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Material World
Element · Se

Selenium

An element that conducts better in light than in darkness — the basis of the first photocopier.

Selenium's photoconductivity was noticed in the 1870s and turned into a product in the 1930s: xerography works by charging a selenium-coated drum, exposing it to an image, and letting the illuminated areas discharge. It was also the basis of early light meters and rectifiers.

It is now largely displaced in those roles, and its remaining uses are as a glass decolouriser, a red pigment, and a trace nutrient with a notably narrow margin between deficiency and toxicity.

Uses

Selenium's photoconductivity — it conducts electricity better in light than in darkness — made it the basis of photocopying, where a selenium drum holds an electrostatic image. That use has largely moved to organic photoreceptors.

Its persistent uses are in glass, where selenium decolourises the green tint iron gives to soda-lime glass or, in quantity, produces a strong red; in pigments; and as an essential trace nutrient. The gap between a necessary dietary amount and a toxic one is unusually narrow.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is used as

  • Pigment — application · selenium reds in glass and ceramic glaze

is produced by

  • Electrorefining — process · from the same slimes, and by the same accident of the refining cell

is sourced from

  • Copper — element · from copper anode slimes, alongside the tellurium it accompanies and closely resembles

is found in

  • Soda-lime glass — material · added in traces as a decolouriser: iron in the sand makes glass green, and selenium's pink cancels it — which is why a clear bottle contains a few grams per tonne of a rare element

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • PubChem
    US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
  • NIST — National Institute of Standards and Technology
    United States Department of Commerce · US Government work — public information, credit requested

Questions this page answers

Where it comes from, and what it becomes

Follow Selenium back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Selenium → is sourced from (from copper anode slimes, alongside the tellurium it accompanies and closely resembles) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Selenium → is produced by (from the same slimes, and by the same accident of the refining cell) → Electrorefining → takes as input (as impure smelted anodes, around 99% copper, which is not pure enough for wire) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Selenium → is sourced from (from copper anode slimes, alongside the tellurium it accompanies and closely resembles) → Copper → is extracted from (the principal copper ore worldwide) → Chalcopyrite
  • Selenium → is sourced from (from copper anode slimes, alongside the tellurium it accompanies and closely resembles) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Selenium → is sourced from (from copper anode slimes, alongside the tellurium it accompanies and closely resembles) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (reduced with carbon to metallic tin) → Cassiterite
  • Selenium → is sourced from (from copper anode slimes, alongside the tellurium it accompanies and closely resembles) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (reduced with coke in a blast furnace) → Hematite

These are the most distinct paths back. Selenium can be traced through others besides.

Downstream — what it becomes

  • Selenium → is used as (selenium reds in glass and ceramic glaze) → Pigment