Metallic Property Changes Upon Heating and Cooling
Date: September 2007
After heating a pure copper penny in a Bunsen burner flame
until it glows orange and then cooling the penny off, its appearance
does not change (except that it looks newer). However, when the
penny is dropped from high up or thrown down on a lab table, it does
not bounce. Comparing it to a normal copper penny shows a dramatic
difference. Why is that?
There are two reasons I can think of that could give this result: (1)
the macro crystal structures of the penny has annealed and made a harder
denser structure, or (2) the filler material in the (modern) penny has
deteriorated and is now more mobile.
In the first case, the crystal macrostructures of metals can be changed
by a process called annealing. It is not so much that the crystal structure -
the way the atoms are put together that changes, but rather, the way the
individual crystal comes together to form larger, macrostructures. The whole
coin is not one complete and uniform crystal of the metal alloys. Rather it
is made up of small crystals that organize themselves into bigger structures.
As such, between these crystal structures are grain boundaries, errors in the
way the crystals came together, or crystal structures that may slip across
the face of other crystals. Heating the coin may cure some of these
imperfections and make bigger, more uniformly joined crystals. This also
means that the coin becomes harder and possibly less elastic.
On the other hand, the heat can change the metal structures in the alloys
and make them more able to disperse the energy as heat - making the collision
less elastic. This can happen in modern coins that have cheaper alloys that do
not retain their crystal structures when heated. Heating can then cause higher
mobility of the atoms or crystals which means that the kinetic energy absorbed
during collision is dispersed through the coin as heat (movement of
particles). This energy is lost and not returned as an elastic collision.
Try to see if there is a difference between how much energy it takes to
bend the penny. If the penny is harder to bend, then annealing has taken
place and the more uniform crystal macrostructures are the reason for the
poorer bounce. If the penny is easier to bend, then the weaker bounce is a
result of better energy dispersion through the coin.
Greg (Roberto Gregorius)
The color is merely a surface condition. The inside of the flame has more
H and C from fuel
than O from air, so it is a reducing environment and will change copper
oxide back to copper.
The surface color goes from dark-tarnished to bright& copper-colored.
Bouncing depends on the mechanical properties of the bulk of the metal,
such as elasticity and stiffness and strength.
When greatly heated the crystallization state of the metal will change.
Probably the metal of you penny was cold-worked to a "hardened" state
when it was coin-stamped, long before you got it.
Cold-working means when a metal is forcibly deformed,
the flow trips up internally and makes defects,
which then tend to act as sticking points that reduce further flow.
So thereafter the metal flexes and recovers in a spring-like fashion,
and is harder to permanently bend or dent.
Then when you heated it the penny the metal was annealed.
All those defects, that made the metal act hard and ringing and bouncing,
were healed as metal atoms moved around, freed by the high temperature.
Now the metal is softer, easier to bend and possibly duller in ringing
So when a force is applied by impact on the table,
the deformation is permanent bending instead of temporary lossless flexing.
The penny will not bounce as high because energy was absorbed by that bending.
Recent pennies are zinc alloy with copper cladding.
If you heated the penny hot enough the zinc and copper may have mixed a
zinc+ copper = brass, which tends to be a little lighter-colored than
Maybe this was partly responsible for the brightening you saw.
And the zinc alloy is probably the bulk metal which was annealed.
Mixing of copper into the zinc could have had some effect on mechanical
Heating "red hot" must have nearly melted the zinc inside...
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Update: June 2012