GOLD ANNEALING CRUCIBLES, PARTING FLASKS AND OTHER ACCESSORIES
PARTING PROCESS IN CUPELLATION
After cupellation, we obtain a bead made of gold and silver. Therefore, it is still necessary to separate the silver from the gold . In order to achieve this, we attack two times the bead with boiling nitric acid and then we anneal it. This process is called “parting” and “annealing”. Gold annealing crucibles and parting flasks are the key instruments to achieve this.
The reason for using nitric acid, is that silver is soluble in it but gold is not. However, to ensure that no silver is secluded into the gold, and therefore protected from being dissolved, it is first necessary to laminate the bead into a cornet.
Finally, it is possible to do the nitric acid attack in quartz parting and annealing crucibles or in borosilicate parting flasks. Particularly, Quartz parting and annealing crucibles have the advantage of requiring less manipulation of the bead because the next step of the process (gold annealing) is done using this same crucible.
GOLD ANNEALLING PROCESS IN CUPELLATION
After the nitric acid attack, the obtained pure gold cornet is very brittle because it is covered by a multitude of holes left by the dissolved silver. At this point, it would be unsafe to weight the gold cornet due to the risk of breaking it and losing a part of the sample. For this reason , it is necessary to anneal the gold cornet in a quartz parting and annealing crucibles , fireclay crucible or porcelain crucible.
Therefore, the Annealing process consists of just heating the gold cornet above its recystalization tempearute (850ºC) but without melting it. After annealing, the cornet can be safely weighted withou risk of breaking it.
Fire Assay SL supplies the parting and annealing accessories to achieve this:
Parting Flask made with low thermal expansion borosilicate glass withstands both nitric acid attack and thermal shock.
Additionally, they have a tall narrow neck designed to fit inside gold annealing cups.
Finally, the dimensions of our parting flasks are: 230 x Ø 48 x Ø 18
Fireclay or Unglazed Porcelain Annealing Cups / annealing crucibles possess a smooth unglazed surface and thin walls that provide them with excellent resistance against thermal shock.
Furthermore, they have a tronco-conical form designed for gold bead annealing and for ignition of precipitates such a barium sulphate and copper sulphide.
|PORCELAIN UNGLAZED CORNET ANNEALING CUPS nº 226||33||35|
|CORNET ANNEALING CUPS nº 227||25||25|
|CORNET ANNEALING CUPS nº 230||32||30|
|CORNET ANNEALING CUPS nº 231||33||34|
Glazed porcelain crucibles to perform the parting and annealing process according to ISO 10378.
|Reference||Heigt / mm||Top Diameter / mm||Volume / mL|