Boron

• 1. Introduction

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  Boron, the element with the symbol B and atomic number 5, owes its existence to events far from any laboratory. Stars do not build it in their cores. Instead it arises when cosmic rays break apart larger atoms and when supernovas explode. That odd birth makes it scarce in the Solar System and in the Earth's crust. Yet it turns up in laboratory glassware, in laundry products, in rocket ignition, and in experimental cancer treatments. As a crystal it is a brittle, dark, lustrous metalloid. As an amorphous solid it is a brown powder. This documentary asks how people used boron compounds for centuries before anyone knew the element existed. It asks who finally separated it, and why a few places dominate its supply. It also asks how a single isotope's talent for catching neutrons reached power plants, hospitals, and spacecraft.

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• 2. Borax Before Boron

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  Around 300 AD in China, a mineral called tincal found a job as a glaze. Tincal is the natural mineral form of borax. Some crude borax traveled westward and was apparently mentioned by the alchemist Jabir ibn Hayyan around 700 AD. In the 13th century, Marco Polo carried some glazes back to Italy. Around 1600, Georgius Agricola reported borax being used as a flux in metallurgy, meaning an additive that eases the working of metal.

  The hot springs near Florence, Italy, called soffioni, entered the story in 1777. Boric acid was recognized there, and it came to be called sal sedativum because of its supposed medical benefits. The mineral form was later named sassolite, after Sasso Pisano. Sasso served as the main source of European borax from 1827 to 1872, when American sources replaced it.

  Francis Marion Smith's Pacific Coast Borax Company changed the picture in the late 1800s. Boron compounds had rarely been used before then. His company popularized them by producing them in volume at low cost. The word boron itself arrived later, coined from borax by analogy with carbon, which boron resembles chemically.

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• 3. Hunting Pure Boron

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  In 1808, Sir Humphry Davy sent electric current through a solution of borates and saw a brown precipitate form on one electrode. He then set electrolysis aside and used potassium to reduce boric acid. That produced enough material to confirm a new element, which he named boracium.

  Joseph Louis Gay-Lussac and Louis Jacques Thénard reached the element by another path, using iron to reduce boric acid at high temperatures. They oxidized their boron with air and showed that boric acid was the result. Jöns Jacob Berzelius identified it as an element in 1824. The American chemist Ezekiel Weintraub arguably produced pure boron for the first time in 1909.

  Purity has stayed difficult. Most studies of so-called boron actually involve samples that contain small amounts of carbon. Early routes reduced boric oxide with magnesium or aluminium, but the product was often contaminated with borides of those metals. Pure boron can be made by reducing volatile boron halides with hydrogen at high temperatures. Ultrapure boron for semiconductors comes from decomposing diborane at high temperatures, then refining it by zone melting or the Czochralski process. Because the element is so hard to make, chemists build boron compounds from borates instead.

  The crystalline material is formidable. It rates 9.3 on the Mohs scale, melts above 2000 °C, and resists boiling hydrofluoric and hydrochloric acid. Boron forms four major allotropes, which are different structural arrangements of the same element. Beta-rhombohedral is the most stable and the most common. Most phases are built from B12 icosahedra, which are twelve-atom cages with twenty triangular faces. Compressing boron above 160 GPa yields a phase whose structure is still unknown, and it becomes a superconductor below 6-12 K.

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• 4. Mining the Borates

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  Boron makes up only about 0.001 percent of the Earth's crust by weight. The wider universe offers little more. The Big Bang formed a negligible amount, and the interiors of stars destroy boron rather than make it. At the oxygen-rich surface of the Earth, boron is always found fully oxidized as borate. Uncombined boron appears only in small amounts in meteoroids, with extremely small traces detected in lunar regolith.

  Water rescues the situation. Many borates dissolve, so water can concentrate them, and industry mines them as evaporites such as borax and kernite. More than a hundred borate minerals are known. Four of them, colemanite, rasorite (also called kernite), ulexite, and tincal, make up 90 percent of mined boron-containing ore.

  Central and western Turkey hold the largest known borax deposits, including those in the provinces of Eskişehir, Kütahya, and Balıkesir. Turkey possesses 72 percent of the world's known deposits. A state-owned company, Eti Maden İşletmeleri, holds a government monopoly on mining borate minerals there. In 2012 it held a 47 percent share of global borate production, ahead of its main competitor, Rio Tinto Group.

  The United States is the other major producer. Almost a quarter of global boron production, 23 percent, comes from the Rio Tinto Borax Mine, also known as the U.S. Borax Boron Mine, near Boron, California.

