Selenium dioxide
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Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium. It is used in making specialized glasses as well
Crystals of selenium dioxide
Bluing (steel)
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Bluing, sometimes spelled as blueing, is a passivation process in which steel is partially protected against rust using a black oxide coating. It is named after the blue-black appearance of the result
Selenium dioxide
…so to create (apparently) colourless glass. In larger quantities, it gives a deep ruby red colour. Selenium dioxide is the active ingredient in some cold-bluing solutions. It was also used as a toner in photographic developing…
A 19th century blued steel button
Acid solution applied to bare metal
After boiling rusted parts
After eight rust, carding and oiling sessions