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PlaidBastard t1_izxqpgo wrote

More likely, it was finished (ground, sanded, polished) differently either on purpose or as a side effect of manufacture (some bullion is just left how it cools in the mold, other bars are hit with torches to melt the surface and get it to re-puddle glassy smooth on the side that was down in the original casting, etc. etc).

That said, sure, different non-gold alloying metals (silver vs copper vs rhodium etc.) can affect how readily a lower-karat gold polishes, I guess, but you're probably seeing a variety of finishes/surface textures independent of the gold content, from what little you gave us to go on?

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SilenceFailed t1_j01ufw7 wrote

This is the same with soldering on circuit boards. The gloss, like in blacksmithing, occurs when all of the slag (impurities) are removed. Hitting metal with a torch is a good way to hand out impure metals.

However, you are right. Having polished stainless steel molds for glassware, it is essentially a corrosive substance like kerosene (when working with substances that stick to each other, like glass in metal), a polishing lacquer (like rouge, French word, it was green stick, not positive on its makeup), and a polishing paste, to make a seal on the metal (ours was pink, not sure if it's makeup either). To hit the full shine, we cleaned it out with alcohol when we were done. Sand paper of varying levels of grit is used to smooth it out. Now, if you want the full process, let me know. I'll post the whole thing. I used to work in a glass factory producing various glass pieces from various companies. From pier 1 to Christmas Shop to dollar stores to Walmart.

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rootofallworlds t1_izxuoiz wrote

Not directly. But an initially polished gold object will dull as it wears; as its surface is roughened by contact with other objects. A harder gold alloy will wear more slowly than a softer one.

Pure gold is rather soft so jewellery (and circulating coinage historically) is typically made of an alloy with the gold content described in either karats (24 is nearly pure) or millesimal fineness (1000 is pure). For example 22 karat / 916 fine and 18 karat / 750 are commonly used, with 18 karat being harder than 22 which is harder than 24. The alloying elements are usually silver and copper but other elements may be used, typically to change the colour of the alloy but the hardness will also be affected.

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Ok_Bookkeeper_3481 t1_j00h8t5 wrote

Gold is very soft, so the initially polished shiny surface easily accumulates abrasions and scratches - and becomes dull. In fact, lower purity alloy, mixed with a harder metal, would maintain the sheen longer than pure gold.

A related point of interest: gold nanoparticles are dark red color, not, well, golden.

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