17·Gold 371 example,with metallic zinc,which causes the gold to precipitate. In another process,hot sulfuric acid or nitric acid is applied to gold alloys which dissolves silver and other constituents(but not gold)and thus separates gold from its alloying components if this is desired.In "cementation"(a process which has been in exis- tence since about 550 B.c.),the gold-silver alloy in the form of thin leaves is heated in the presence of salt(NaCl),whereby sil- ver chloride forms,which can be washed out.Gold is so noble that it needs a mixture of nitric and hydrochloric acids (aqua re- gia)to change it into a salt(AuCl3). For jewelry and many utensils and coins,pure gold would be too soft and therefore needs to be alloyed with silver,or possi- bly palladium or platinum,or left unpurified,as was done in an- tiquity;see below.Thus,goods made of gold alloys usually have a broad range of colors.(A 50%gold alloy is known by the term “12 karat'”whereas pure gold is called“24 karat'".) A large number of skilled gold artisans emerged in different cultures through the millennia,as witnessed through splendid artifacts which have been found in numerous burial sites.Among them are finds near Varna (Bulgaria)on the coast of the Black Sea (see Plate 1.4)dating from 4300 B.C.,artifacts from the Sume- rians(4000 B.c.),and particularly from the Egyptians who skill- fully covered wooden or copper articles with gold leaves(called gilding).But solid gold articles were also created by the Egyp- tians.One of the finest examples of this is the inner coffin be- longing to the pharaoh Tutankhamen(1350 B.c.),which was made of solid gold and weighs 110 kg.A number of Irishmen during the second millennium B.c.were likewise gifted gold ar- tisans.They traded their goods between the British islands and Central Europe.This considerably enhanced the wealth of the in- volved people,particularly the British.Other places of gold min- ing and artistic activities emerged in India,China,Greece,and Asia Minor,to mention a few.By the time of the Middle Ages (around A.D.1500),London counted 52 goldsmith shops who had to maintain a certain standard in workmanship before they were accepted in a trades guild.A few examples of art work made of gold are depicted in Plates 17.2-17.5. As implied above,gold coins played an important role in early trade.(To a lesser extent,coins made of silver and even bronze were likewise used.)Probably the first true coins,that is,cast disks of standard weight and value,were manufactured by the Lydians of Anatolia at about 640 B.C.using a natural alloy con- sisting of gold with 20 to 35%silver,which was known by the name electrum.The coins were embossed with the aid of a die that bore the image of a lion.However,the trade value of theseexample, with metallic zinc, which causes the gold to precipitate. In another process, hot sulfuric acid or nitric acid is applied to gold alloys which dissolves silver and other constituents (but not gold) and thus separates gold from its alloying components if this is desired. In “cementation” (a process which has been in exis￾tence since about 550 B.C.), the gold–silver alloy in the form of thin leaves is heated in the presence of salt (NaCl), whereby sil￾ver chloride forms, which can be washed out. Gold is so noble that it needs a mixture of nitric and hydrochloric acids (aqua re￾gia) to change it into a salt (AuCl3). For jewelry and many utensils and coins, pure gold would be too soft and therefore needs to be alloyed with silver, or possi￾bly palladium or platinum, or left unpurified, as was done in an￾tiquity; see below. Thus, goods made of gold alloys usually have a broad range of colors. (A 50% gold alloy is known by the term “12 karat” whereas pure gold is called “24 karat”.) A large number of skilled gold artisans emerged in different cultures through the millennia, as witnessed through splendid artifacts which have been found in numerous burial sites. Among them are finds near Varna (Bulgaria) on the coast of the Black Sea (see Plate 1.4) dating from 4300 B.C., artifacts from the Sume￾rians (4000 B.C.), and particularly from the Egyptians who skill￾fully covered wooden or copper articles with gold leaves (called gilding). But solid gold articles were also created by the Egyp￾tians. One of the finest examples of this is the inner coffin be￾longing to the pharaoh Tutankhamen (1350 B.C.), which was made of solid gold and weighs 110 kg. A number of Irishmen during the second millennium B.C. were likewise gifted gold ar￾tisans. They traded their goods between the British islands and Central Europe. This considerably enhanced the wealth of the in￾volved people, particularly the British. Other places of gold min￾ing and artistic activities emerged in India, China, Greece, and Asia Minor, to mention a few. By the time of the Middle Ages (around A.D. 1500), London counted 52 goldsmith shops who had to maintain a certain standard in workmanship before they were accepted in a trades guild. A few examples of art work made of gold are depicted in Plates 17.2–17.5. As implied above, gold coins played an important role in early trade. (To a lesser extent, coins made of silver and even bronze were likewise used.) Probably the first true coins, that is, cast disks of standard weight and value, were manufactured by the Lydians of Anatolia at about 640 B.C. using a natural alloy con￾sisting of gold with 20 to 35% silver, which was known by the name electrum. The coins were embossed with the aid of a die that bore the image of a lion. However, the trade value of these 17 • Gold 371