Restoration of Bronze

Cleaning and Treatment of Ancient Metal Artifacts and Coins

Part - 1 - One

Introduction:
Why does ancient metal deteriorate and what is that green and black
stuff that covers artifacts? The metals from which most artifacts are
made are not naturally occurring in the ground. Instead they are
alloys, or combinations of several different refined metals melted
together and mixed up to create a new metal with certain useful
properties such as low melting temperature, resistance to corrosion, or
flexibility. For Example: Bronze is generally something like 85% copper
and 15% tin melted together and thus combined.
So, the metals from which artifacts have been made have been altered in
two ways. First by refining, or using heat to make pure metal from
metal ores, and second by alloying, or combining refined metals, again
with heat, to create new metals with improved useful properties.

Metals in nature, the way they are found in the ground, are generally
fairly stable. Malachite, the gemstone, is for instance, a stable form
of copper found in nature. It has naturally combined with things in the
environment to create a substance that looks almost nothing like the
metal copper, yet it is made of more than 70% copper and can be refined
to create metallic copper. When the metal copper, which is not stable,
is returned to the earth, it will unrefine itself, slowly, recombining
naturally with elements in the soil, and the result, within a few
hundred years will be a layer of malachite and other related minerals
on the surface of the metal. This is the type of deterioration known as
verdi-gris. (Another example of this is the black or gray tarnish that
you see on silver items. This is silver combining with sulfur in the
environment, and copper alloyed into the silver, combining with oxygen,
both returning to a stable natural condition, and at the same time,
becoming less attractive and useful.)

So, to clarify, metal ores are, through heat, refined and purified into
pure metals that must eventually, at normal temperatures, combine with
elements in their environment and return to their more stable natural
states. This process can take hundreds or even thousands of years, and
is what we know as patination, verdi-gris, corrosion, and the other
properties of aged metal.

The second important thing that happens as metals age is that, those
which are alloyed, or made of a combination of two or more metals, may
separate slowly into their components. An example of this is ancient
silver coins which become brittle. Silver used in coins is almost
always a combination of silver with about 1.5 to 15% copper. Adding a
little bit of copper to silver makes the normally soft silver harder,
and more resistant to wearing down. Silver and copper don't really mix
all that well, however, and over time (300-500 years or more), at
normal temperatures, the copper will sometimes begin to again separate
itself from the silver. The technical name for this is precipitation of
copper at the grain boundaries, which means copper coming out of the
alloy at the edges of the natural crystals of the metal. This is known
as crystallization of the metal, to coin collectors, all though it is
really just the crystals of the metal becoming visible as the copper
comes out of the alloy and begins to corrode, thus weakening the metal.
To clarify this point, some alloys are not stable, and, over hundreds
or thousands of years, they will begin to separate back into more
stable natural states.

Bronze
There are a limited number of major chemical changes that happen to
bronze as it deteriorates, depending on age, soil conditions and a few
other things that are collectively referred to as 'the conditions of
preservation.' The visible results of the changes in the bronze are
collectively referred to as the patination, patina, encrustation, or
verdigris. I will use the terms patina and patination and encrustation
interchangeably.
Pretty much all of the changes that occur in bronze over time are the
result of interactions of the copper in the alloy with the environment.
The tin is relatively inert and is stable in alloy with copper. That
is, it won't separate like silver and copper.

Following are enumerated the different types of copper patina you are likely to encounter:
What is this crusty stuff on my coin or artifact and can it be cleaned:
Let's start with an important note: Most patinas are desirable,
valuable, and attractive, and should not be removed. Some, however are
not. You will need to use your own judgement in this regard, paying
attention to esthetic, and the potential destructiveness of the
particular elements present. Bear in mind that most artifacts and coins
have multiple of the following reactions occurring simultaneously on
their surfaces, and that, cumulatively, they are the irreplaceable
signature of the ages.

Pretty: Copper Oxide, Cupric Oxide, Cuprous Oxide: (red, brown or
black) - Cuprous oxide is generally reddish in color and tends to form
first. It quickly converts to cupric oxide which is dark brown or black
in color. Virtually all ancient bronze coins have at least a thin layer
of brown copper oxide directly on the metal surface.

---

Orignal From: Restoration of Bronze