What is Bismuth?
Bismuth is a brittle, crystalline metal that has a silvery white hue when freshly produced. Over time, surface oxidation lends the metal a pinkish or iridescent tinge. It can occur naturally in elemental form, as sulphide and oxide ores, and as mineral deposits. It is also obtained as a by-product of mining and refining other metals such as Copper, Silver, Gold, Lead and Tin.
As a heavy metal, it shares physical properties with other such metals such as Lead and Tin, however, it has unusually low toxicity, prompting the use of Bismuth and its alloys as a replacement for Lead in recent years.
Bismuth material properties
Among the most significant physical properties of this material are its iridescent oxide coating – exhibiting colours from yellow to blue – created on heating the metal, and the unique spiral, stair-stepped structure of Bismuth’s crystals. Though virtually unseen in nature, high-purity Bismuth can form these distinctive, colourful hopper crystals, using moderate heat, even in a home, stove-top setting. It is also denser in its liquid phase than its solid form, a characteristic it shares with materials such as Germanium, Silicon, Gallium and Water. This is because it forms a crystalline structure and expands 3.32% on solidification, a behaviour that has made it a long-time component of low melting typesetting alloys, where it compensates for the contraction of other alloying components.
Bismuth is the most naturally diamagnetic material, meaning it resists being magnetised and is repelled by a magnetic field. It also has one of the lowest values of thermal conductivity among metals and low electric conductivity. It has high electrical resistivity and a very low melting point. When Bismuth is burned in oxygen, it has a distinct blue flame, while its oxide forms yellow fumes. Bismuth is relatively nontoxic, especially compared to other heavy metals.
Bismuth melting point and crystals
It is known for its unusually low melting point, enabling the metal to form alloys easily, which in turn are used abundantly for applications such as moulds, fire detectors and fire extinguishers.
The low melting point – 271.5 °C or 520.7 °F – along with the material’s low toxicity, means that it can be heated at home, in stovetop settings as well. As it cools down, highly pure Bismuth is able to form stepped pyramids and other interesting forms – a type of crystal known as a ‘hopper’. These hopper crystals form because as Bismuth cools, it is strongly driven to quickly create hard edges in preparation for a cube-shaped crystal. However, these strong driving forces do not allow the faces to grow entirely. Instead, the forces impatiently form more and more edges, leaving behind metal spirals.
Bismuth metal is brittle on its own and is usually mixed with other metals for its applications. Its alloys with Tin or Cadmium have low melting points, owed to Bismuth’s properties, and are used in fire detectors and extinguishers, electric fuses and solders.
Used principally in alloys, metallic Bismuth contributes its own special properties of low and expansion on solidification. This means that it is a useful component of type-metal alloys, which are used to make neat, clean castings; and low-melting alloys or fusible alloys, which have a large range of applications especially in fire-detection equipment. Small concentrations of Bismuth are also used to improve the machinability of other metals such as Steel, Stainless Steel, Aluminium, other alloys.
Compounds of Bismuth have diverse uses, ranging from cosmetics to pharmaceuticals, and medicine to nuclear energy. Bismuth oxide is used as a yellow pigment for cosmetics and paints, while many cosmetics including lipsticks, eyeshadow and nail polish contain bismuth oxychloride, a pearly powder which makes these products shine. More familiarly, Bismuth subsalicylate, sold under brand names such as Pepto-Bismol and Kaopectate, is a well-known remedy for indigestion and stomach aches.
Bismuth in art and design
The physical properties of Bismuth – its iridescent hues on oxidation and unusual crystal forms – greatly attract artists and designers. This, along with its low melting point and low toxicity, mean that the material is easy to mould and shape. It also reveals aesthetically pleasing inherent hues and crystals forms. Bismuth is being used in applications ranging from jewellery and homeware, to sculptures, framed artworks and even as healing crystals.
The rainbow-like iridescent hues of the material are the result of a phenomena called thin film interference where variations in the thickness of Bismuth’s oxide layer, which form on the surface of the crystal, cause different wavelengths of light to interfere upon reflection, thus displaying a rainbow of colours. Growing these crystals and triggering such an iridescent effect it relatively easy, even at home, paving the way for increased use by creatives.