Pique the Geek 20110814: Tin, Less Common than You Think

(9 pm. – promoted by ek hornbeck)

One metal that we take for granted is tin, something that most of us see and handle every day in the form of “tin” cans, long used to store food. Actually, this invention dates only from the early 1800s when canning itself was invented, although tin plated iron and steel date back much earlier.

Tin is used it lots more than cans, however. Much of the tin used today is in the form of solder, used for joining other metals, particularly copper and brass, together. Formerly a 1:1 by mass mixture of tin and lead was used for soldering copper water pipes, but because of increased awareness of the dangers of lead, other solder compositions are now used for potable water.

Let us take a few minutes to explore this interesting metal.

Tin has been used since antiquity (since around 3000 BCE) as a component of bronze, an alloy of tin and copper (the proportions are variable) that is much harder than pure copper, which had been the metal of choice for earlier work. Bronze in not only harder than copper, it also melts at a lower temperature than pure copper, making it easier to cast. A sword made of bronze could easily cut through a similar one made of copper, so there was a real advantage to bronze.

Bronze was discovered by accident, when copper ores that contained tin as a significant impurity was used instead of more pure ores. This got people to try to figure out what was going on, and finally by purely empirical work tin ores were discovered and from then on tin was intentionally added to the copper.

Bronze is not much used now except for a very few industrial purposes. Most bronze is now used for decorative work, in particular statues and the like, and for bells. Bell bronze is particularly expensive, because it contains around 22% tin, (the balance copper) and tin is very expensive. For example, the latest price that I could find for copper is about $4 per pound, whilst the price for tin is around $11 per pound. Other bronzes range from around 4% tin up to 12% tin, the rest being copper.

Tin is element 50 in the periodic table and is notable for having 10 stable isotopes, more than any other element. As expected, the more abundant isotopes have an even number of neutrons, so that the isotopes 112, 114, 116, 118, 120, 122, and 124 account for 83.51% of all natural tin. Tin is fairly stable to air and is good at wetting many other metals in the molten state, so it is used widely for plating and has been for thousands of years.

Tin is a fairly rare metal, and is not found in too many places. Historically, the Cornwall region of the United Kingdom was a prime producer, but the mines have pretty much been worked out now. As with many other metals, China seems to have the greatest reserves of tin. However, the combined reserves of Peru, Brazil, and Bolivia are about equal to the Chinese reserves, so we are not in any danger of being held hostage for it.

In the old days, iron and steel were tinned by dipping clean sheet metal in molten tin either before or after fabrication. This is still the method used for artisan work, but the tinsmith is just about a dead profession now, with industrial scale manufacturing techniques being utilized now. However, in the 17th and 18th centuries tinsmiths crafted many household items. Since tin is very resistant to corrosion, iron and steel covered with tin is quite rustproof if proper precautions are taken. However, tin is fairly soft and is easily abraded, yielding bare steel that is very prone to rust.

Actually, zinc is a much better protective coating for steel. This has to do with the electromotive series in chemical physics. In this series, a metal that is higher in the chart than another one will tend to corrode until it is all gone, then the lower metal will start to corrode. Zinc is higher in the table than iron, and iron is higher than tin. Thus, when zinc coated steel is scratched, the zinc still corrodes before the steel does, thus acting as a sacrificial electrode. The steel will not begin to rust until almost all of the zinc is gone. However, since tin is below iron, when tin plating is scratched, the steel that is exposed corrodes much faster than if not coated at all.

This is all well and good, but zinc dissolves easily in acidic foods and zinc salts are quite toxic (not like lead, but still toxic) so zinc coated steel is useless for food applications. Even tin will dissolve in highly acidic environments, so tin cans are almost always lined with some sort of lacquer to keep the tin from coming in contact with the food. This is controversial, because many of the lining formulations contain bisphenol-A, a known estrogen mimic and potential endocrine system disruptor. Glass is best.

The United States produces no tin at all, so we are dependent mostly on South America for all of our supply. Tin is also absolutely needed for some applications that have to do with national defense, so it is a strategic material. During the buildup to World War II the old hot dip process for producing tin plate for steel cans was abandoned in favor of electroplating. By using electroplating, a much thinner coating of tin can be applied to the steel, with similar protective properties. Flash forward to today and there are a number of companies, such as Rex Plating, who specialise in electroplating and supplying plating machines and solutions for the process of plastic and metal plating industry.

So what are these strategic uses? Solder, for one. Almost all electronics have solder connexions, and this is probably the largest single use for tin now. I need not point out how essential electronics are for modern warfare, using components such as an HDI pcb can make all the difference when constructing electronics. Another large use is for bearings in automotive applications. Without bearings, no engine would run for more than a few minutes, because bearings are a variety of anti-friction materials. As for electronics, engines of all types are essential for modern warfare.

There lots of other uses for tin. Back before aluminum foil became widely available, tinfoil was actually used, but is much too expensive for modern usage. Aluminum is just over $1 per pound, 11 times less expensive than tin. Aluminum is also much less dense, so a pound of aluminum produces more square feet of foil than tin does. Back before the advent of plastic toothpaste and ointment tubes, the tubes were actually made of tin. I am old enough to remember those.

