Linstrum
07-29-2007, 12:37 AM
Yesterday I was down at the industrial scrap recycling yard that has become my own little private gold mine. I found a newly arrived “gold deposit” that looked very much like tin metal. It was bright shiny new stuff that was in the form of easily bent 1/2-inch wide strips cut from thin sheets. Besides being easily bent, it was also light silver or aluminum-colored like tin is, too hard to scratch with my fingernail but still soft enough to be scratched with a penny just like tin. When I bent it, though, it did not squeak, crackle, or groan like tin does, which is the best way to identify tin when pure. Zinc will also make a crunching sound when bent, but the sound as well as the appearances of the two metals are quite different, zinc making a noise more like a pecan shell cracking than a squeak when bent as well as being a darker bluish colored metal with the typical gray color reminiscent of galvanized steel since zinc is what is used for galvanizing. However, just because some light silvery colored metal does not squeak and squeal when bent is not proof positive that the stuff in question does not contain a high concentration of tin metal since a small amount of alloying metal such as copper, silver, lead, antimony, bismuth, phosphorus, arsenic, etc, will prevent the phenomenon referred to as “tin cry”. The only way to tell for sure if an alloy has tin in it is to do a chemical test, which is actually not that difficult for tin, but the reagents needed are unfortunately no longer available without going to a chemical supply house, and those businesses will not sell to a stranger walking in from the street. But there is one test that will work in this particular case that is real easy and anyone who reloads or casts boolits already has all the right stuff to do it with just sitting there ready to go!
In this case, the only other thing my mystery metal could be is lead since it superficially resembles tin rather closely except for being softer. Freshly cut lead is very nearly the same silvery color as tin and the two can only be told apart after a few days when the lead begins to darken from the formation of lead rust, which is a dull dark gray, and tin does not rust. If alloyed to make solder or Babbitt with tin or any of the metals listed in the example above for tin “cry”, though, the color match is impossible to tell apart even after a few days since the alloying metals prevent the lead from rusting and keep it bright. The only means left to tell them apart very quickly at this point is to find their densities, or specific gravities, since lead is a substantially denser element compared to tin. Lead is 11.34 times as dense as water and tin is 7.265 times as dense.
To find the density of a sample of metal it is simply weighed while under water and then this weight is compared to its weight out of water. Theoretically the weight should be checked in a vacuum since air also supports and buoys-up all objects as well, but its buoying properties are tiny compared to water and for our purposes for extremely dense materials like metals it can be conveniently ignored. I tied one end of a six-inch length of fine sewing thread onto my piece of mystery metal and the other end onto the pan mount pivot of my 509.9-grain capacity Hornady-Pacific powder scale and weighed the piece with it dangling down below the top of my workbench. After weighing it I got a glass of water and carefully lifted the glass up underneath the piece of metal until I could see through the glass that it was totally submerged. I got a plastic drink straw and teased the air bubbles loose that were hanging onto the thread and then weighed the piece under water. I weighed it three times and the three wet weights all agreed, so I was done. All that was left to do was the math part, which is straightforward. The dry weight was 167.2 grains and the wet weight was 151.5 grains. Subtracting the two weights left 15.7 grains, which is the weight of water equivalent to the volume of the piece of metal. We want to find how many times more the weight of the metal is than the weight of its volume of water, so dividing the dry weight by the difference of the weights gives this number. The specific gravity of the piece is 167.2 divided by 15.7, which gives the number 10.64968. Rounding off that number to a legitimate value gives us a specific gravity of 10.65. Since it is a whole bunch more than 7.265 but pretty close to 11.34, it is a good educated guess that the mystery metal is mostly lead with a smidgen of something in it to make it harder than my fingernail, probably antimony, and judging from its shape and where I found it, its use was as tare weights for balancing some kind of aerospace devices. Since I get wheel weights free but this stuff was selling for $0.50 per pound, I left it.
In this case, the only other thing my mystery metal could be is lead since it superficially resembles tin rather closely except for being softer. Freshly cut lead is very nearly the same silvery color as tin and the two can only be told apart after a few days when the lead begins to darken from the formation of lead rust, which is a dull dark gray, and tin does not rust. If alloyed to make solder or Babbitt with tin or any of the metals listed in the example above for tin “cry”, though, the color match is impossible to tell apart even after a few days since the alloying metals prevent the lead from rusting and keep it bright. The only means left to tell them apart very quickly at this point is to find their densities, or specific gravities, since lead is a substantially denser element compared to tin. Lead is 11.34 times as dense as water and tin is 7.265 times as dense.
To find the density of a sample of metal it is simply weighed while under water and then this weight is compared to its weight out of water. Theoretically the weight should be checked in a vacuum since air also supports and buoys-up all objects as well, but its buoying properties are tiny compared to water and for our purposes for extremely dense materials like metals it can be conveniently ignored. I tied one end of a six-inch length of fine sewing thread onto my piece of mystery metal and the other end onto the pan mount pivot of my 509.9-grain capacity Hornady-Pacific powder scale and weighed the piece with it dangling down below the top of my workbench. After weighing it I got a glass of water and carefully lifted the glass up underneath the piece of metal until I could see through the glass that it was totally submerged. I got a plastic drink straw and teased the air bubbles loose that were hanging onto the thread and then weighed the piece under water. I weighed it three times and the three wet weights all agreed, so I was done. All that was left to do was the math part, which is straightforward. The dry weight was 167.2 grains and the wet weight was 151.5 grains. Subtracting the two weights left 15.7 grains, which is the weight of water equivalent to the volume of the piece of metal. We want to find how many times more the weight of the metal is than the weight of its volume of water, so dividing the dry weight by the difference of the weights gives this number. The specific gravity of the piece is 167.2 divided by 15.7, which gives the number 10.64968. Rounding off that number to a legitimate value gives us a specific gravity of 10.65. Since it is a whole bunch more than 7.265 but pretty close to 11.34, it is a good educated guess that the mystery metal is mostly lead with a smidgen of something in it to make it harder than my fingernail, probably antimony, and judging from its shape and where I found it, its use was as tare weights for balancing some kind of aerospace devices. Since I get wheel weights free but this stuff was selling for $0.50 per pound, I left it.