Tempered glass, like these jars, is usually rather thick-walled and owes its stability to the tension created between the layers of glass - the outer and inner shell of the jar - during the manufacturing process. If it were to shatter, it would only break into thousands of small, almost blunt fragments that pose little risk of injury.

Duralex in central France has been producing utility glass from tempered glass for decades. The process of making glass less impact-sensitive and at the same time heat-resistant by alternating between heating - to around 600 °C - and rapid cooling in the annealing lehr has now also been mastered by others. However, hardly any glass has achieved such worldwide fame as the bistro glass from the "Picardie" series, which has long been an integral part of French pubs.

Tempered glass is considered impact-resistant and shatterproof. Nevertheless, you should handle these drinking glasses with the same care as any other "normal" glass, as tempered glass is under tension so that the vessels are also suitable for hot drinks. Microscopic cracks in the material - caused by knocks or blows, for example during the cleaning process in the dishwasher - or material fatigue due to ageing can cause the stress ratios to become unbalanced and the glass to break.