0.441 lb. or 129 dwt. per cu. in. It is practically unworkable.
Ruthenium is an excellent hardener for palladium or platinum, being approximately twice as effective as iridium. The hardness and strength of an alloy of 4.5% ruthenium 95.5% palladium are about the same as the alloy made of 10% iridium and 90% platinum.

Airplane magnetos utilize a 10% ruthenium. and 90% platinum alloy for the contact points that continuously make and break the primary electrical circuit. Unusually high resistance to wear and spark erosion are required of contact points in this service.

IRIDIUM

Pure iridium, a very heavy element of white color, weighs 0.813 lb. or 237 dwt. per cu. in. and melts at approximately 4449°F.

Being harder than rhodium, iridium is very difficult to roll into wire or strip. It is used principally as an alloying material, usually as a hardener for platinum. An iridium-platinum alloy containing 10% iridium is used in fine jewelry. A proportion as high as 30% iridium is used to harden platinum for hypodermic needles and other special purposes.

of karat gold alloy to a base metal and then rolling or drawing the composite metal to the dimensions required for fabrication into finished articles.

The quality standard for "Gold Filled" and "Rolled Gold Plate" articles (except watch cases) is based on the proportion of karat gold used, the proportion being measured by weight. The mark "1/20 14K Gold Filled" indicates that 95% of the weight of the material is the base metal and 5%, or 1/20, is the 14K gold alloy. The "1/20 14K Gold Filled" material contains a little less than 3% of pure gold. When the proportion of the karat gold alloy is less than 1/20 or 5%, the composite material must be described as "Rolled Gold Plate".

Special quality standards apply to watch cases, the standards being based on the coating thickness rather than the proportion by weight. The thickness of the karat gold coating on the outside of a watch case is required to be at least 0.003 in. for "Gold Filled" and 0.0015 in. for "Rolled Gold Plate."

The old "Sheffield Plate," a composite metal made by brazing silver to copper, was at the height of its popularity around 1800. Tea services, water jugs and many other articles were made of Sheffield Plate, until the process for electroplating silver was discovered in 1838.

The modern "Silver Plate" is made of a base metal coated with silver, by electroplating. For flat-ware of the better quality, a common practice is to make spoons, forks, etc. of "Nickel Silver"-a white alloy of copper, nickel and zinc-and then electroplate the articles with a heavy coating of silver. By special processing, the coating thickness may be increased at points of wear. Terms such as "Quadruple Plate," and "Standard Plate" are used sometimes to describe silver plate. For "Quadruple Plate," the quantity of silver applied is four times greater than for "Standard Plate," which has an average thickness of approximately 0.0003 in.

FINISHES

Inexpensive costume jewelry, compacts, lighters, etc. made of a base metal or other substance, frequently are finished with thin coatings of gold or other precious metal. The coating thickness may be only a few millionths of an inch, the thicker coatings being more serviceable.

A finish of gold often is applied to costume jewelry, etc., by electroplating. Small embellishments, incorporating imitation stones, may be finished with rhodium to give sparkle to the embellishments.