What 3 Elements Are in Pyrex Glass?
Pyrex brand cookware has been around since 1915. It was originally owned and manufactured by Corning Glass, but they sold it in 1998 to World Kitchen. Corning Glass used mainly borosilicate glass for its Pyrex products, but World Kitchen uses tempered soda lime glass. The laboratory glassware is still made with borosilicate glass. The European manufacturer of Pyrex glassware, Arc International, also uses borosilicate glass for the glassware it produces.
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Function
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Borosilicate glass was used originally by Corning Glass because of its high resistance to thermal shock. The new tempered soda lime glass does not have the same resistance. It is normal window glass that has been tempered to give it heat resistance. Tempered glass, though tougher than normal glass, can still be scratched. This compromises the integrity of the temper and, if moisture gets in, the scratches can cause problems. Borosilicate glass can be scratched as well, but it does not break in the same manner as tempered glass: instead of shattering into many small fragments, it snaps in large pieces.
Ingredients
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Borosilicate glass is composed of 10-12 percent boric oxide and 70-80 percent silica, with small amounts of other oxides such as sodium oxide, potassium oxide and calcium oxide. This combination melts at a higher temperature than normal glass. The main ingredients of soda lime glass made for containers are about 74 percent silicon dioxide, 13 percent sodium oxide and 10 percent calcium oxide or lime.
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Features
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Borosilicate Pyrex is better able to withstand changes in temperature because it has a very low thermal expansion coefficient, though it will still crack if exposed to rapid or uneven temperature changes. The maximum safe operating temperature for borosilicate Pyrex laboratory glassware is 515 degrees C or 959 degrees F. It is also inert to almost all substances except hydrofluoric acid, heated phosphoric acid and hot alkalies. Tempered soda lime glass has a maximum safe temperature of 495 degrees C (923F). This is only if there is nothing in the environment that will stress or damage the glass, such as abrupt temperature changes.
Misconceptions
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There are stories circulating that may contain some misconceptions about Pyrex glassware. It has been reported by some consumers that they experienced a Pyrex dish exploding while hot. This can happen with any glass bakeware if not handled with care. There are also concerns that the reason this happens is because the new manufacturer uses tempered soda lime glass instead of borosilicate glass. According to the new Pyrex producer, World Kitchen, the plant that produces U.S. Pyrex has been using soda lime glass since approximately the 1940s.
Warning
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All glass, regardless of composition and heat treatment, can break and should be handled with care. Repeated heating and cooling also causes the glass to deteriorate over time. Scratches can interfere with the integrity of the dish, which is exacerbated by heating and cooling as well. Dishes should be handled with dry pot holders, and they should not be introduced to drastic changes in temperature. Let them reach room temperature before moving them from a freezer to a hot oven or vice versa. Place them on soft place mats rather than hard surfaces to avoid scratching or damaging the glass. Do not place them on direct heat such as stove tops, or use them with toaster ovens or broilers. Make sure to read the manufacturer's instructions so that the temperature specifications can be followed.
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