Gold (Au) is a precious metal primarily used in jewellery production; as much as 50 weight percent (wt. %) of annual consumption. Because pure gold is very soft and ductile, we add different metals as alloying accessories. The most important are silver and copper, but nickel, zinc and palladium are also used. In jewellery, caratage is very important. Carat (ct) is the unit of measurement for the purity of gold. Pure gold is defined as 24 carat gold. One carat is the equivalent of 41.66 particles of pure gold in 1000 particles of alloy. In Europe, jewellery is mostly made from 14 or 18 carat gold, but it can, of course, be made from alloys with higher or lower gold content. Precisely because of the wide variety of alloying options for gold, it is important to find out the impact of alloy elements on properties of gold such as the colour shade etc.
As part of my thesis, we researched the changes in hardness and colour shade depending on the fraction of alloy elements in 14 ct gold. The experimental work was carried out on the premises of the department of materials and metallurgy, and the department of textiles, graphic arts and design. We made three samples of 14 ct gold alloys with different contents of silver and copper in each individual sample. 14 ct gold contains 58.8 wt. % of pure gold (Au), what remains are alloy elements which are added accordingly to the desired colour of gold. The material for the fourth sample was prepared at my family business premises “Zlatarski atelje Kodre”. We added 16.66 wt. % Ag and 8.33 wt. % Cu to pure gold since 18 ct yellow gold contains 75 wt. % Au. The usual ratio of alloy elements in yellow gold is 2/3 silver and 1/3 copper. The weighted samples were placed in a furnace at 1138 °C until they melted, and then they were left in the furnace to slowly cool down, after which they were put under a microscope. Later, we also measured the samples’ harness according to Vickers. Each sample was subjected to several measurements on the most appropriate spots of which the mean value of the measurement was determined.
To determine the colour shade of each sample, we photographed the samples with a colour check sheet in a light booth. The photographs were corrected with Adobe Illustrator and their CIELAB system measurements were taken using a different program in order to get the coordinates of the system that represents the colour shade.
We came to a conclusion that copper is best at hardening the gold and it also gives gold a reddish shade. The lowest hardness value came from the first sample, to which silver was added, which has a similar hardness value as gold. We measured that 18 K has a higher hardness of 14 K, which is contrary to expectations. This can be justified by a different manufacturing process that had 18 K gold sample from other samples.
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