Re: Does the age of a drinking glass effect its brittleness?

Hi Rebecca,

That's a really good question, and you've touched on a major focus of 
Materials Science and Engineering: Material Properties.

The short answer is that both you and your mom are right, but for 
different reasons.  Glass should not change with age, but older glasses 
will tend to be more fragile than new ones.

A drinking glass, if you left it "alone" (I'll get this later) your entire 
life, would not become any more brittle than the last day you used it.  
Over a long enough period of time, (I'm talking hundreds to thousands of 
years) the glass may flow and become thinner at the top and thicker at the 
bottom, which may make it more "fragile", but not more "brittle".  Brittle 
describes how a material resists bending and how it will crack rather than 
tear.  

In reality, the place where you and your mom store your glasses is not 
leaving it "alone".  The temperature cycles hot and cold between night and 
day, they vibrate anytime someone walks by, moisture in the air will vary 
according to the season, all sorts of things will be affecting it and 
possibly making it more fragile.  (Not the same as brittle.)  For example, 
when it's a warm, sunny, Florida day, and the glass expands from the size 
it was at night, it undergoes just a little bit of stress and strain.  Do 
this enough times, and the glass gets weaker.

However, the biggest source for the glass to weaken would be handling.  
Each time you touch it, tap it, or handle it, you probably break some of 
the bonds between the molecules.  Break enough of them, and your glass 
will break too.

The bottom line is that old glasses will normally be more fragile than new 
glasses, (Material Property) but often used glasses will be break more 
easily than rarely used glasses. (Wear and Fatigue)  So your mom's old 
glasses will be more fragile if you started using them, but not exactly 
for the reasons why she said so.

I hope this helps!
Jeff Yap
Mad Scientist

Other cool info:

You see the glass flow effect on window panes in colonial era houses.  
This is because most glass is an amorphous solid, which means that it 
doesn't have a specific melting temperature, it will just get softer as it 
gets warmer.  Some scientists describe glass as a "Supercooled 
liquid" but that's still under debate...

 
SweeTARTs contain Silicon Oxide, also known as Silica, also known as 
Sand, which when you melt it, makes glass.  The candy doesn't contain 
enough Silica to have any effect on you, but it's sometimes fun to read 
the ingredients...

Most Brittle materials fail by cracking.  The way a crack works is that 
when you break a chemical bond between two adjacent molecules of the 
material, it releases energy.  If the amount of energy released is less 
than the energy it takes to break another bond, then nothing happens.  If 
the amount of energy released is more than the required amount, then a 
crack will form in the material, because it will break the next bond, 
which breaks the next one, and so on.  This is called "Crack 
Propagation".  If the released energy is MUCH more than the required 
amount for the next bond, then it may break two or three more bonds, which 
may break a bunch more.  Cracks go in all directions, and the object 
shatters.  This is known as "Catastrophic Failure".

Most plastics have a glassy state and a plastic state and what is known 
as a Glassy Transition Temperature.  Above this temperature, the material 
bends when you deform it.  Below this temperature, it will crack.  
The "new car smell" is actually a chemical known as a plasticizer, which 
lowers the glassy transition temperature, making the upholstery and other 
plastic parts soft and flexible.  That's why old cars often have cracks in 
the dashboard or in the seats.  The plasticizer evaporates out of the 
plastic, and it becomes more brittle.

References:
Varshneya, "Fundamentals of Inorganic Glasses", Academic Press, Inc, 1994

Corning Museum of Glass
Encyclopaedia Brittanica