ISO 11426:2014 pdf free
ISO 11426:2014 pdf free.Jewellery一Determination of gold in gold jewellery alloys Cupellation method (fire assay)
Weigh, as in 7.2.1, at least two proof assay samples of pure gold (4.5) and pure silver (4.4) in masses which correspond to the expected gold and silver contents (including the inquartation addition) of the assay sample. The total content of base metals in the assay samples is taken into consideration by the addition of a corresponding quantity of copper.
It is difficult to extract all the nickel in the alloy into the cupel by using the standard quantity of lead.Effective cupellation requires an additional 4 g of lead (4.3) and the use of larger cupels. This extra lead may be incorporated at the start of the test, if the cupel is large enough to contain the increased volume of melt. Alternatively, (preferably) a button of lead is added to the hot precious metal bead in the cupel after the lead oxide fumes from the initial operation have ceased. Care is needed if the cupellation furnace (5.2) is not adapted for this addition.The proof assays should contain approximately the same proportion of nickel as the sample.
For white gold alloys containing nickel, pre-treatment of the sample by scorification involves wrapping it in 2 g of lead foil (4.3). The sample consists of 125 mg to 250 mg of gold, and is inquarted with silver equivalent to 2,3 to 3 times the mass of fine gold present. Place this capsule in a scorification dish (5.10),together with 15 g of lead and 1,5 g to 2 g of Sodiumtetraborate (4.9) and heat to 1 000 °C in the furnace.
An increased air supply may be needed to oxidize the large quantity of lead. After 20 min to 30 min, when a liquid slag covers the surface of the dish, raise the temperature briefly to 1 100°C (for about 2 min). Remove the dish with the tongs, cool and separate the lead button from the slag. This button,which contains the original gold and silver, is cupelled as described in 7.2.3.ISO 11426 pdf download.
