Friday, March 29, 2019

On the value of Mercury

In my Yoon-Suin game, the party came in posession of a few pounds of quicksilver (aka mercury}, and wanted to know how much it was worth.   I think we can all agree that quicksilver would be considered to have significant value... but that's vague. Is it as valuable as copper? As gold? More than gold? etc etc.

Now, I could have just made something up, but what's the fun in that?  Surely there must be some ancient value we can look at?  I tried again and again to find information on the price of mercury in the middle ages, but to no avail. However, I have just found a quote from Pliny the Elder on the value of cinnabar - a pigment in its own right, but also a mercury-based mineral:

"Nothing is more carefully guarded. It is forbidden to break up or refine the cinnabar on the spot. They send it to Rome in its natural condition, under seal, to the extent of some ten thousand pounds a year. The sales price is fixed by law to keep it from becoming impossibly expensive, and the price fixed is seventy sesterces a pound."

Ok, so the price of cinnabar - which has the chemical formula of HgS, is 70 sesterces a pound. 

First, some roman unit conversions. 1 roman pound = 328.9 grams, so cinnabar is 96.6 sesterces an imperial pound.

The value of the sesterce varies, but I believe that around the time of Pliny it was worth 1/4 of a denarius, which is a common silver coin (3.9 grams/coin). So 1 imperial pound of cinnabar = 24.2 silver pieces.

All right! but what about quicksilver... Well here we have to do some fiddling.

1: Purity: I'm going to assume that this ore isn't completely pure since it's in natural condition, but that also it's not garbage, so 75% pure

2: Processing efficiency. This is is a *wild* guess, but I'm going to assume that the process is 50% efficient - there are some losses due to evaporation of mercury and other errors.

3: Processing cost: this is a reflection of the cost of refining the cinnabar into mercury. Alchemists and retorts aren't cheap (see this wikipedia file for a neat illustration). I'm going to say this doubles the cost.

4: Amount of mercury in cinnabar: ah well this one we have exact numbers based on atomic weight and the HgS formulae: 200.6/232.6, or 86% This is *not* the same as #1, that was for pieces of rocks and other impurities.

So the cost of a pound of quicksilver is: (24.2 X 2)/(0.86 X 0.5 X 0.75) = 150.1 silver pieces (for pieces of silver of 3.9 grams), or 585 grams of silver.

This means that quicksilver is almost 1.5 times as valuable as silver by weight, which makes it fairly precious, but not insanely so.  I'm not sure if this is the correct number, but at least we know it's a plausible one.

Lastly, I note that is we look at Pliny the Elder's quote again, we can conclude that the cinnabar business was worth over 175 000 sp a year- not a bad gig.