  Supply is not close to running out. Proven reserves exceed one billion metric tonnes, against yearly production of about four million tonnes. Producers have still kept expanding. Eti opened a boric acid plant at Emet in 2003 with a capacity of 100,000 tonnes per year. Rio Tinto raised its boron plant from 260,000 tonnes per year in 2003 to 310,000 by May 2005.

  The price gap between forms is striking. Crystalline elemental boron cost US$5 per gram in 2008, while borax and kernite together were priced at US$377 per tonne in 2019.

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• 5. Glass and Hard Materials

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  About 46 percent of boron use worldwide goes into glass fiber for insulating and structural fiberglass, especially in Asia. Boron enters the glass as borax pentahydrate or boron oxide and shapes the strength or fluxing qualities of the fibers. E-glass, rich in boron, is named for Electrical use. C-glass also carries a high boron oxide content and serves glass staple fibers and insulation. D-glass is named for its low dielectric constant.

  Another 10 percent of production goes into borosilicate glass, which is typically 12-15 percent boron oxide and 80 percent silica, with about 2 percent alumina. Its low coefficient of thermal expansion gives it good resistance to thermal shock. Schott AG's Duran and Owens-Corning's trademarked Pyrex are two major brand names, used in laboratory glassware and in consumer cookware and bakeware. Ceramics take about 15 percent of global boron, agriculture 11 percent, and bleaches and detergents about 6 percent. In the United States, 70 percent of boron goes into glass and ceramics. Sodium perborate has served as a bleach, although the laundry product Borateem no longer contains boron and uses sodium percarbonate instead.

  Cubic boron nitride, sold under the trade name Borazon, is a superior abrasive. Its hardness is only slightly below that of diamond, and its chemical stability is greater. The resemblance to carbon is no accident, since a boron atom and a nitrogen atom together have eight valence electrons, like a pair of carbon atoms. Hexagonal boron nitride mimics graphite and serves as a high-temperature lubricant. Boron carbide is a ceramic made by heating boron oxide with carbon in an electric furnace. Its complex structure gives it great strength for its weight. Metal borides such as TiB2, ZrB2, and HfB2 melt above 3000 °C and find use as hard materials for cutting tools.

  Boron fibers are made by depositing boron onto a tungsten filament through chemical vapor deposition. They go chiefly into advanced aerospace structures, with limited production for golf clubs and fishing rods. Laser-assisted deposition can even grow sub-millimeter crystalline boron springs, with an elastic modulus of 450 GPa and a fracture stress of 17 GPa.

  Boron also sits inside neodymium magnets, written Nd2Fe14B, which are among the strongest permanent magnets. They appear in MRI systems, computer hard disk drives, CD and DVD players, and the tiny speakers of mobile phones. In steel, a few parts per million of boron increase hardenability.

  Semiconductors use boron as a dopant. Having one fewer valence electron than silicon, it donates a hole and produces p-type conductivity. After the 1970s, ion implantation, which relies mostly on boron trifluoride, largely replaced high-temperature atomic diffusion for adding it.

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• 6. Explosive Chemistry

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  One widely used laboratory procedure warns that pentaborane will "spontaneously inflame or explode in air." Pentaborane belongs to the boranes, compounds of boron and hydrogen. Diborane, the dimer of borane BH3, has a formula akin to ethane's, C2H6. Its structure is very different, with a pair of bridging hydrogen atoms. That arrangement was deduced only in the 1940s, and it was an early sign of how strange boron chemistry could be.

  William N. Lipscomb received the 1976 Nobel Prize in Chemistry for "studies on the structure of boranes illuminating problems of chemical bonding." His subject included large cluster compounds, in which boron atoms have a coordination number greater than four. H. C. Brown won the Nobel Prize in Chemistry in 1979 for hydroboration, a reaction that adds boron-hydrogen bonds to carbon bonds and opened many routes to complex organic compounds. Akira Suzuki shared in the 2010 prize for a reaction that bears his name and uses boron compounds in the synthesis of pharmaceuticals.

  The volatility of boranes tempted fuel researchers. In the 1950s, several studies examined boranes as energy-increasing additives, called "Zip fuel," for jet fuel.

  Triethylborane found a lasting job as an igniter. It ignites the JP-7 fuel of the Pratt & Whitney J58 engines that powered the Lockheed SR-71 Blackbird. It also lit the F-1 engines of the Saturn V rocket used in NASA's Apollo and Skylab programs from 1967 to 1973. Today SpaceX uses it to ignite the engines of its Falcon 9 rocket. The reason is its pyrophoric nature, meaning it ignites on its own in air, and it burns at a very high temperature. Amorphous boron, for its part, gives pyrotechnic flares a distinctive green flame.

  New bonding patterns are still being found. A compound with a boron-carbon triple bond was synthesized for the first time in 2025.

