Ghosts in Aluminum
Aluminum does have ghosts — a faint carry-through pattern lurking in the grain.
A Word for Something Rarely Seen
In any craft, enterprise, or business, words are used to try to describe some unique aspect of that profession. When associated with a particular profession, this is known as jargon. Sometimes jargon is used simply to keep things "within" the community and confuse outsiders. But other times, words are really created and applied because there are no other terms to properly describe the phenomenon.
An Observation, Not (Necessarily) a Proof
The metallurgical community is known to develop jargon for both of those reasons. There's one term
that rarely gets beyond the technical community, because it describes an observation rather than
something necessarily validated by scientific proof.
Ghosting
Aluminum is produced by solidifying liquid aluminum into molds. This process yields large ingots, each with a unique solidification pattern, or "signature" — the pattern established depends on the cooling conditions present as the aluminum went from liquid to solid.
Take a look at a tray of clear ice cubes in your refrigerator — like snowflakes, each one has a different internal structure depending on how it solidified.
A similar pattern in aluminum may be visually observed if you take a slice of the ingot, polish the face, and acid- or caustic-etch the surface, then examine it under good lighting. This process is called a macro inspection.
A Faint Carry-Through
When we obtain the ingot and produce a hand forging, a faint image of the original casting pattern sometimes remains — if you look hard enough and know it's there. Depending on the heat treatment, amount of reduction, the direction you're looking, and whether a similar macro examination is performed on a slice of the hand forging, the original ingot pattern may show up very faintly in the background of the grain structure.
This leads to the question: is something "there," or not? It's like a ghost image on the macro examination specimen. Since the pattern didn't originate with the forging process, metallurgists call this carry-through a "Ghost" image.
Aluminum does have ghosts. They are lurking way in the background and can be seen if you want to look hard enough — and are considered mostly harmless.