We all know about metallurgical assay and Cupellation is one of the oldest assay method used. Do you have any idea that this old method is still in use.
But what is this Cupellation exactly means???
It’s one kind of refining process in metallurgy. In this, ore or alloy are treated in a high temperature and with some controlled operations it extract the noble metal such as silver from the ore as for lead.
As the base metal does not react with the chemical and the ore does, it gets separated easily.
This method is used from the early Bronze Age to get silver and then in Renaissance period, it became one of the most popular method to obtain and refine the precious metals. And you know, this process is still hit in the market.
The primary tools for the Cupellation are knows as Cupels. They are finely made with lots of care. They are small vessels with the shape of inverted truncated cone. These Cupels are made out of bone ash. The ash has to be mixed with fine and homogenous powder and some sticky substance to mould the vessels. These moulds are mainly made of brass. These structure are still maintained today.
This Cupellation has mainly two categories, let’s have a look on them.
Large Scale Cupellation
Often the native silver is found in combination with other element or mineral. Hence at the primary steps it requires smelting and then the alloy is heated at the temperature of 960°C to 1000°C, the lead oxidises and this liquid lead is removed or absorbed into the hearth lining by capillary action.
The base of the hearth was dug in the form of a saucepan, and covered with an inert and porous material rich in calcium or magnesium such as shells, lime, or bone ash. The lining had to be calcareous because lead reacts with silica (clay compounds) to form viscous lead silicate that prevents the needed absorption of litharge. Some of the litharge evaporates, and the rest is absorbed by the porous earth lining to form “litharge cakes”
Now, come to the other part:
Small Scale Cupellation
This is based on the same principle as the one done in a cupellation hearth; the main difference lies between the both process in the amount of material to be tested or obtained. The minerals have to be crushed, roasted and smelted to concentrate the metallic components in order to separate the noble metals.
As the main purpose of small scale cupellation was to assay and test minerals and metals, the matter to be tested has to be carefully weighed.
Let’s have a look on a diagram of an assay furnace of Cupellation
Now, have a look on the movement of time period and the use of Cupellation for noble metal
- Near East in Anatolia and Mesopotamia during the 4th and 3rd millennium BC.- first known for use of silver
- Early Bronze Age – silver and lead found in litharage
- Iron Age – Cupellation done by fusing
- Roman Times – opened silver lead mines
- 12th Century A.D – written reference to cupels
- Renaissance – written in details the art of mining and testing of ores
- Some kind of blast furnace known as huayrachinas used in Peru and Bolivia to smelt the ores.
Have a look of few this age Cupellation Furnaces
Isn’t this Assay process amazing and quite primitive too. The most interesting part that it still didn’t loose it’s charm, it still has it’s fame in the market.