We came across this post on Twitter comparing 2 similar glass measuring cups, the main difference being the capitalisation of the brand name.
The post claims that the measuring cups are made of different glass, and that “pyrex” cookware may crack, shatter, or explode when heated and/or cooled while “PYREX” cookware does not.
Pyrex cookware
Pyrex is a brand of cookware, popular in the United States (US), with glass cooking utensils that are advertised as being heat-resistant. Its glass ovenware is allegedly safe for use in the oven or microwave.
The brand also claims that its ovenware can be heated in the microwave or a preheated oven, straight from the refrigerator or freezer.
Thermal Shock
Thermal shock, in relation to glass, occurs when a rapid change in temperature causes different parts of the glass to expand by different amounts, creating tension in the material and leading to cracks and breakage.
For example, if a hot glass is submerged in very cold water, the glass is more likely to crack or shatter due to the sudden change in temperature. However, the damage to the glass may vary according to its composition.
Common types of glass
There are 3 types of glassware commonly found in one’s kitchen (i.e. soda-lime glass, tempered glass, and borosilicate).
Soda-lime glass has low resistance to thermal shock and breaks easily when dropped. Whereas tempered glassware is glass that has been heat-treated to increase its resistance to thermal shock, and breaking when dropped.
On the other hand, borosilicate glassware is made with boric oxide. Compared to tempered glass, borosilicate is more resistant to thermal shock but is more likely to break when dropped, although to a lesser extent than soda-lime glass.
PYREX vs pyrex
Besides the Twitter post, there have been others who have claimed that “PYREX” and “pyrex” are made of different types of glass, albeit being from the same brand.
In particular, it is alleged that Pyrex’s European manufacturers still use borosilicate glass (“PYREX”) whereas Pyrex’s US manufacturers use soda-lime glass (“pyrex”).
Pyrex has confirmed that its glass bakeware was initially made of borosilicate glass in 1915 but subsequently, its glass bakeware sold in the US has been made of tempered soda-lime glass. In some European countries, the glassware sold under the Pyrex brand is made from borosilicate glass.
In addition, Pyrex stated that tempered soda-lime glass is comparably resistant to breakage caused by thermal shock and therefore isn’t inferior to borosilicate glass. It also highlighted that the manufacture of tempered soda-lime glass is more environmentally friendly compared to borosilicate glass such as releasing fewer harmful emissions.
Based on Pyrex’s / its parent company’s websites across various countries, Pyrex glassware manufactured in France contains the “PYREX” logo and is still made of borosilicate glass while Pyrex glassware manufactured in the US contains the “pyrex” logo and is made of tempered glass. However, Pyrex does not appear to have confirmed that the different logos correspond to different glass types.
Therefore, it is likely true that “PYREX” and “pyrex” cookware are made of different glass, the former from borosilicate glass and the latter from tempered glass.
Is PYREX shatter-proof?
There have been incidents in the US of Pyrex brand glassware shattering or exploding whilst used for cooking.
According to the US Consumer Product Safety Commission (CPSC), which works towards reducing the risk of injuries and deaths from consumer products, only a small percentage of glass cookware shatter unexpectedly.
In some instances, the breakage of the Pyrex brand glassware has been associated with failure to follow the manufacturer’s instructions such as not placing the hot glassware on a cool surface.
In relation to the Twitter post, it is imprecise to say that tempered soda-lime glass might crack, shatter, or explode when heated or cooled but not borosilicate glass. Generally, tempered glass is able to withstand temperatures up to about 243 degrees Celsius.
While borosilicate glass is more resistant to thermal shock, it is not completely immune to sudden temperature changes. Therefore, it is inaccurate that borosilicate glass will not crack, shatter, or explode when heated or cooled.
