All That Glitters

« Scientific Bargain Shopping | Home | What on Earth? »

By Linda | November 7, 2008

My young Science Junkie is fascinated with precious metals and gems. When we visited the Smithsonian this last summer, his favorite exhibit was the hall of gems. He was particularly awed by the Hope Diamond. Well, who wouldn’t be?

Over time he has asked me many questions about gold. Some I could answer off the cuff, others I had to look up. This post contains those collected “nuggets” of golden knowledge.

Gold jewelry is usually marked with a number followed by the letter “k”. This number refers to the gold’s “karat” - a measure of its purity. The higher the percentage of pure gold, the higher the karat. Pure (24k) gold is generally considered too soft for use in jewelry so it is mixed or alloyed with other harder metals for strength and durability.

24 karat = 100% gold
18 karat = 75% gold
14 karat = 58.3% gold
10 karat = 41.6% gold

“Plumb Gold”refers to karat gold that is precisely mixed to contain exactly the amount of gold reflected in its karat measurement. In jewelry-making, the ratio of gold to alloy metals can sometimes be approximate. 14k plumb gold would contain EXACTLY 58.3% gold. The term “plumb gold” has often been confused with “plum gold” describing the color perhaps? There is no such thing as plum gold.

To add to the confusion. there are two jewelry terms that are pronounced the same but are spelled differently and have two different meanings. Although both are pronounced like “carrot” (the vegetable), “karat gold” is different than “carat weight” which is a measure used to weigh diamonds and gemstones.

In its pure form, gold has a metallic luster and is sun yellow, but when mixed or alloyed with other metals, gold’s yellow can be modified to achieve a variety of hues ranging from silver-white to green and orange-red.

These are some of the more common gold colors along with the metals used to achieve them:

White Gold - Nickel can be mixed with gold to create a white or gray color, but some people have an allergy to nickel. Palladium is another metal used to create white gold. Palladium is more desirable but also more expensive.

Pink and Rose Gold - Copper is added to the gold. The more the copper, the deeper the red color effect will be.

Green Gold - Created by adding silver to gold.

Purple Gold - Referred to as amethyst or violet gold. Purple gold is obtained by mixing gold and aluminium in a certain fixed ratio. Gold content is almost 79% and therefore it is qualified to be referred to as 18K gold.

Blue Gold - Made as an inter-metallic compound between gold and indium . The gold takes on a bluish hue with this process.

Black Gold - Can be created using a few techniques. Electro-deposition using black rhodium or ruthenium is the first technique. Controlled oxidation of karat gold containing cobalt or chromium can also be made to create black gold. Amorphous carbon is also used sometimes, with the Plasma Assisted Chemical Vapor Deposition process.

One of gold’s unique and useful qualities is its virtual indestructibility. Gold is the most non-reactive of all metals. It is called a “noble” metal (an alchemy term) because it does not oxidize under ordinary conditions, meaning that it will never rust and never tarnish.

Gold’s physical properties of high electrical conductivity and chemical inertness make it an excellent and reliable conductor, particularly in harsh environments, where temperatures can range from -55°C to 200°C. The use of gold in circuitry ensures reliability of equipment operation, particularly in the vital activation of safety airbag mechanisms in motor vehicles or deployment of satellites and spacecraft.

No other metal is as ductile (easily workable) as gold. A single ounce of the metal can be drawn into a wire five miles long! Gold can be hammered into sheets so thin that light can pass through. High purity gold reflects infrared (heat) energy almost completely, making it ideal for heat and radiation reflection. Gold-coated visors protected astronauts’ eyes from searing sunlight on the Apollo 11 moon landing.

Gold is also an excellent conductor of thermal energy. It is used in many electronic processes to draw heat away from delicate instruments. For example, the main engine nozzle of the space shuttle uses a 35% gold alloy.

Gold is the only yellow metal and bears its name from the Old English word for yellow, (geolu). Gold’s chemical symbol, Au, comes from the latin word for gold (aurum). In the Periodic Table of Elements, gold is classified as a transitional metal with the following characteristics;

Atomic Number: 79
Atomic Mass: 196.96655 amu
Number of Protons/Electrons: 79
Number of Neutrons: 118
Melting Point: 1,947.97°F
Boiling Point: 5,084.6°F
Density @ 293°K: 19.32 grams per cubic centimeter
Crystal Structure: cubic