Once the concentrate had been collected, unwanted and contaminating minerals such as arsenic had to be removed.
Cleaning the tin ore removes any of the remaining impurities. It was roasted either in an open furnace or in a building known as a Burning House. This later developed into a mechanised Calciner. This process produced the final tin concentrate which could be sold and then smelted.
The earliest cleaning process was roasting or calcining. A calciner was a type of furnace, tin concentrate is fed through a hole in the roof and raked through the flames. This means the tin ore was roasted and burnt. This process was very polluting and labour intensive.
A new method of cleaning was developed in the 1930’s called froth flotation. Flotation uses chemicals and air bubbles. Tin particles, and sometimes iron, sink to the bottom and the waste material attaches to the air bubbles, making them float to the top.
The final stage of cleaning used a powerful magnet. Tin is not magnetic, but other metals, like iron, which is not picked up by froth floatation, gets caught by the magnet.