Analysis of information sources in references of the Wikipedia article "Nines (notation)" in English language version.
For example, the difference between a purity of aluminum of "four nines" and "six nines" is understandable to a scientist, but not necessarily to a politician. (These are 99.99 percent pure and 99.9999 percent pure).
For example, good commercial-grade gases for plasma spraying are stated to be at least 99.995% ("four-nines-five" purity).
Therefore, in reality, it's virtually impossible to have a 100% chemically pure substance that maintains its purity for an extended period of time.
At "five nines pure" (99.999%) the gold contained less than 10 parts per million of other elements like silver, aluminum and zirconium, one decimal place purer and hundreds of thousands of dollars more valuable than any gold coinage minted in Vienna or elsewhere, ever.
We achieved "arguably the purest of all gold" in 1957. Refinery Officer Leo Hickey and Senior Craftsman Alexander Osborne produced a proof 'plate' of almost six nines - 999.999 parts of gold per thousand - as measured by the Worshipful Company of Goldsmiths in London.
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