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Mon, 28 Feb 2005 Bismuth

So, we received our bismuth metal and proceeded to do some informal experiments. I call them "informal" because we did not keep strict records, form a precise hypothesis, and work hard to vary only one variable at a time. However, we did form some interesting conclusions and observations.

Experiment 1: Diamagnetism

The first thing we tried to observe was the diamagnetic effect in bismuth. The idea behind "diamagnetism" is that a magnet will induce in some materials an opposing magnetic field. We tried to observe this using a set of small but very powerful neodymium magnets. I am sorry to say that we were unable to observe any noticeable magnetic repulsion using any amount of bismuth from tiny splatter fragments to a solid ingot of perhaps 200g in mass. I am not sure why we could not observe this effect, which is supposed to be quite strong. One hypothesis is that our bismuth was not pure bismuth, but I think this is very unlikely since in all other respects (melting temperature, formation of crystals, oxidation, color tint of the oxidizing melt, and behavior when the liquid metal was dripped into water) it behaved exactly as expected). Also, it is my understanding that the bismuth would not need to be absolutely pure to show the diamagnetic effect. We will have to do a little more reading and perhaps ask some other people.

Safety

For the hot metal experiments, we made sure that we were wearing reasonably durable clothes that covered most of our skin, that we both had eye protection, and that we had a ready supply of water nearby to put out any fires, including a 32-ounce cup pre-filled and ready to go. For eye protection, I have prescription polycarbonate lenses which I judged to be adequate. Isaac put on a pair of polycarbonate sunglasses when he came near the melted metal. We used a thick cloth potholder to grab the handles of the cups, which was adequate, since the metal handles acted as heat-sinks anyway, although a silicone glove might have been provided a better grip.

Experiment 2: Melting

The second informal experiment was done to answer the question "can we melt bismuth on our stovetop?"

Our stove is a rather low-end home stove, and the oven is not very powerful, frequently doing a poor job getting the right amount of heat for baking, so I was not completely confident that we'd be able to get the metal hot enough.

Our methodology was to place 5 10-gram cylinders of the metal, as it arrived from the supplier, into a new stainless steel 3/4-cup measure. We placed this directly on the heating element and set the temperature control to 75% of the way to maximum. I was prepared to wait a while for melting to start, but Isaac noticed that the metal began melting almost immediately. So the answer was "yes." It did not even seem necessary to set the heat to maximum.

Experiment 3: Crystals

The third informal experiment was done to answer the question "can we form macroscopic bismuth crystals?"

The methodology was to melt 100 grams of bismuth and then remove the cup from the heat, allowing it to cool for several minutes without disturbing it, then pouring off the remaining melted metal into another heated cup.

We ran into complications because I had thought that we could place the hot cup on a heavy wood cutting-board in order to to allow it to cool slowly, rather than on a more heat-conductiven metal surface. The heat was too much for the wood, though, and the cutting board smoked and steamed underneath the cup. We increased the ventilation, but this was not really satisfactory. The smoke detectors did not go off, but it was at this point that Grace took the baby from the living room downstairs with her to the laundry room so that the smoke would not irritate her.

Despite this complication, after the first pour-off, we noticed visible crystals. After the bismuth and cup had cooled for several minutes, it was cool enough to try scraping some of the tiny crystals out of the cup, which I did using a wire hanger.

We broke one small crystal, about 3 millimeters across, that exhibited "hopper" structure. This was not very impressive, but it proved the concept. I threw this one back in the melted metal in the hopes of getting larger crystals on another pass. In retrospect, I should have saved it, for we were not able to get good crystal specimens after this.

The plan was to try using different cooling times to determine the cooling time which produced the most crystals. Because I was trying to find a solution to the smoke problem, we were not able to repeat the experimental conditions accurately while changing only one variable. I tried using ten layers of aluminum foil between the steel cup and the wooden board, reasoning that the foil would deflect some of the heat. This did not really solve the scorching problem; it just scorched more slowly. Moreover, the use of the foil dramatically changed the characteristics of the cooling metal. It no longer cooled first from the sides and bottom, but seemed to cool from both the top and bottom equally, so that when we attempted to pour off the remaining melted metal, it poured not from the top but from "holes" in what had become a semi-crystalline slab of brittle, oxidized bismuth. This brittle mass was clearly formed of a crystallized form of the metal, but it did not form attractive individual crystals.

