Soda Lime Glass
The most common type of glass is Soda-Lime(-Silica) glass, it accounts for about 90% of glass production and is made of mainly silica sand, soda ash and limestone, though the composition may vary to meet specific requirements.
Soda lime glasses are sensitive to changes in temperature, and, because of their large coefficient of thermal expansion (9,0 x 10⁻⁶/K), they may develop high thermal stresses that can induce cracking, which is why they undergo different processes to mitigate those issues.
Just to quickly clarify, thermal expansion is the process in which the volume of a material increases as its temperature increases.
At low temperatures the atoms of a material move at a slower pace, taking up less space as opposed to when moving quicker and consequently taking up more space at higher temperatures.
This applied to all materials, however not to all of them to the same extent, which is why different materials have different thermal expansion coefficients.
Borocosilicate Glass
It has a high resistance to chemical corrosion and temperature changes and is used for applications where thermal, mechanical and chemical conditions are too harsh for standard soda-lime glass.
Its owes its high thermal resistance to its very low thermal expansion coefficient (3,3 x 10⁻⁶/K), which is about a third of soda-lime glass, however that doesn’t mean that it can’t crack under any circumstances. It is used in flasks, bake and laboratory ware among others.
To learn more about the make up of different glass types click here https://www.cmog.org/article/types-glass
GLASS PRODUCTION
There are two methods of glass production, the float glass process which produces sheet glass and more importantly for our purposes, glassblowing which produces bottles and other containers.
So since the vast majority of glassware is made from the same type of glass, soda lime, why can’t we just pour our candles into whatever glass we find?
Well because due to it’s relatively high thermal expansion coefficient glass is under a lot of internal stress, so it can’t deal with additional stresses, like a candle flame for example, very well and would likely crack pretty easily.
So to mitigate the risk of fracture, different processes of glass strenghtening have been developed to make glass as a material less susceptible to external stresses by reducing its internal stresses.
Internal stresses can be made visible with a Polariscope.
GLASS STRENGHTENING METHODS
1) Annealed Glass
The molten glass is allowed to cool slowly in a controlled way until it reaches room temperature, relieving any internal stresses in the glass. Without this controlled slow cooling, glass would crack with relatively little change in temperature or slight mechanical shock.
Used in for example: tabletops, cabinet doors, picture frames
2) Heat Strengthened Glass
is semi tempered or semi toughened glass. The heat strengthening process involves heating annealed glass back up to about 650 to 700 degrees Celsius and then cooling it quickly, although not as fast as with toughened glass. The heat strengthening process increases the mechanical and thermal strength of annealed glass, making it twice as tough as annealed glass.
When it breaks the fragments are similar in size to annealed glass, but with a greater likelihood of staying together.
Heat strengthened glass provides higher resistance to thermal stress than annealed glass but is not as strong as fully tempered glass. As a result, it is often used in applications where a higher level of thermal resistance is required, but the full strength of tempered glass is not necessary.
3) Chemically Strengthened Glass
The chemically strengthening process involves first bringing annealed glass into the desired size and shape, then submerging the glass in a solution that promotes the ion exchange where the smaller ions are replaced by larger ions. This exchange causes a compressive layer on the glass surfaces and a tension layer in the core. The strengthening process increases the mechanical and thermal strength of the glass by a factor of 3.
4**) Tempered or Toughened Glass**
Annealed glass is heated to about 700 degrees Celsius by conduction, convection and radiation. The cooling process is accelerated by a uniform and simultaneous blast of air on both surfaces.
The inner layer cools slower than the outer layer, which causes the tension that makes tempered glass four to five times stronger than annealed or untreated glass.
The counteracting stresses or surface compression gives toughened glass its increased mechanical resistance to breakage, and when it does break, causes it to produce small, regular, typically square fragments rather than long, dangerous shards that are far more likely to lead to injuries.
Most of the glass that you see in commercial and residential spaces has been tempered, skylights, office walls, railings, appliances
5**) Laminated Glass**
For the sake of completeness, the 5th type of glass is laminated glass, which is more so relevant for sheet glass applications. should the glass shatter it is held together by specialty plastic, any one of the above types of glass can be laminated.
So to conclude, the characteristics of different types of glasses depend mostly on their respective heat treatment, additionally different chemicals/ingredients may be used to achieve the desired material outcome.