Silver bowl and silver plant on acid etched steel. Miranda Meilleur Designer silversmith UK 2007.
Silver objects by Paul Revere, American silversmith.
Gold was the first precious metal to be worked by man. It was available as nuggets that required no smelting. Because of its rarity, its luster and its ability to resist corrosion, gold has been valued by many cultures.
Gold melting point: 1063 degrees C. Gold specific gravity cast: 19.2 . Gold specific gravity worked: 19.3. Gold atomic weight:197.3.
Gold was probably the second metal to be worked by man being discovered after copper. Quality gold work can be found as early as 3000BC. If all the gold ever found (about 20000 tons) were cast into a single ingot, it would make only a 20 yard cube. Gold has a face centered cubic crystal. Pickles for gold can be a mixture of 1 part nitric acid with 8 parts of water. Gold dissolve aqua regia and solutions of chlorine and potassium cyanide or sodium cyanide.
Gold filled: this term refers to a material on which a layer of gold has been bonded by fusing. The resulting ingot is rolled or drawn to make sheet and wire. A standard practice is to clad the base with 10% (weight) 12 Karat gold. Since 12 Karat is half pure it means the final result , if it were melted down and assayed, would equal 1/20 or 5% pure gold, marked as 1/20 G.F
Fineness of gold: Since fine pure gold is too soft for most of its uses it is alloyed with other metals to achieve a desired hardness. During this process the colour, hardness, malleability and melting point can also be altered. Silver and copper are the two most common additives but many other metals can be used. The relative amount of gold in an alloy is called the Karat. This word signifies proportion and should not be confused with Carat which is a unit of weight.
24 Karat gold: 1.000
18 Karat gold: .7500
9 Karat gold: .3750
One ounce of gold can be flattened to a sheet that will cover 100 square feet or drawn to a wire almost a mile long. Gold can be made into a foil that is less than 5 millionths of an inch thick at which point it is virtually transparent.
The testing of gold: To be scientifically accuarate a sample must be assayed ina testing laboratory but these two tests have been used for many years and are often sufficiently accurate for the crafsperson.
A small file makes a scratch in an inconspicious spot. While wearing rubber gloves, a drop of nitric acid is applied with a wooden, glass or plastic spoon. The reaction is observed and the sample is then rinse under clear water. If no reaction occurs the sample is gold. If a bright green colour occurs the sample is base metal. If the green colour resides in the scratch the sample is gold layer over base metal. If the reaction produces a milky scratch the sample is gold over silver.
Finding out what Karat the gold sample is:
A testing kit is needed: nitric acid, aqua regia, samples of known carat and a touchstone of slate or ceramic. The object to be tested is rubbed on the stone to leave a streak. A parallel line is made with one of the test needles. Both marks are flooded with acid and the reaction is observed. When the sample colours at the same rate as the test streak, a match has been made. Nitric acid is used for low Karat golds, while aqua regis is needed for higher Karats.
To lower Karats: Time the amount to be lowered by its Karat. The same amount is then times by the desired Karat. The difference between the first and the second is found and then divided by the quality being made.
To raise Karat: The amount to be raised is timed by its Karat. The same amount is timed by the desired Karat. The second sum is substracted from the first. The difference is found between pure Karat (24) and the desired Karat. The substracted sum is now divided by the sum found between the pure Karat and the desired Karat.
Alloys: A mixture of roughly equal parts of gold and silver is called electrum. This alloy shows maximum hardness at a 50/50 mix. The hardest alloy of gold ,silver and copper is reached at 50/25/25. This is 12 Karat yellow. Any increase of the copper content in a gold alloy will lower its melting point up to 18% copper 9melts 880 degrees C). To continue lowering add silver. Many kinds of colours of gold solders are commercially available, but in a pinch a gold of a lower Karat may be used.
Fluxes: When pouring gold ingots use an even mixture of powdered charcoal and ammonium chloride (sal ammoniac) during melting. This will yoeld a bright tough ingot that will withstand rolling. Dangerous fumes are produced. If iron or steel are present for instance as a result of file wear, purify the scraps by melting with a flux of 1 part potassium nitrate (salpetre) and 2 parts potassium carbonate. After cooling, remelt with the sal ammoniac flux and pour the metal into a warm mold.
Gold comedy mask Italy.
3000Bc Repousse gold.