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18kt Medici gold-plated bracelet

Measuring Layer Thickness on Gold Plated Surfaces

One of the most confusing issues for the consumer new to the electroplating field, is the specifications and measurements that are utilized to classify plating thickness. Because specifications play a key role in determining pricing, quality and durability of plated finishes, it is important to clearly understand the definitions, particularly when measuring layer thickness on gold plated surfaces.

As a starting point it will be necessary to define the “unit of measurement” that describe electroplated layer thicknesses. The most common unit of measurement in the U.S. is the “microinch”. In layman’s terms, a microinch is actually the same as one millionth of an inch (see chart a).

For Europe and some U. S. industries (i.e. jewelry trade), the unit of measurement is often expressed in “microns.” To get an idea of just how thick one micron is, it takes 1000 microns to make one millimeter. For comparison, a U.S. dime coin is 1250 microns or 1.25 millimeters in thickness.

Chart A: Units of Measure

Unit of Measure US InchMetric
Chart ABase conversion of inches to metrics
One Micro-Inch=one millionth of an inch.0254 microns
One Millimeter=one thousandths of an inch25.4 microns
One Micron=39.37 micro-inchesOne millionth
of a meter

To better understand why there are different units of measurement for plating, it is necessary to define the different applications for gold electroplating. It is generally acknowledged by most regulatory groups in the U.S., that gold and other precious metal electroplating can be divided into two distinctly different categories; Decorative applications and Engineering applications.

Decorative Applications

Decorative electroplating traces its roots back to the early 1800’s. It has the oldest applications in jewelry, watches, silverware and their related trades. The description “decorative” can include all the applications for precious metal plating where the primary requirements is surface beauty, luster, color uniformity, wearability and enhanced aesthetics.

Since plating thickness can affect these characteristics, some basic specifications and common standards were necessary to help eliminate fraud and confusion within the respective watch, jewelry trade and consumer groups.

Plating thicknesses for precious metal objects and related trade is regulated by specific rules that are administered by the U.S. Federal Trade Commission. This can be found in the Code of Federal Regulations, Title 16, Commercial practices Part 23 January 1999. Precious metals under F.T.C. regulations include gold, silver, and the platinum metals group. F.T.C. requirements for plating thickness are expressed in both microinches and microns (see chart b).

Chart B: FTC Classification

FTC Classification Micro-Inches
(US inches)
Microns
(Metric)
Chart BUS Gov. FTC classification for gold plated thicknesses
Gold Flash (10 kt.)=7.0 micro-inches.175 microns
Gold Electroplate=7.0 micro-inches.175 microns
Gold Plate=20 micro-inches.50 microns
Heavy Gold Plate=100 micro-inches2.5 microns

For a clearer comparison, chart c describes the average thickness in microinches and microns of some common objects most all of us are familiar with. As you can see most electroplated gold finishes are quite thin by comparison.

Chart C: Base conversion of inches to metrics

Unit of Measure Micro-Inches
(u.s. inch)
Mils
(US Inches)
Microns
(Metric)
Chart CBase conversion of inches to metrics
Coating of Varnish=393.7.393710
Human Hair Follicle=3,9373.93100
US Dollar Bill=7,8747.87200
US Dime=49,21249.21,250

After comparing chart b and chart c, the first thing one would ask is how can electroplating so thin last very long? As a clear example, lets use a gold plated watch case to illustrate a point. Luxury quality watch cases from some manufacturers have been plated in gold (18 kt color) up to 20 microns (1/50 of a millimeter) in thickness. This by far exceeds the 2.5 microns required for “heavy gold plate” under F.T.C. rules.

As thin as 20 microns may seem (1/5th of a human hair), the expected durability of this type of plated gold finish, with reasonable care, could nearly exceed the functional lifetime of the watch itself!

Surface Hardness of Gold

Its not enough to have a thick gold plated layer if it is easily rubbed off from casual contact with clothing, skin and other surfaces. Hardness of the goldplated layer is an extremely important consideration when durability is the prime concern.

Newer electroplated gold formulas are becoming available that produce finishes that are extremely hard and scratch resistant. Electroplating formulas, which include trace amounts of exotic metal alloys, have reduced the need for gold plated finishes over 5 microns in thickness. While this may still seem very thin, the thickness is more than adequate for long term durability and luster for most high wear objects.

