|9ct gold is harder
than 18ct. This is a classical myth!!
Actually 9ct could be harder but normally it is not. The hardness
depends on many factors including both the mixture of alloys and the way
the product is treated during manufacture.
In general most 18ct carat
alloys are both harder and more durable than their 9ct counterparts. Typical
figures for white alloys are 150 (hv) and 130 for 9ct, but the variables
are so wide between differing alloys and material preparation, they are
9ct is also usually more brittle than 18ct. If the gold
is worked by rolling, hammering, bending and stretching it becomes 'work
hardened'. This usually leaves the metal in a more brittle state and less
suitable for wearing. By a process of heating called annealing which removes
internal stress, crystal defects, and dislocations, the metal regains
its former malleability. This may be carried out many times during the
manufacturing of a hand made piece, but as most jewellery is cast today,
this happens only once.
As well as affecting physical properties, alloying gold generally increases
the strength and hardness, with some reduction in malleability / ductility.
The silver atom is slightly larger than that of gold, so alloying gold
with silver gives a moderate improvement in strength and hardness. The
copper atom is significantly smaller than that of gold and so it has a
greater effect on strengthening gold than silver, as it distorts the gold
crystal lattice more. Thus reducing caratage from 24 carats through 22
ct down to 18 carat gold results in stronger and harder alloys, but alloying
beyond 18 ct down to 9 carat does not have much further effect.