The separating and purifying of precious metals
from other metals is called .
is the separation
of precious metal from non-metallic impurities.
Midwest Refineries is committed in using correct and accurate evaluation
methods of all precious metal shipments from our valued customers.
Our modern assay laboratory guarantees that all precious metals
are tested using the most suitable methods of evaluation. Gold,
silver, platinum, palladium and rhodium are assayed (determination
of precious metal purity) in various ways to accurately and precisely
evaluate each metal’s percentage of purity.
method known as “fire assay” is the oldest known
method of assaying gold and continues to be one of the most accurate.
When gold is melted, a sample of the gold is taken, either by drilling
through the poured bar or by capillary tube sampling while the
gold is in a molten state. The sample is weighed very precisely
and the amount recorded. The sample is wrapped in assay lead foil
along with a quantity of pure silver. This wrapped ball is placed
in the furnace in a cupel (a special kind of disposable crucible).
All the non-precious metals are absorbed by the hot cupel. The
precious metal forms a button within the cupel. The
cupel is removed from the furnace and the button is brushed to
remove any remaining bits of the cupel. It is hammered flat, rolled
thin and then heated in a porcelain crucible containing a weak
nitric acid solution. The acid removes the silver, which is poured
off and the silver recovered from solution. The gold is then rinsed
in distilled water to remove any residual acid and then dried.
The sample of gold is now at least 99.999% pure! The sample is
then weighed again. The original weight of the impure sample is
divided into the weight of the now pure sample. The result is the
assay. In the analysis of platinum group metals, roughly the same
procedure is followed, but the final element readings are normally
done using ICP.
|ICP-OES Inductively coupled plasma optical emissions spectroscopy
is a major technique for elemental analysis. The sample to
be analyzed, if solid, is normally first dissolved and then
mixed with water before being fed into the plasma. Atoms in
the plasma emit light (photons) with characteristic wavelengths
for each element. This light is recorded by one or more optical
spectrometers and when calibrated against standards the technique
provides a quantitative analysis of the original sample. ICP
instruments comprise of various optical spectrometers, nebulizers,
spray chambers, ICP torch and RF generators.
Typical ICP Torch
X-Ray fluorescence (XRF) spectrometry is a non-destructive analytical
technique used to identify and determine the concentrations of
elements present in solid, powered and liquid samples. The spectrometer
measures the individual component wavelengths of the fluorescent
emission produced by a sample when irradiated with X-rays.
Midwest Refineries accepts all amounts of precious metals for
purchase. In cases where customers may have just a few items they
wish to sell, like an old gold school ring, or a couple of gold
dental crowns etc., we will test the items and purchase them outright,
eliminating the need for a melt and assay. Our trained staff can
accurately determine the purity of most precious metals either
by electronic means or by basic scratch and touch stone testing
with acid, which compare scratch marks from unknown karat gold
items with known karat gold needle marks. The touch stone is a
smooth fine grained, slightly abrasive black stone, usually quartz
or jasper. It was used as the first method of assaying gold and
was used at least as early as 500 BC. Touch stones and karat needles
are still used today and can be purchased from jewelry supply houses.
They obviously are not anywhere near the accuracy of a fire assay,
but still serve as a valuable tool to test the karat value of unknown
Touchstone set used in the 19th century
for testing the fineness of gold Individuals can
test their own gold and silver items with a few basic tools and
acid. Nitric acid can be obtained from most jewelry supply houses
and used to determine whether an item is gold or gold-plated on
steel or copper. A good strong magnet, available at any hardware
store, is useful to test whether the item is just gold-plated over
steel. Gold is not magnetic. Gold-filled items are not magnetic,
because the gold is usually over copper. Gold-filled items can
be tested by filing a deep notch in the surface and putting a drop
of nitric acid over the filed notch. If it starts to bubble green
color, it is the copper reacting to the acid and it is gold-filled.
When nitric acid is applied to 10 KT. gold, it will turn a reddish
brown in color. 14 KT. will not change color and the acid will
just look like a drop of water on it. From 18 KT. to 24 KT will
stay bright on a touch stone, but is hard to tell any difference
between the karat value. There are numerous electronic testers
on the market that will tell the karat value of gold items and
some that can test for platinum. Electronic testers are not nearly
as accurate as an actual fire assay, but can serve as a tool to
get a basic idea of what an unknown piece of metal is, or is not.
Electronic gold testers are available through jewelry supply companies.
You can also determine whether a silver colored piece of flatware
or jewelry is sterling silver or silver plated over base metal
or copper by filing a deep notch in it and applying acid in the
notch. If the item is sterling silver it will turn an ashy light
gray in color, but if it starts to bubble green at the notch, it
is silver plated over copper or other base metal.
When it is time for you to sell gold, silver, platinum, palladium
or rhodium, it is important that your precious metals are evaluated
correctly and accurately. Midwest Refineries maintains a modern
assay laboratory that is committed to properly evaluating all precious
metal materials to insure our customers are compensated honestly
and receive the highest settlements they are entitled to. We appreciate
all of our customers business and continually strive to provide
the best dependable service in the quickest possible turn-around
time. While we often may be imitated, no precious metal refiner
can equal our determination, commitment and integrity in serving
our customers. Repeat, loyal customers are the main reason Midwest
Refineries has been in operation for over 65 years. Give us a try
on your next batch of scrap gold, platinum, rhodium, palladium
or silver. We know your transaction with us will be a positive
one. Our reputation depends on it!