The Fundamentals of Plating

General Physical Properties of Gold

Atomic Number: 79
Atomic Weight: 196.97
Density: 19.3 grams per cubic centimeter
Melting Point: 1063 C
Boiling Point: 2970 C
Thermal Conductivity: 0.71 cgs units (20 C)
Chemically Resistant to: Sulfuric acid (100 C), fuming sulfuric acid, persulfuric acid, fuming nitric acid, 30% hydrochloric acid, perchloric acid, hydriodic acid, phosphoric acid, acetic acid, tartaric acid, citric acid, selenic acid
Chemically Attacked by: Sodium and potassium cyanide plus oxygen, aqua regia, chlorine, chlorine water, bromine and iodine in alcohol, selenic acid (+230 C)

 Because of its unique physical properties and lasting beautiful color gold has always been one of mans favorite elements. Since the beginning of recorded history gold has served as an art medium and a symbol for wealth, eternity, the Sun, life, love and many other desirable parts of the human experience. Aside from the psychological, metaphysical, and religious appeal of gold it has certain physical properties that make it a desired medium. From the ancient goldsmith to todays scientist, gold is unique in is use.

What is Plating?

Plating refers to covering a surface with a metal.

Our technology is based on electroplating.  Electroplating is a specific plating process in which positively charged metal ions in a solution are deposited onto an electrically conductive, negatively charged surface.

How Does Plating Change the Properties of the Surface?

Plating has been a critical element in the advancement of modern technology. Plating is used for corrosion resistance, to improve solder-ability of a surface, hardening, to improve wear-ability, reduce friction, to improve paint adhesion, alter conductivity, for decorative and other purposes.

Common Materials That Can Be Plated

  • Copper
  • Copper Alloys (including brass)
  • Nickel
  • Steel (stainless steel requires special activation)
  • Silver
  • Pewter

The Process of Electroplating Gold onto properly prepared conductive surfaces electrochemically deposits a very thin1 layer of metallic gold over the surface of the item to be plated.  This process can impart some of the desired properties of the gold to the entire piece being plated at a fraction of the cost of solid gold.  In many cases you can maintain the important mechanical properties of the substrate and obtain the desirable benefit of an exposed gold surface.  An example of this is gold plating stainless steel or copper electrodes for the fuel cell industry.  In this case you can have the strength or conductive properties of the substrate and the corrosion resistance and/or catalytic, conductive or chemical effect of the gold surface.

For a hardened gold plate, (Type I & II - Grade B, C or D),  the gold layer typically ranges in thickness from 5 micro-inches, (0.125m), to 100 micro-inches, (2.55m).  Hard gold plate deposit thicknesses greater than 100 micro-inches can result in deposit stresses that may be detrimental to the value of the gold plating.  Pure soft gold, (Type III, Grade A), Can be plated much thicker.  We commonly plate Type III gold to a thicknesses greater than 500 micro-inches, (12.55m).  The trade off is that the grain structure is more course which allows greater diffusion and generally makes the surface more "matt" in appearance.

The density of 24K gold plating is ~ 12.45mg for each square inch of area plated to a thickness of one micron,(~0.00004 inch)..  Therefore, if evenly distributed, 1 gram of 24K gold will plate approximately 160 square inches of area to a thickness of 1/2 micron, (0.00002" or 20 micro-inches).  This thickness is the minimum thickness of gold plating for jewelry items to be sold as "gold plated" in the United States under the US Federal Trade Commission standards. Note: The density of gold plating can range from 11.3mg/in2 per micron thickness through 12.45mg/in2 per micron thickness. the upper end of the range is for a very fine grain gold plate. 

Measuring Units for the thickness of gold plate:  The English term is "micro-inches" (millionth of an inch). If Metric terms are preferred then  "microns" or "micro-meters" (millionth of a meter), are used.  The most frequently used term when discussing the thickness of gold plate is "micron".  A "micron" is about 40 times greater than a "micro-inch".

Less frequently the term "mil" is used when "micro-inch" is intended, this can be a much bigger problem since there are 1000 micro-inches in a mil.  One example of such a problem was recently noticed when a seller at a popular on-line auction was offering 20 gold plated pennies for $4.95.  This is a good deal but then they went on to say "Each coin is specially layered with 7 mils of Genuine Pure 24 Karat Gold".  Using the density information above and a little arithmetic* shows that this seller is offering 1.25 troy ounces of gold for $4.95.  This is not likely since as of this writing the London bullion market was at $1,875.00/troy oz. 

*A one mil thickness of 24K gold plate will weight 0.316 grams per square inch.  The surface area of a US penny is ~0.88 square inches.  Therefore - 20pennies * 0.88 in2/penny * 0.316 grams 24K gold/in2/mil thickness * 7mil thickness = 38.93 grams 24K gold.

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