One use for tin that I bet that very few of the readers are aware is making glass. Yes, glass! In particular, flat window and plate glass. Have you ever been in a very old house and notices that the windows were sort of “wavy”? That is NOT because of the glass slowly “running” (glass sometimes called a supercooled liquid, meaning that it has no organized crystal structure, not that it flows). Back 100 years ago, window glass was manufactured by blowing a sphere (just like you see glassblowers do on TeeVee) and then cutting the upper surface of the sphere and allowing it to flatten on an insulated work surface. This produced the waves. I have an antique walnut curio cabinet with wavy glass doors, but I could not get a picture to turn out well enough to show you what I mean. Just take a look at the windows that next time you are in a really old house with the original ones. The house in which I grew up was built in 1912 and it has wavy windows.

Modern window glass is made by pouring molten glass on top of a molten tin bath, thus allowing the glass to become very flat. The glass is then pulled to an insulated surface and allowed to harden, with very few waves. So much for the “old glass is wavy because glass flows” myth. There is another variation of that myth, by the way. If you go into old churches with stained glass work, the thickness of the glass is almost always greater at the bottom of each piece than at the top. That is not about glass flowing, either. Centuries ago glass was not very uniform, and stained glass artisans almost always but the thicker part at the bottom just for stability.

Since solder is the major use for tin, let us discuss it a bit further. For the sake of brevity, we shall consider only tin/lead solders, but there are many others. These solders are good for joining several metals together, notably copper, silver, iron, and steel either to themselves of to other metals in the list. Gold is right out, because tin and gold form a brittle, difficult to handle allow, so tin free solders have to be used for gold.

There are two basic types of tin/lead solders: the eutectic and everything else. A eutectic mixture of two metals is a unique composition having the lowest (usually, but sometimes the highest) melting point, and I mean a point, not a range. For the tin/lead system, the low melting eutectic mixture is 63% tin and the 37% lead. It has a sharp melting/freezing point of 361.4 degrees F, about that of a medium oven. This is very often used in electronics for a couple of reasons. Since it is the lowest melter of any other combination, thermal damage to delicate electronic components is minimized. I have soldered many leads to many electric matches (used in pyrotechnics as an initiator), and was always thankful for 63/37 solder! Another advantage is that is either fluid or not, with no transition. Thus, as soon as the heat source is removed and the temperature of the work fall below 361.4 degrees, the entire solder joint solidifies. This is a real advantage for electronic and electrical work, because there is no “mushy” phase.

However, 63/37 solder is expensive! Remember, tin is $11 per pound, and lead is only about $1 per pound. The more lead in the solder, the cheaper it is. Common compositions are 40/60 and 50/50 (by convention, the tin content is always first, and all compositions are by mass). For general use, 50/50 is the most common. This is the solder that was used until recently for plumbing connexions, and as a joint cools, there is a definite “mushy” or “slushy” phase. Since the melting point of lead is well above that of the eutectic, crystals of lead begin to form in the liquid phase that is nearer the eutectic, making the solidifying joint slushy. In electronics, this can be a real problem, because until the temperature falls to below the melting point of the eutectic, the joint is very weak. However, this is an advantage in soldering plumbing pieces, because it gives you a little time to work. This is something you can talk about with plumbing companies in surrey if they are doing work in your home.

When “wiping” a joint for plumbing, solder is applied to both the female and the male pieces first, and then the larger of the two is reheated until the solder remelts, and then some. Sometimes the other piece has to be heated as well. Then the smaller piece is twisted (wiped) onto the larger piece until the temperature falls enough for the entire joint to solidify. This allows the joint to be completely water tight and resistant to high pressures. With 63/37, the solidification is so fast that gaps are likely because there is not enough time to give the joint a proper twist.

Before modern plastic automotive body filler (Bondo is one trade name) was available, body work was done with 40/60 solder, called body lead in the trade. Solder fills are much more robust than plastic ones, and since the metal actually alloys with the steel in the automobile body, there is no chance of water infiltration and thus eventual rust behind the patch. Very few people go to the trouble of using body lead now, but a few artisans still do. My neighbor’s son specialized in making low rider pickup trucks for income, and he uses body lead even now. He is a perfectionist.

Finally, there is one more routine use for tin that many of you are likely to encounter on a daily basis. It has to do with tooth decay and gum disease. Many dentifrices contain calcium carbonate (ground chalk) as a mild abrasive to remove stains and plaque when people brush their teeth. It is known that fluoride prevents cavities and kills germs that cause gingivitis, and that is fine. However, the cheapest form of fluoride, sodium fluoride, reacts with the calcium in the toothpaste and forms the highly insoluble calcium fluoride, with very little beneficial effect. Stannous (the +2 oxidation state of tin) fluoride is very resistant to reacting with calcium, so many toothpastes use the more expensive stannous fluoride to maintain their effectiveness. Check the label. If your toothpaste has calcium in it, make sure that the fluoride source is stannous fluoride. If it does not have calcium in it, then sodium fluoride is fine.

Oh, I mentioned that tin is not very common. Out of the 92 natural elements, tin is 49th in abundance in the crust of the earth, making is more rare than many other elements. I did not have time to discuss tin in US coins, so please ask me in the comments!

Well, you have done it again! You have wasted many einsteins of perfectly good photons reading this tin ear piece. And even though Rick Santorum comes to his senses and drops out of his windmill tipping when he reads me say it, I always learn much more than I ever could hope to teach by writing this series, so keep those comments, questions, corrections, and other feedback coming! Tips and recs are also welcome. I shall hand around as long as comments warrant tonight, and shall return tomorrow after Keith’s show for Review Time. Bill Nye indeed!

Warmest regards,

Doc, aka Dr. David W. Smith