People have always been interested in gold.
Mainly it attracts attention being bulk bars,
coins, jewelry.
Besides, gold is money and money is power.
But as we know all that glitters is not gold,
so beautiful and expensive at first glance
jewelry may appear just a fake.
The well-known legend about Archimedes tells
that Syracuse King suspected his jeweler of
cheating him in making golden crown by substituting
gold in it with silver.
The king gave Archimedes an order to reveal
the fraud and to prove that the crown is not
full gold.
Archimedes made a lot of efforts in order
to solve this problem until the solution came
to him accidentally when he was taking a bath.
Water started overflowing when he was diving
into the bath and realized the volume of displaced
water is equal to the volume of his submerged
body.
At the moment of sudden insight he rapidly
ran home naked exclaiming eureka which means
I have found it in order to check theory.
Then he made the demonstration for the king
by diving the crown and a golden bar of the
same weight into water.
As a result the crown submerged bigger volume
of water than golden bar did.
That was direct evidence of gold substitution
by silver.
The latter has smaller weight but bigger volume.
Almost everyone wears golden jewelry but only
a few of us are aware of its total composition
which can be varied by jewelers.
Because of that some of gold jewelry can be
unsafe for owner’s health resulting in allergic
skin reactions.
Let’s consider it more precisely.
Gold by itself is inert, very soft metal which
barely fits for making jewelry which is why
jewelers started to melt it with copper, silver
and even platinum.
Because of application in minting coins, gold
copper alloys for many years were produced
much more often than other gold containing
alloys.
After a while people started adding other
metals such as nickel, platinum, zinc to gold
for obtaining different shades.
In fine, white, red, pink, green, blue and
even black shades of gold jewelry can be found
at markets and in stores.
The main condition for making golden jewelry
is easy processing in order to produce complex forms.
And along with this, crafted items should
be quite hard, steady and corrosion stable.
Hardness of products is increasing as gold
content is decreasing.
However such reduction makes golden alloys
lose their environmental resistance which
leads to the formation of oxides as well as
hydrogenation.
As a result, jewelry may fade because of air
influence and interaction with human sweet.
After all it may bring troubles and even chemical
skin burns to jewelry owner.
How can all these be prevented and how many
karats should gold jewelry contain?
Fineness of gold can be expressed in two common
ways: millesimal fineness and karats.
Millesimal fineness denotes the purity of
gold alloys as the number of weight parts
of the base noble metal per 1000 weight parts
of the alloy.
For instance, a fineness of 750 translates
to 75% gold, or 18k.
Karats measure the parts of gold alloy per
24, so that 100% pure gold is 24 karat, when
18 karat = 18⁄24 = 75% of pure gold in an
alloy.
Rest of conversions between the percentage
of gold and karats is displayed on the slide.
In ancient times the main method to verify
if gold is real was bending gold with teeth.
In our times a scientific approach is used
for this.
Let us pretend to be big scientists and use
phase diagrams of main gold based alloys.
There are two main components used in gold
jewelry, copper and silver.
Let’s refer to gold copper diagram and consider
stability of the most widespread alloy from
a scientific point of view.
Gold and copper form continuous series of solid solutions.
The highlighted lines are the Liquidus Lines above which only liquid is present and the
Solidus Lines below which only solid is present.
The diagram has non variant equilibrium points.
In this case it is gold and copper.
So the diagram displays the points for amounts of gold about 58% and 96%.
The alloy formed at these points has maximal stability and doesn't decompose with time.
Thus, according to phase diagram we can say that the most equilibrium and stable are gold
copper jewelry of 14 carat and 23 karat.
This is really important for people who plan to wear golden jewelry constantly or often
or even use it as a dental crown etc.
Other gold copper compositions can start decaying with time due to its interaction with human
skin and sweat leading to allergic reactions and even to chemical skin burns.
After copper, silver is the second main component of golden jewelry according to its percentage
in an alloy.
Silver makes an alloy more plastic and ductile.
Also it changes a shade of alloy making it
whiter.
By reaching silver phase over 30% in an alloy, shade of yellow alloy becomes lighter and
by its further increase up to 65% gold becomes white.
The phase diagram for gold silver alloy is
presented on the slide.
It is a Cigar type diagram.
Thus, the alloy has a complete solubility.
Gold silver alloys are soft, ductile and homogeneous.
These alloys are equilibrium at the whole
range of the diagram.
The main conclusion.
If you are going to wear copper based gold jewelry often or constantly, please, think
of getting only 14 and 23 karats.
In case of silver based gold jewelry no allergic reaction is expected, since silver is inert
metal just like gold.
If you missed our previous historical review devoted to gold, please, follow the link.
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