Pure gold (fine gold) is softer than pure silver but harder than tin. Its beauty and luster are unmatched by any alloyed golds. The extreme malleability, ductility, and softness of pure gold make it practically useless for jewelry applications. Alloying elements (other metals) are added to gold to increase the toughness and hardness of the gold alloy. While almost any metal can be alloyed (melted) with gold, only certain metals will not dramatically change the color or make the metal brittle. The addition of indium, for instance, turns gold purple and gives gold alloy the workability of glass.

Over time, certain percentages of gold have become legally recognized "karats." The karat indicates the amount of gold as a percentage of the total, i.e. 24 karat is 100 percent gold. Thus 14 karat is 14/24 gold or 58.33 percent gold. Gold standards vary around the world. In the United States, 24, 22, 18, 14, and 10 karat gold are the only karats allowed to be sold as karated gold.

In karated gold, there is a known proportion of metals in the non-gold percentage. These metals provide the various colors and hardness of karated golds. Typical alloying elements and their color effect are

• Gold Bars
• Copper Reddening
• Silver Greening
• Zinc Bleaching
• Nickel Whitening
• Palladium Whitening

Examples of the compositions of different colors are

• Yellow: Gold, copper, silver, zinc
• White: Gold, copper, nickel (or palladium), zinc
• Red: Gold, copper
• Green: Gold, silver

Adjusting the proportions of base metals (non-precious metals) provides the array of colors on the market. They also enhance properties such as castability, grain size, hardness, corrosion resistance, workability, ultimate strength, and ductility. These additions can dramatically change the properties of the karated gold for better or worse.

Knowing how the additions will affect the karated gold greatly enhances the possibility of a superior final product. In deep drawing of metals, it is important to have a metal which will elongate or stretch a great deal before fracturing, thus high ductility. The requirement for an earring post would be a high tensile strength (a great deal of force needed to get the material to permanently deform or bend). It is imperative to select the proper karated composition for the desired application

What is the definition for white gold-filled, yellow gold-filled, nickel-plated, and yellow-plated?

Gold-Filled

The term gold-filled refers to the manufacturing process in which a sheet of base metal, usually brass, is mechanically bonded with thinner sheets of gold. A "sandwich" is formed by mechanically bonding a layer of gold on both sides of brass. This "sandwich" is then cold worked by rolling until a much thinner gauge metal is achieved. Products are then formed or die-struck from this layered material.

When a layer of gold is affixed on all surfaces by any mechanical means and the weight of the gold is a minimum of 1/20 of the total weight of the metal in the article, it may be marked Gold-Filled (GF). The quality of the gold used is typically 10, 12 or 14 karat gold with 10 karat being minimum. Hallmarking would look like “10K GF” or “14K GF”. Yellow gold-filled or white gold-filled simply indicates the color of the karat gold used in the making of the gold-filled product.

The terms “Rolled Gold Plate” and “Gold Overlay” refer to the same bonding process; however, the weight of karat gold is less than 1/20 but not less than 1/40 of total weight of metal.  Hallmarking would look like “1/30 12K R.G.P”, “1/40 10K Gold Overlay”.

Gold-Plated

When a product is referred to as gold, rhodium, or nickel-plated, this indicates that it has been electroplated with a thin layer of that particular metal. An article of jewelry is Gold-Plated when gold is electroplated or mechanically sheathed with a mini­mum thickness of 1/2 micron (20 millionths of an inch) of fine gold. The quality of the gold used is typically 10, 12, or 14 karat. The karat quality of the gold plating must be disclosed, and it can be described as 12K Gold Plate or 2μ 12k G.P. for an item plated with two microns of 12 karat gold.

The table below lists all the different plating terms and their associated thicknesses.

Want to learn more about Rhodium plating? Watch these tutorials.

