Before the advent of electric lighting, theaters used heated calcium oxide (quicklime)

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to illuminate their stages.

Quicklime, when heated to a high temperature, emits a brilliant white light. This was achieved by placing quicklime in a special lamp known as a "limelight," which was often fueled by burning oxyhydrogen gas (a mixture of oxygen and hydrogen). The intense light produced by the heated quicklime was used to spotlight actors and performers, making it one of the most effective ways to illuminate the stage before the widespread use of electric lighting.

This is where the phrase 'in the limelight' comes from.

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1. Some interesting physics and chemistry associated with that ...

The Limelight

The famous limelight, produced by heating a block of quicklime (calcium oxide), became available in the 1820s. It was quickly replaced – before the end of the 19th century – with much more convenient arc lighting, which is understandable given the need for a supply of hydrogen-oxygen mixed gas to allow a limelight to produce a sufficiently high temperature. There are interesting quirks to the technology in as much as the calcium oxide does not glow only because it is hot, but also due to a phenomenon called candoluminescence. This causes it to emit light at a higher frequency (that is, bluer) than would be expected simply because of the temperature. The oxygen-hydrogen flame heats the ceramic to only around 2000 degrees kelvin, which, for a perfect blackbody radiator, would imply that the colour temperature of the light would also be 2000K. However, candoluminescence adds enough blue light to the output that most limelights emitted at over 4000K, which would have looked very blue compared to contemporary light sources based on flame. Limelight, as well as carbon arc, was the point source of light which made really decent projection possible, as well as projection-type ellipsoidal or profile lighting devices such as followspots.

A latecomer to the field was the gas mantle, something which had been attempted since the earliest days of gas lighting. Austrian physicist Carl Auer von Welsbach finally figured it out, soaking an assembly of cellulose fibres in a solution of thorium (plus 1% cerium) oxides. This is almost precisely the same arrangement that gas camping lights still use, although the faintly-radioactive thorium oxide has been replaced with that of zirconium. In use, the fibres burn away, leaving an almost impossibly delicate structure of the oxides. When heated, a bright, slightly greenish light is produced. Using a gas burner similar to the bunsen burner used in laboratories, which produce a blue-hot flame which itself has almost no light output, the mantle can be heated to create light with much greater efficiency than by using a flame to create light directly.

https://www.redsharknews.com/technology-computing/item/1212-magic-lantern

A little more: https://en.wikipedia.org/wiki/Candoluminescence