Ask A Scientist Materials Science Archive

name         Jennah
status       student
grade        K-3
location     CA

Question -   My name is Jennah, I am in 1st grade at Las Juntas
school. My question is What is an eraser made of and why does it only
fall apart when you rub it on something?
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Jennah,

Pencil erasers are made up of a material that is very similar to natural 
rubber, like materials found in running shoes, padding or even car tires. 
What makes erasers work, and what makes them fall apart when you rub it 
against something is that the eraser is made in such a way so that tiny 
globes or spheres of rubber are bunched together to make the eraser. 
Because these spheres are held together loosely, they can break away from 
the main body of the eraser really easily. So the reason erasers work is 
that the lead is attracted to the rubber in the eraser, and then, to make 
sure that your eraser does not become full of lead, bits of it come off 
(because they have been designed to be these little spheres that are stuck 
together) when you rub it.

You can contrast the normal pencil eraser to the clay or moldable erasers 
that you can find in arts supply stores. These erasers are not designed to 
break apart and so after a while the whole eraser becomes full of the 
pencil graphite (what we call lead). The way you get a new surface for 
erasing in these type of erasers is to stretch and fold the rubber so that 
a fresh surface becomes revealed and ready for use.

Greg (Roberto Gregorius)
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