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• 7. The Neutron Catcher

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  Natural boron is a mixture of two stable isotopes, boron-11 at 80.1 percent and boron-10 at 19.9 percent. Thirteen isotopes are known in all. The shortest-lived, boron-7, decays by proton emission and alpha decay with a half-life of 3.5 times ten to the minus 22 seconds.

  Boron-10 is the isotope that matters to the nuclear industry, because it captures neutrons. Boron carbide absorbs neutrons without forming long-lived radionuclides, especially when doped with extra boron-10. That makes it attractive for shielding, control rods, and shut-down pellets in nuclear power plants. Inside control rods, boron carbide is often powdered to increase its surface area. The industry enriches natural boron to nearly pure boron-10, and the less valuable by-product, depleted boron, is nearly pure boron-11.

  In pressurized water reactors, boric acid in the cooling water acts as a neutron poison that offsets the changing reactivity of the fuel. Its concentration is highest when new rods are inserted and falls over their lifetime. After a shutdown for refueling, boron-10 boric acid is added to the coolant. At restart it is slowly filtered out over many months as fissile material is used up.

  Medicine borrowed the same trick. In boron neutron capture therapy, a compound containing boron-10 is built into a pharmaceutical that a tumor selectively takes up. The patient is then treated with a beam of low-energy neutrons, which trigger short-range alpha particles and lithium-7 ions inside the tumor cells. BSH and BPA have been used clinically, and accelerator-based neutron sources have been developed as an alternative to reactor-based sources. Trials have covered glioblastoma, head and neck cancer, cutaneous melanoma, hepatocellular carcinoma, lung cancer, and extramammary Paget's disease. The main challenge is finding boron agents with higher boron content and better targeting.

  In space, the same neutron appetite becomes a hazard. Cosmic radiation striking a spacecraft produces secondary neutrons, and boron-10 in the chips captures them, releasing a gamma ray, an alpha particle, and a lithium ion. These products can flip bits and cause data loss. Radiation-hardened designs use depleted boron, which is greatly enriched in boron-11.

  Boron-11 has its own ambitions. Struck by a proton with energy of about 500 keV, it produces three alpha particles and 8.7 MeV of energy. Because the reaction avoids penetrating neutron radiation, it is a candidate fuel for aneutronic fusion. In existing fusion research, boron coatings on reactor walls reduce the release of hydrogen and impurities.

  Magnesium diboride is a superconductor with a transition temperature of 39 K. A CERN project to make cables from it produced test cables able to carry 20,000 amperes, intended for extremely high current distribution in a contemplated high luminosity version of the Large Hadron Collider.

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• 8. Boron and Life

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  For plants, boron is an essential nutrient, needed primarily to keep cell walls intact, and the margin for error is narrow. Soil concentrations above 1.0 ppm cause tip and marginal necrosis in leaves and poor growth. Sensitive plants show the same symptoms at 0.8 ppm. Above 1.8 ppm nearly all plants show some toxicity, and above 2.0 ppm few perform well. Excess boron occurs particularly in acidic soil, and in barley it appears as black spots on the leaves.

  Humans sit in a murkier position. Boron is not classified as an essential human nutrient, because research has not established a clear biological function for it. Deficiency has only been clearly established in livestock. Adult dietary intake in the Third National Health and Nutrition Examination Survey was recorded at 0.9 to 1.4 mg per day. Foods rich in boron include avocados, raisins, peanuts, pecans, prune juice, grape juice, wine, and chocolate powder. A small trial published in 1987 gave postmenopausal women 3 mg per day after first making them boron deficient. Supplementation markedly reduced urinary calcium excretion and raised serum levels of 17 beta-estradiol and testosterone.

  Toxicity is modest. Borates are about as toxic to mammals as table salt, with an animal LD50 of about 6 g per kg of body weight. An intake of 4 g per day of boric acid was reported without incident. More than 0.5 grams per day for 50 days causes minor digestive and other problems. Insects fare worse, which is why boric acid is routinely used as an insecticide. The boranes, by contrast, are quite poisonous, showing that toxicity depends on structure rather than on boron itself.

  Nature also makes boron-containing antibiotics. The first one found, boromycin, was isolated from streptomyces in the 1960s. The tartrolons were discovered in the 1990s in culture broth of the myxobacterium Sorangium cellulosum. Human-made drugs followed. Bortezomib treats myeloma and one form of lymphoma by binding the catalytic site of the 26S proteasome. Tavaborole, which treats toenail fungus, gained FDA approval in July 2014.

  In 2013, the chemist and synthetic biologist Steve Benner suggested that conditions on Mars three billion years ago favored the stability of RNA and the formation of oxygen-containing boron and molybdenum catalysts found in life. In his theory, primitive life formed first on Mars and later migrated to Earth.

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