One could easily speculate that in the future, the improvements in gold plating finishes relative to hardness and durability, could make the F.T.C. rules for heavy gold plate obsolete!

For repairing worn and damaged gold filled jewelry and watch items, gold electroplating done properly, can provide an excellent low cost and durable refinishing alternative to the consumer.

Engineering Applications

Electroplating for engineering applications are primarily concerned with functional, chemical, electrical, and mechanical performance on discreet components. In the U.S., the rapid growth of the semiconductor and electronics industry has introduced major innovations to the electroplating field. Gold electroplating enjoys the dominant role in advancing this new technology.

Rigorous performance standards along with specific classifications are utilized by industry to control the engineering properties of gold plated deposits.

The American Society of Testing and Materials (ASTM) have defined classifications and standards. They are defined in document B488-95 titled “Electroplated Coatings of Gold for Engineering Uses” In addition to the ASTM standards, various segments of industry utilize military specifications or “Mil specs”. This term refers to the body of specifications utilized by the U.S. military, NASA, and the National labs. For gold plating performance standards MIL-G 45204C defines the engineering specifications including purity (type), hardness (grade), thickness (class).

Type

For engineering applications,"type"refers to the percent purity of a gold plated layer relative to the "theoretical purity" of 100%. For decorative plating applications "theoretical purity" for pure gold expressed as 24 karat.
Grade refers to the standard for the hardness of the gold plated layer. Different surface hardnesses are needed for specific performances relative to conductivity, solderability, and abrasion resistance. In contrast, for decorative plating applications, surface hardness of gold plated finishes are not regulated or defined by US FTC regulations as of August 1999.
TYPEPercent of Pure GoldGrade
Type A99.7 % gold minimumgrades a, b, or c
Type B99.0 % gold minimumgrades b, c, or d
Type C99.9 % gold minimumgrade a only

Grade

Grade refers to the standard for the hardness of the gold plated layer. Different surface hardnesses are needed for specific performances relative to conductivity, solderability, and abrasion resistance. In contrast, for decorative plating applications, surface hardness of gold plated finishes are not regulated or defined by U.S. F.T.C. regulations as of August 1999.
* Knoop hardness is calculated by measuring the indentation produced by an elongated four-sided pyramid shaped diamond point. Pressed into an electroplated surface under loads (usualy less than 1 kilogram-force) the indenter creates a 4 sided impression. Knoop hardness (HK) is calculated using the formula HK=14.229(F/D2). F is the applied load, D2 is the area of the indentation measured in square millimeters. Hardness numbers are cited relative to specific load values that were used in the test.
GRADEKnoop Hardness*Type
Grade A90 minimumtype 1 and type 3
Grade B91–129type 1 and type 2
Grade C130–200type 1 and type 2
Grade D201 and overtype 2

Class

"Class" refers to the thickness of electroplated gold layer. Shown here in microinches and microns, different layer thicknesses for specific engineering applications need to be controled in order to meet performance requirements (circuit boards, connectors etc.)
* These two classes of gold plated finishes would meet or exceed the FTC requirements for jewelry items designated and sold as "heavy gold plate".
CLASSMicro-InchesMicrons
Class 0020 micro-inches.005
Class 030 micro-inches.762
Class 150 micro-inches1.25
Class 2100 micro-inches2.54 *
Class 3200 micro-inches5.06 *

For practical applications and additional definition, if you saw a gold plated specification for a plumbing fixture that read: Type 1, Grade C, Class 2; it would mean that it is 99.7% in gold purity, with a hardness of at least 130 knoop, and has a gold plated thickness of at least 2.5 microns. Color can vary depending on the electroplating formula that was used in the process.

Depending on the application and customer requirements, there are numerous other electroplating standards that are observed in the U.S. by different segments of private industry. Automotive, marine, and other transportation industries have their own specifications, with subtle differences to satisfy specific technical requirements.

In summary, to prevent misunderstandings, it is important to know the differences between decorative electroplating and engineering electroplating. As you can see, each of these applications have their own specifications designed to satisfy two different sets of priorities.

If you are attempting to select a company to gold plate an object, be sure and define what your needs are. Most electroplating companies will be happy to help you select the best finish for your plating needs.

For additional Information…

For additional information regarding electroplating and metal refinishing, be sure to visit our faqs section located on this site. If we can assist you in your custom plating and metal refinishing needs, please contact us.

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