Bismuth (known in German as Bismut or Wismut) melts at around 271 degrees Celsius and expands when it solidifies, similar to water when it freezes. Pure bismuth is brittle and is therefore usually alloyed, but iridescent hopper crystals can also be grown from its melt.
Bismuth as an element and raw material
Bismuth is the chemical element with the symbol Bi and atomic number 83. In our shop, you will find bismuth as a pure metal with 99.99% purity—referred to in the industry as 4N—available as lumps, granules, pellets, and blocks.
Bismuth is primarily obtained as a by-product of lead processing, as well as during the refining of copper, tin, silver, and gold ores. It is technically in demand as a component of low-melting alloys and as a substitute for lead, for example in shot.
Overview of bismuth properties
| Property | Value |
|---|---|
| Melting point | approx. 271 degrees Celsius |
| Boiling point | 1,564 degrees Celsius |
| Solid density at 20 degrees Celsius | 9.78 g/cm³ |
| Liquid density at melting point | approx. 10.05 g/cm³ |
| Volume during solidification | Increase of about 3.3% |
The ratio of the two densities is striking. Liquid bismuth is denser than solid bismuth, which is why solidified metal floats on its own melt, just like ice on water. Alloys consisting of at least 50% bismuth also expand upon solidification, such as BiSn42 with 58% or Rose’s metal with 50% bismuth.
Toxicity and radioactivity
Bismuth is considered a heavy metal with low acute toxicity and is used specifically where lead needs to be replaced. However, this does not imply suitability for food contact or applications on the body.
Natural bismuth consists 100% of the isotope bismuth-209. This isotope is radioactive but decays with a half-life of about 19 quintillion years. Its specific activity is therefore extremely low and cannot be detected with standard measuring instruments.
Powder has its own hazard classification. Bismuth powder is considered a flammable solid, and airborne dust can explode under certain conditions. This classification applies to powder, not to the compact metal in the form of lumps, granules, pellets, or blocks. However, anyone grinding or filing bismuth creates fine dust themselves. In this case, follow the safety recommendations for bismuth powder: use a P1 class particulate filter and safety glasses with side shields.
Working safely with liquid metal
When melting, first watch out for water. If the melt encounters moisture, the water evaporates instantly; the steam builds up pressure and causes liquid metal to splash. Crucibles, ladles, moulds, and metal must therefore be dry before use. As long as the metal is liquid, do not cool the mould or the metal with water.
Heat a casting ladle before it touches the melt so that any adhering water evaporates. If the melt is already waiting in the crucible, dip the preheated ladle in and leave it there briefly until it has reached the temperature of the melt.
For steelworks and foundries, occupational safety regulations require safety goggles, non-flammable heat-insulating gloves, and clothing without easily melting synthetic fibers. In your own workshop, this is the equipment you should use as a guide.
Furthermore, do not heat bismuth more than necessary. When heated in air, it forms bismuth oxide, and if heated strongly, it burns with a blue flame to form yellow oxide vapors.
Melting and casting bismuth
Melt small quantities directly in a steel casting ladle, and larger quantities in a melting crucible. Our casting ladles are designed for tin, bismuth, zinc, and aluminum and can withstand temperatures up to about 800 degrees Celsius—well above what bismuth requires.
When it comes to the mould, the most important factor is that bismuth grows as it solidifies. The casting will wedge itself in steel or aluminum moulds, and it will warp in sand. Silicone is therefore best suited; graphite also works.
For the mould, our guide on building an oil-sand mould suggests using silicone rubber moulds as permanent moulds for metals with a casting temperature up to 400 degrees Celsius. A sand mould requires too much effort relative to the number of casts. Our heat-resistant silicone rubber HB is designated for casting tin, lead, and bismuth. The load limit for this rubber is 400 degrees Celsius, and at most 450 degrees Celsius briefly during pouring. You can read about how to set up and pour such a mould in our guide for pewter figures.
Keep the melt far enough above the melting point of around 271 degrees Celsius so that it does not solidify in the sprue, but at the same time below the stated limit of the silicone.
