass for shelves, casings and other architectural purposes, since decorative glass, such as cathedral or obscure glass, aren't strong enough to support much weight. Float glass, also called annealed glass, is a more durable glass type; it undergoes a heating and slow cooling process to strengthen the surface by removing interior tensions. Two to three times stronger than annealed glass, heat-treated glass undergoes a rapid cooling process that partially tempers the material. Tempered glass, like heat-treated glass, is rapidly cooled after heating; it's four times stronger than annealed glass. Base your expectations of strength on your shelf's glass type.
Generally, thicker glass is better able to hold heavier weights without shattering. Annealed glass shelves ¼-inch thick won't hold anything heavier than about 70 pounds, even with a lot of strong supports. Tempered glass can hold significantly more weight, but usually glass shelves this thin are completely safe only for lightweight bric-a-brac and other small decorative items. Most commercial glass shelves are between 3/8-inch to 5/8-inch thick, which allows for moderately heavy items, such as a couple of books, while keeping support distance reasonable.
While thicker pieces of glass can safely support large amounts of weight, thick glass with short distances between support brackets can support even more. For example, a 1-inch-thick length of tempered glass holds almost 200 pounds if its supports are 5 feet apart; the same glass shelf can hold more than 5,000 pounds if supports are 1 foot apart.
Even though thicker, heat-treated glass can better support heavy weights, breakage is still a risk if you improperly support the shelves' bracketing. Before you install your shelves, weigh them. Include the weight of the shelves along with the weight of the load they'll carry when you purchase your installation hardware. Once you've properly anchored or otherwise supported your brackets and installed the shelving, avoid loading the glass shelves with their maximum load. The stress on the glass over extended periods will increase the risk of breakage.