I'm still slowly working my way towards making nice Bismuth crystals. In the mean time, I have made some small experimental progress. It seems there is little information online about this, possibly because some folks have healthy businesses selling crystals. In fact, I have found some information on the web that is exactly wrong.
I have limited Bismuth to work with currently. Above is a 2" diameter geode on the left, filled with mostly blue oxides and a number of beautiful hopper crystals that the photo does not clearly show. On the right is a "swish" of a smaller amount of molten bismuth in the same sized crucible.
We have three things to consider here: color, size of crystals, and the habit.
Let's start with color. I personally think any color is nice and care more about the size and habit, but it seems color is a major factor in desirability for others. I mentioned earlier in Pt I that the colors are iridescence caused by oxides forming as the metal cools. Different thicknesses of this oxide layer yield different colors as it causes certain wavelengths of light to be cancelled out and others to pass. That is physics.
How do you control the oxide thickness? I can think of two primary ways:
1) Expose to more or less oxygen. The more oxygen, the thicker the oxide. No oxygen exposure at all yields no oxidation and you get the metal itself. I don't recommend exposing pure oxygen to molten metal, but the amount in the air that the melt is exposed to will help determine oxide thickness (think of it as a limiting reactant). When using a blow torch, an oxidizing flame should provide more oxygen than a reducing flame. Humidity is also able to give up its oxygen with enough heat. I suspect Colorado and The Gulf Coast, where I am, would show different behavior due to humidity and amount of oxygen in a given space of air.
2) Control the heat. The longer it takes to solidify, the more opportunity there is for exposed surfaces to form oxide layers, so thicker they will be. In the small experiment on the right, I mentioned that I "swished" the molten metal around in a circular pattern until dry. While the edges contained the bismuth that cooled last, the part ultimately exposed to air was only momentarily molten as it had already cooled a bit. Clearly a quicker cooling gives the golden color. There are a number of ways to control the length of time to cool. These include amount of material, how hot it is heated to, the environment that it cools in (including insulation), whether forced air or water is used, and so forth.
How does oxide thickness relate to color? I read online that it follows the electromagnetic spectrum. We probably remember the weird acronym ROYGBIV, or if you are British, the more straightforward, "Richard Of York Gave Battle In Vain," which also helps you remember your history and Shakespeare all at once.
The source light matters, as only that light is possible to be reflected back from the metal under the oxide layer. Your eyes also matter since we "loosely" have receptors for red, green, and blue. A television does not make yellow wavelengths, but makes our brains think it does by emitting green and red light. In nature, that could be yellow, or it could be green and red. We don't know, our brains get the same signal. Just ignore that if it doesn't make sense; we are combining chemistry, physics, and biology here. The point is that color we see is usually not a single wavelength and is very different than mixing crayola colors in elementary school, where we all learned that blue and yellow make green. Expect when it doesn't.
Anyway, I believe that the golds cool the fastest (forming thinner oxides) and then the reddish-golds (almost a copper color), then purples, then blues, and then a pale green. Since they are all metallic, they are hard to describe. I do not believe it corresponds directly to the spectrum, in either direction. I am sure that the silver color means little to no oxide layer so almost everything is reflected back. I'd love to do a formal study on this, but lack the equipment. Even if I am wrong on the order of the colors, you can play with temperatures and oxygen richness of the environment and make a variety of colors. That much we know.
What is the exact oxide formed? Bi2O3. Here is the reaction with humidity:
2Bi + 3H2O -> Bi2O3 + 3H2
You could possibly bathe your colored crystals in water with CO2 and change the chemical composition of the external layers. I am not sure what that would do to the iridescence.
OK, let's tackle size. This one is pretty simple. The longer it takes molten material to cool, the more chance the crystals have to form and grow. So get it very hot, and then try to cool it as slowly as possible (except for and parts you are trying to color a certain way). A bigger pot of molten Bi will take longer to cool than a small one (and givens more room for growth too). You may be able to allow for slow cooling by keeping a nominal amount of heat on (not enough to melt it). You may also consider insulating the molten material so it cools more slowly. It is just like Lego building. The more time you have, the bigger you can replicate a cubic structure out of the same size blocks. This is true for all crystals, and explains why you can't find any decent igneous crystals in Hawaii. The good stuff is way way below where the magma is cooling slowly for millions of years and the molecules have the time they need to find their friends and create large structures. Up top, the volcano spews out billions of billions of atoms regularly and they immediately cooled in the air and water, leaving lava rocks instead of mineral crystals.
Now we learned in the previous post that crystals form around a center of nucleation. Bismuth is good at providing this wherever it cools. Maybe it is the side of a dish. More likely it starts with a film then crust across the top where exposed to air. This is just what our sodium acetate trihydrate did. In this case, crystals will grow down from the top into the liquid Bismuth. Getting them out at the precise time without damaging them is tricky. My experiments show that I have not tackled this yet. My geode was nothing but a partially cooled dish of Bismuth with the molten middle dumped out. The crystallization started on the edges of the crucible where it cooled more quickly. Just like our volcano, the center of the melt will cool last.
Finally the most interesting thing to me about these crystals is their hoppered psuedocubic habit. Psuedocubic is a big word that means the crystals are rhombohedric in shape, but so close to 90 degree angles that it looks cubic. So we get a bunch of shapes that look like little buildings, some tall, some short, some long, some almost perfectly cubic. These shapes are simple, pleasing, and easy to recognize. What is not simple, and really wows people is the hoppered element. Hoppered crystals have grown so fast that they can't fill-in their centers as they expand. So they end up leaving stairstep vacancies as they tend to spread along their edges. It is really an amazing phenomenon. While my geode is full of nice obviously hoppered crystals, it is hard to photograph. You might see a little bit of it if you look carefully. I hope to make some bigger crystals in the future that show this more clearly. I will likely grow these from the surface down into the melt with a larger container and more bismuth.
Now you may wonder why Bismuth forms psuedocubic crystals and copper and silver and gold form dendritic shapes. I do, but I don't know yet. Maybe that can be in Pt III along with better crystals.
Thanks for reading,
Paul
p.s. Ignore anything about Bismuth being radioactive. It is so stable that they just recently proved it is technically radioactive. About .0000001g out of 100g will decay away every 14 billion years. I'm not really worried about that. My measurements are not that precise and the age of the universe is less than that.
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