Glass is not resistant to heat or temperature shocks (with the exception of borosilicate glasses like Pyrex etc.). You should never let candles burn all the way down. When glass gets overheated, it cracks or develops an inner stress. Thick glass in particular should only be washed in lukewarm water and by hand. If the water is too hot the glass may crack or stress may develop. Direct sunlight over an extended period of time can also produce stress in the glass. This can build up to a point where the glass breaks seemingly of its own accord. Drinking glasses and glass plates and bowls should only be machine-washed on a gentle programme or one meant for glass vessels, with water temperature not exceeding 55° C (actually 45° C is hot enough). The water temperature should in any case be checked by a specialist, and adjusted if necessary – especially in the case of older machines. Use a modern good-quality brand detergent, and follow the quantity recommendations of the manufacturer. Check the state of the rinse gloss and regeneration salt at all times (after filling the salt compartment, you should run the pre-wash programme). Glass should not be washed along with pots and pans. Some metals will oxidise or may be attacked by the detergent agents – this results in an base, which dulls the glass surface – often in this case just one wash is enough. Inappropriate positioning in the dish or glass basket is one of the most common of glass breakages. Never have glass products touching one another – this is indispensable to avoid scratching. Glass products are tested for stress after production. Basically, no marketed product leaves the facotry containing inner stress. So a crack in the glass is generally the result of inappropriate handling.
As an exceptionally sensitive detergent for glass cleaning, we recommend SOMAT (www.somat.ch).