Variable stator vane · 7 cm, darkened all round

On all the others one side stayed bright — this is the only one evenly darkened on both faces. Otherwise the same design: 7 centimetres, eye at the head, spigot behind.

Inside a jet engine's compressor, rotating and stationary blade rows alternate. The stationary ones — the stator vanes — straighten the air for the next stage. Some of them are adjustable: they sit in bearings and are turned together by a ring, so the compressor works as cleanly at idle as it does at full power. That is why this piece has an eye with a bore at its head, where the lever engages, and a spigot on the reverse that runs in the casing.

The even grey-brown tone is a temper colour: metal that has run hot for a long time forms a thin oxide layer which shades differently with heat and duration. That both sides look alike suggests this vane sat further back in the compressor, where the air is already much warmer. It cannot be proven — but it is the obvious explanation, and it does not make the piece worse, only different.

Which engine this came from is not known. There are no papers with it, and apart from its shape nothing identifies the type. The material cannot be named without testing either: compressor blades use titanium, steel or a nickel alloy depending on the stage. What is certain is in the key facts — the rest stays open.

• Length 7 cm
• Flat bearing eye with bore at the head
• Guide spigot on the reverse
• Twisted aerofoil, sharp edges
• Metal, alloy undetermined
• Loose single piece, no stand

Used. Both sides evenly grey-brown, without patches or flaking — the tone sits in the metal and does not wipe off. Edges without nicks, bore clear, no distortion.