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The fineness of a precious metal refers to the ratio by weight of the primary metal to any added base metals or impurities. Many precious metals are used in the form of alloys. Other metals are added to increase hardness, to make the metal more practical for use in such items as coins and jewelry, or to decrease the cost of the alloy. For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares and jewelry. Coin silver, which was used for making silver coins in the past, contains 90% silver and 10% copper, by mass. Sterling silver contains 92.5% silver and 7.5%, by mass, of other metals, usually copper. Whilst various ways of expressing fineness have been used, two remain in common use: Millesimal fineness expressed in units of parts per 1,000〔(【引用サイトリンク】title=Definitions )〕 and carats or karats - used only for gold. Carats measure the parts per 24, so that 18 carat = 18/24 = 75% and 24 carat gold is considered 100% gold. ==Millesimal fineness== Millesimal fineness is a system of denoting the purity of platinum, gold and silver alloys by parts per thousand of pure metal by mass in the alloy. For example, an alloy containing 75% gold is denoted as "750". Many European countries use decimal hallmark stamps (i.e. '585', '750', etc.) rather than '14K', '18K', etc., which is used in the United Kingdom and United States. It is an extension of the older carat (karat in North American spelling) system of denoting the purity of gold by fractions of 24, such as "18 carat" for an alloy with 75% (18 parts per 24) pure gold by mass. The millesimal fineness is usually rounded to a three figure number, particularly where used as a hallmark, and the fineness may vary slightly from the traditional versions of purity. The most common millesimal finenesses used for precious metals (and the most common terms associated with them): 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fineness」の詳細全文を読む スポンサード リンク
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