Melt Temperature Recommendations for Various Casting Methods

• Vacuum assist casting: 100 degrees to 200 degrees Fahrenheit over Liquidus Temperature
• Centrifugal/sling casting: 50 degrees to 125 degrees Fahrenheit over Liquidus Temperature
• Pieces with less detail require lower superheats.
• Experience is the best guide
• Flask Temperature Versus Casting Method and Piece Size and Design
• Smaller pieces such as small findings or filigree rings require higher flask temperatures 1100 degrees to 1300 degrees Fahrenheit
• Large pieces (gent's rings) and pieces with few details require a lower flask temperature, 800 degrees to 1000 degrees Fahrenheit
• White gold generally requires flask temperatures on the high end of the recommendations
• Centrifugal casting tends to have flask temperatures on the lower end of the recommendations
• Large and small pieces on a single tree require flask temperatures closer to those needed for light pieces
• Experience is the best guide

About the Metal

• Ratios in excess of 50% scrap are not recommended
• Clean metal thoroughly before using (no investment)
• Purge system periodically. Start fresh every six months
• The weight of the metal in the button and tree should at least equal the weight of the pieces cast
• Investing Procedures
• Ratios of investment to Water
• Follow instructions of the supplier
• Pieces with great detail may need a slightly greater amount of water for investment

Vacuuming investment

• Make sure you have a good vacuum
• Vibrate table to help remove trapped air

Burnout Cycle

• Don't move invested trees too early (wait one hour)
• Follow instructions of investment supplier
• Check that temperature read was accurate
• The casting flask temperature (final step) should be maintained for at least two hours before the first flask is removed

Proper Spruing and Gating

• Don't use choke gates. They cut off the feed metal
• Place sprue (gates) at heaviest cross sections of the piece
• Use runners to heavy areas if they are separated by thin cross sections
• Flair the gate at the connection to the piece. This will help reduce turbulence in the metal flow
• Don't put pieces too close together
• Smooth all surfaces to avoid sharp corners which may break off during casting (i.e. connections between main sprue and gates; and gates and piece)

Breakout

• Don't rush to break out. Wait at least fifteen minutes for most alloys before quenching in water
• Some alloys require specific handling, follow instructions of alloy supplier
• Clean investment off thoroughly if you plan on reusing the scrap. Investment in the melt will cause porosity problems over time
• If breaking out dry by hammering, be sure flask is well below 600 degrees Fahrenheit to help reduce potential cracking

Your customers may think that faulty manufacturing or underkarating might be the problem when a ring "turns," blackening or discoloring the skin, clothing, or the jewelry itself. However, that is not the case. You can help them understand the causes and how to prevent them.

The most common reason is metallic abrasion caused by makeup on skin or clothing. Cosmetics often contain compounds harder than the jewelry itself, which wear or rub off very tiny particles. Very finely divided metal always appears black rather than metallic, so it looks like a jet-black dust. When this dust comes into contact with absorbent surfaces such as skin or clothing, it sticks forming a black smudge.

To prevent this, your customers should try switching cosmetics. If this is not possible, recommend that they remove rings and other jewelry while applying them, and clean skin areas in contact with jewelry with soap and water.

Another cause is actual corrosion of the metals. Gold itself does not corrode, but its primary alloys of silver or copper will do so (forming very dark chemical compounds) under moist or wet conditions.

When your customer perspires, fats and fatty acids released can cause corrosion of 14 karat gold, especially when exposed to warmth and air. This problem can be worse in seacoast and semitropical areas, where chlorides combine with perspiration to form a corrosive element that discolors skin. Smog fumes gradually attack jewelry and are evident as a tarnish that rubs off on the skin.

Suggest that your customer remove jewelry often and use an absorbent powder, free of abrasives, on skin that comes into contact with jewelry.

Even the design of jewelry can be an influence. Wide shanks have more surface area to contact abrasives or corrosives. Concave surfaces inside a shank form collection points that trap moisture and contaminants, causing a type of dermatitis.

Have customers remove all rings before using soaps, cleaning compounds, or detergents, and clean their rings frequently. As well as solving the problem, they'll be amazed at how much better their rings look.

In addition to these corrective actions, recommend that customers switch to 18 karat gold or platinum. The lower alloy content of 18 karat gold (25% versus almost 42%) significantly reduces the problem, and the use of platinum should eliminate it completely.