I should say a few words about oxidation. We observed as we poured off, melted, and re-poured off the metal, it developed a "sheen" of various colors, most notably a vivid blue and green, as well as an oxidized, ugly "skin" of powder-gray metal, which would not melt. We were able to skim this off using the wire hanger and collect a heap of powdery oxidized bismuth. After working with the metal for a while, the cups became scarred and pocked with a yellowish oxide on the bottom, and lumps of scaly oxide on the sides, which we were unable to scrape off using the wire. At higher temperatures the liquid metal could be skimmed with the hanger, which would leave behind a clean, shiny surface, which would then immediately begin to develop a powdery-gray appearance.

This bismuth oxide, which we were not able to melt, could presumably be "reduced" using charcoal and a crucible, but we did not have an apparatus to do this, and so our shiny melted bismuth became gradually more and more contaminated with oxide. The smoke from the scorching cutting board, and possible varnish on the wire hanger, may have contributed to this contamination.

So, result was that we were able to grow at least a few small macroscopic crystals, but we were not able to successfully experiment with melting and cooling conditions or improve the results. I was not terribly disappointed by this as I was not really expecting to be able to grow large, beautiful crystals with this simple setup.

Experiment 4: Casting

A fourth informal experiment was done to answer the question "Can we cast the molten bismuth into a solid ingot using a mold made from folded aluminum foil?"

This part was very interesting. The answer was technically "yes," although with extreme reservations.

I folded a dozen layers of foil into a small box shape, and placed it on the cutting board, and poured about 100 grams of molten bismuth into the mold.

The result was somewhat startling. The combination of hot bismuth and aluminum foil immediately produced a thick black smoke, so we cleared the area and increased the ventilation. The smoke was not coming from the wood underneath, which did scorch as expected, but seemed to come the mold itself. The foil did not visibly burn, but may have either burned under the molten bismuth or have somehow allowed the bismuth itself to oxidize with some violence.

The molten bismuth did form a solid ingot, which upon cooling we were able to remove from the foil, which did not appear particularly burned. The top of the ingot was coated with a black powdery residue, which was much more difficult to remove from my fingers than the bismuth oxide. We washed and dried the ingot. Some of the aluminum foil on the bottom had stuck to the metal and could not easily be removed. We decided to melt the ingot back down and remove the foil from the melt. However, once it was melted, we were not able to find the foil, so I assume that it burned up. Our melt was now presumably contaminated with aluminum oxide, which probably would have ruled out any further crystal growth.

So, in conclusion, it was possible to use foil to form a mold and create an ingot, but the heat produced a potentially dangerous reaction, so I would not want to try this again. Powdered aluminum is used as an explosive, and aluminum is apparently too prone to oxidation to use safely for such a purpose, although I imagine that a solid piece would not produce smoke so readily. We are not certain of the toxicity of the generated smoke, but presumably it was not a good idea to be breathing it. I was glad that the baby was out of the room and that we had good ventilation.

Experiment 5: Spatters

A fifth informal experiment was done to answer the question "what happens when you drop the molten bismuth metal into water?"

We tried using several different size containers and dropping the metal in different ways, ranging from pouring individual drops into the water up close, to pouring a stream from high up.

For these experiments we used a small steel bowl on the counter, and also a large bucket on the floor. This part made it clear how important it was to have eye protection. The hot metal produced small steam explosions, which resulted in sprays of water droplets as well as occasionally very small droplets of molten or near-molten bismuth. The result was somewhat like the spatters that can happen while cooking bacon in a frying pan. I received some very minor burns on my forearms, but I had expected that this might happen, and none of the burns were severe enough to require treatment.

Pouring pure bismuth into a small container in a steady stream produced very elaborate spatter shapes that would stick together in a semi-solid "forest." Doing this raised the temperature of the water to the point where it was steaming, and produced a lot of spatter.

Dropping bismuth into the larger bucket produced at least four somewhat distinct results.

Dropping the metal from several feet produced "exploded" shapes, where it appeared that the molten metal actually splashed upon hitting the water. The shapes ranged from very thin foil-like fragments to spheres and teardrops that seemed "exploded" -- hollowed out.

Pouring the bismuth very close to the water surface resulted in elongated, needle-like shapes with points at both ends, some several inches long.

