TIN

Tin

Chemical symbol Sn
Atomic number 50
Melting point 231.91 C boiling point 2625 C
density 7.29 grams per cubic centimetre

 

 


Where does tin come from?

Tin is obtained from the mineral cassiterite which is tin oxide SnO2. Brazil, Bolivia, China, Indonesia, Thailand and Malaysia and now Peru are significant producers. The Cornish mines were almost completely wiped out by the price collapse of 1985 with only South Crofty still working in recent years. Whilst idle and almost completely flooded negotiations continue to purchase and restart the mine (November 2000)

Smelting

(tin smelting furnace)

Cassiterite is smelted to metal by reduction with carbon most commonly in a reverberatory furnace. Temperatures in excess of 1200°C are required. The difficulty is that cassiterite is hardly ever produced entirely free from other minerals many of these are reduced to metal at the same time forming alloys with the tin. It is therfore necessary to refine the tin to make it commercially useful. Fire refining involves various procedures on the molten metal. Iron is removed by passing steam through the molten metal, arsenic and antimony are removed by additions of aluminium alloy and copper is removed with sulphur. Very impure tin can be refined by electrolysis to very high purity.

The furnace pictured in section is a fairly small Cornish one probably around 1900, modern furnaces are generally larger and oil or gas is usually used to heat the charge of concentrate and anthracite together with limestone for flux. As the tin metal forms it sinks to the bottom of the furnace and is eventually removed via a tap hole


Tin and its Alloys

One of the most important properties of tin is the ease with which it alloys or mixes with the majority of other metals, it is this quality together with the low melting point which makes it an essential ingredient of most solders. It is not toxic and it does not corrode all that rapidly making it ideal as a protection for steel for food and drinks 'tin cans' properties which are also important in pewter. The very high boiling point allows it to be used as a smooth molten surface to make 'float' glass.

The Tin wedding anniversary celebrates 10 years of marriage making tin or pewter the perfect gift for such an occasion.

The 'cry' of tin is a property that when bent a cracking noise is emitted, some high tin alloys exhibit this property to a lesser extent. We have a pewter composition Carn H1 that gives off a pinging sound as it cools after solidification, this particular alloy contains tin antimony copper and bismuth.

Some remarkable tin alloys

Did you know that an alloy of 80% gold 20% tin melts at around 280°C this would easily allow casting into rubber moulds! Has anyone tried this? I've got the tin if you've got the gold! apparently this alloy is used as a special solder and has a slight green colour.

An alloy of tin, bismuth, lead and cadmium in the right proportions will melt well below the boiling point of water at just 70C

Pewter is a tin alloy which has had many compositions through the ages the only common ingredient being a high tin content, pewter exploits the beauty and ease of working of tin with additions of other metals being made mostly to strengthen it.

Bronze is regarded as an alloy of tin and copper usually less than 12% tin although it often includes other metals to meet specific requirements. Bronze is the first tin alloy used by man but there is much debate about when and how we first deliberately mixed tin and copper as an alloy. Many early 'bronzes' don't contain much tin. Higher tin bronzes are used to cast bells. Brass does not normally contain tin being an alloy of copper and zinc

Solder alloy compositions are numerous but the most important are still the tin lead solders which were used by the Romans. Tin melts at around 232°C and lead at about 327°C in the combination 62Sn 38Pb the resulting alloy melts at 183°C. This composition is known as the eutectic. Solders used in electronics account for a significant proportion of tin consumption. Lead free plumbing solder is often tin with about 0.5% copper although many other compositions have been developed.

Further Reading

Tin and its alloys and compounds - B.T.K. Barry & C.J. Thwaites
Tin in Antiquity - R.D. Penhallurick
Extractive Metallurgy of Tin 2nd edition P.A. Wright (the first edition is very useful as well)

We can supply tin of high purity
Cornish Tin Vase
About Tin Mines : Carn Metals Home Page
Mould making for tin alloys : Tin Alloy Casting Service

LINKS

Bronze Age Craft - Information on early metallurgy - Courses etc.

Tin Mines Under the Atlantic Ocean!

Visit the online EXHIBITION

St Just Mines Research Group

 


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