Growing your own hopper crystals
In nature, bismuth rarely forms well-defined crystals; it usually appears as foliated-granular aggregates. In contrast, colorful, step-shaped crystals are formed when molten bismuth cools slowly. This growth form is called a hopper crystal. Our bismuth lumps with 99.99% purity are suitable as starting material, as are the pellets. In our experience, you need at least one to two kilograms of bismuth for beautiful crystals. Let the melt cool slowly and seed it.
A common procedure looks like this:
- Completely melt the bismuth in a crucible or ladle. Impurities will separate and mostly float on top.
- Pour the melt into a second, dry vessel while holding back the dross on the surface.
- Allow the melt to cool slowly, for example by gradually turning down the heat. Generally, the slower this happens, the better the crystals will be.
- As soon as the bismuth begins to solidify, pour off the remaining liquid melt from the growing crystals. A good moment is when the surface looks solid but still wobbles slightly when tapped.
- Allow the crystals to cool completely before removing them from the vessel.
Freshly solidified crystals are silver and oxidize quickly on the hot surface. The thin oxide layer creates iridescent colors through interference; which color appears in a particular spot depends on the thickness of the layer. You can remelt the poured-off residual melt for your next attempt.
Low-melting alloys with bismuth
Alloys of bismuth with tin or cadmium have low melting points and are therefore used in fire alarms, fire extinguishing systems, electrical fuses, and solders. Our range includes bismuth alloys with tin, lead, indium, or silver as partners, which can be found under low-melting bismuth alloys.
For example, BiSn42, made of 58% bismuth and 42% tin, is lead-free. The alloy is eutectic and therefore melts at a fixed point instead of over a temperature range, specifically at 138 degrees Celsius, just like pure bismuth. The BiSn42 alloy is suitable for casting small parts but is brittle and can hardly be cold-formed. Too much bismuth quickly makes an alloy brittle. For parts that will be subjected to stress later, you should therefore check the strength values of the alloy against your application beforehand. With low-melting bismuth-based alloys like this one, you do not need a dross binder. In our guide to building a silicone rubber mould, the casting result shown—a five-centimeter-high soldier—is made of BiSn42. For small figures in silicone, this alloy is therefore an obvious starting point.
Our Rose’s metal, consisting of 50% bismuth, 25% tin, and 25% lead, melts at 94 degrees Celsius and is very hard and brittle due to the high bismuth content. It can only be ordered with a verified business or institutional account. If you want to cast below 100 degrees Celsius without lead and cadmium, you can use Field’s metal made of indium, bismuth, and tin, which melts at 62 degrees Celsius. It serves as a substitute for Wood’s metal, which contains lead and cadmium in addition to bismuth and tin.
Choosing the right delivery form
The right delivery form depends on your project. For crystals, use lumps or bismuth pellets. For weighing small quantities and for alloying, bismuth granules with a particle size of 2 to 4 mm and pellets weighing around 25 g each are suitable. Both are easier to dose than a block. For small castings, a ready-made alloy like BiSn42 is often a better choice than pure bismuth because it melts at 138 degrees Celsius instead of around 271 degrees Celsius.
Blocks of around 15.5 kg are intended for large-scale buyers. Small quantities up to a gross value of 6,000 euros can be ordered directly in the shop; larger quantities and special forms are available on request via the contact form.
Frequently asked questions about bismuth
Is bismuth magnetic?
No, not in the usual sense. Bismuth is not attracted to a magnet because it is the most strongly diamagnetic of all elements. Diamagnetic substances are weakly repelled by a magnetic field instead of moving toward it. Therefore, a magnet that sticks to iron will not stick to a bismuth bar.
How much bismuth fits in a crucible?
That depends on the volume of the crucible. Our crucibles are specified in cubic centimeters; the amount of metal is determined by the density of the metal. For solid bismuth at 9.78 grams per cubic centimeter, a 100 cc crucible theoretically holds just under one kilogram. Because bismuth is denser than copper, slightly more bismuth will fit than the copper specification in our crucible table indicates.
Are Bismut and Wismut the same thing?
Yes. Bismut and Wismut are two German names for the same chemical element with the symbol Bi and atomic number 83. You might encounter ‘Wismut’ in mineralogy, for example, when referring to native bismuth. Some product names also list ‘Wismut’ in parentheses; it refers to the same metal. The English name is bismuth.
