Gold is compared to everything, continuously. That is the very point of using the standardized unit of value measurements that we call 'currency', to have a common reference to base those comparisions on. Since we are no longer on a gold standard, that reference floats, so there must be a 'gold price' in the fiat currency in order to make said comparision. Steel, copper, wool, maize, industrial chemicals of all sorts, most agricultural products, and coal have dropped in their relative value due to massive improvements in productivity over the past 100 years. Thus, using any or all of these things would give you a distorted perspective, as the value of the currency dependent upon them would change with every significant improvement in any of those industries, thus making the economic calculations (and thus predictive capacity of economic calculations) vastly more complex and inaccurate.
The Whiskey and Gunpowder post you used to start this thread says
Largely forgotten the silver version of the currency is keeping its value relative to things you buy. A gallon of gas is still less than $0.20. Twenty REAL cents. Not the forgeries that pass for money in the minds of the unwary. If you think thats something, realize that a gallon of gas is just five or six cents in terms of the old dollar bills that were also gold certificates. (One pre-1934 dollar was good for 1/20 ounce of gold, or about 80 of todays dollars.) Thats an even more impressive holding of value than the silver coins. That's a claim that is subject to empirical scrutiny. It is not a distortion to cross-correlate exchange rates of different commodities, precious metals, and currencies. It's a test of the hypothesis that silver and gold have characteristically high exchange rate stabilities that justify their use for savings and for backing currencies used in day to day transactions. If you have a better test, please explain it and your reasoning.
The fact that no nation has ever backed a paper currency with mercury is actually irrelevent. Mercury has been used as a trade medium on occasions. However, mercury does not have better properties for commerce than gold silver or copper. The first and most obvious, is that the liquied state of mercury is a problem for trade, as it requires that those who trade in it have both vessels to hold it and scales to measure it, but also a means to verify that it's all mercury in the bottles. You can weight the bottles full, and then empty them into another container using a strainer to make sure that tere is no lead dust mixed in, but that is a lot of work for the common trade. The minted coin, with a recognizable size and weight, and the mark of a trusted (bank, government, king, whatever) tells the casual trader the value of the thing pretty fast.
Coins could be and were debased, forged, and clipped. If the trader actually measured coin mass and density to check its composition and value, there is no reason he could not do the same with mercury. In any case, I have the (perhaps mistaken) impression that enthusiasm for a return to gold or silver standards does not literally imply that every transaction would involve the trading of metals in person. It simply means that the issuing agency of a currency would maintain a fixed metal:currency unit exchange rate, and would trade its currency (paper, coins, electronic records...) for the metal on demand. There's no reason that we would have to give up electronic bank transfers or lightweight paper bills for everyday use as long as users always have the option to exchange for metals.
By that same reasoning, it is strange that enthusiasm for silver does not extend to other materials whose availability is limited by geological facts. Why no bismuth, indium, or mercury standard cheerleaders? Silver is more abundant and currently mined at a higher rate than any of those elements. Geologically speaking none of them is any more likely to go inflationary than silver. I think there is a substantial amount of magical thinking that leads people to
specifically value gold or silver, and advance justifications for those metals as currency standards after they've already made up their minds. If they were starting from their claimed justifications, and simply looking for durable, rare substances whose production is limited and cannot be rapidly increased, then I'd expect as many cries for the Selenium Standard as the Silver Standard.