Pouring the bismuth carefully drop-by-drop resulted in a large number of small, nearly identical "teardrop" shapes. These were so remarkably uniform and attractive that I separated these out and set them aside to put in a jar for display.

In the bottom of the bucket we also collected a very fine "sand" or "grit" of dark, oxidized-looking bismuth. It is not clear whether this was solid oxide, or how it was formed. Some hypotheses include: it oxidized by the dissolved oxygen in the water itself; the hot metal actually released oxygen from the water; it was produced by the explosion within the steam bubbles; it was somehow separated from the actual bismuth metal by the dropping process.

We collected up some of the less attractive pieces, dried them, and put them back to melt again. We discovered that it was extremely important to dry the bismuth thoroughly. Some of the hollowed-out shapes still contained enough water to cause a more violent steam explosion, which blew tiny molten metal droplets everywhere. This served as a good warning; if we try to melt down more of the spatter we will bake it at a low temperature first to drive off the water and then heat it gradually to melt it to avoid this violent steam release.

Experiment 6: Casting (again)

As a final experiment, we poured some of the remaining oxidized and unattractive melted bismuth into a cup and allowed it to cool to room temperature, in another attempt to make an ingot or slab for examination later. When it was cool I was able to remove it from the cup by knocking the cup hard against the wooden cutting board.

End Products

The experiments left us with several end-products:

  1. A highly oxidized, ugly, irregular disc of bismuth combined with whatever oxides or other contaminants are present, with a powdery crust, partially yellow on the outside. I broke this into several pieces and put it into a bag. The broken edges exhibit a very shiny silicon-like appearance. The slab is extremely brittle.

  2. A badly scorched wooden cutting board. I'll save this for now in case weneed to use it again, but we will probably discard it eventually because it smells very smokey.

  3. Several "roasted" steel measuring cups. These are discolored on the outside from the heat. I scrubbed most of the oxide out with steel wool, but some remains that is too hard for me to remove. We will save these for possible use in melting down some of the spatter, or making more spatter, but they may be too contaminated to grow crystals.

  4. One more clean unused steel measuring cup. We may use this for a future attempt to grow crystals.

  5. Perhaps 300 or 400 grams of unused bismuth. We'll save this for a future experiment.

  6. Several baggies with different kinds of spatter: one with very attractive teardrop shapes, which I would like to save; one with "exploded" shapes, one with "needle" shapes, and one that is a jumble of all the rest, most of which is in the form extremely small irregular foils, lumps, crumbs, particles, and fragments.

Paul's Conclusions:

This was a fun way to get the urge out of my system to play with molten metal. Although it was more a demonstration than a formal experiment, we did learn some interesting things and it brought up some open questions for further, more formal, experimentation.

I did wind up making a couple of blunders that caused safety risks. The biggest of these were super-heating the aluminum foil, and putting spatter products that were still damp back into the melt. Fortunatley these do not seem to have caused any long-term harm, although I regret the exposure to the smoke generated by the combination of the hot bismuth and aluminum foil. The scorching of the cutting board was unfortunate but I don't believe it represented a serious risk of fire, because the wooden board was too large and flat to ignite.

Questions for Isaac:
  1. Read the chapter on bismuth in the Elmsley element book.

  2. Examine the contents of the various baggies, particularly the broken "ingot" or solid disc, and the long "needle" shapes. (Wash your hands with soap and water after handling this oxide, since it will stick to your hands).

  3. We can describe metals as ductile (bendable) or brittle (breakable). How would you describe the needle shapes? How would you describe the ingot?

  4. Can you explain the difference in color between the somewhat shiny teardrop shapes and the grayish surface of the ingot? How about the shiny broken edges of the ingot?

  5. Are the needle shapes crystallized metal? Explain. Why or why not?

  6. Is the ingot crystallized metal? Explain. If so, is it one crystal or many crystals? Can you see the crystals? Why or why not?

  7. Explain how to convert the melting temperature of bismuth from Celsius to Fahrenheit.

  8. What would you do differently if you had a chance to do more experiments with bismuth? Particularly, address how to improve safety, how to cool the molten metal without using the wooden board. Which experiment would you like to do?

  9. For the experiment you would like to do, write down the hypothesis, the planned method, the expected result, the reason the expected result would confirm the hypothesis, and an alternative explanation for the expected result that does not confirm the hypothesis.

Isaac's Conclusions:

(to be done!)

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