Kinro’s Gold Spear
In Ch 19, Senku tries to bribe win Kinro over by making him a golden spear to fit his name, done by a process called fire gilding. This is an ancient art used to coat things like metals in a layer of gold, with more intensely saturated results than from modern day electroplating. However, the process is pretty involved, and comes with various health risks due to the use of mercury.
You can’t mix mercury and gold by melting them together because mercury vaporizes (turns into gas) at a relatively low temperature. Instead, you’d mix them by grinding them together with a mortar and pestle, like Senku is doing above. This is to create a paste or amalgam of gold which, once cooled to a certain point, can be spread onto something like a spear tip. It looks like Senku just dips the spear tip into the amalgam, but it’s actually applied to things by using a brush or spatula-shaped rod.
Then to get rid of the excess mercury, Senku heats the spear tip in a fire to evaporate it out. The smoke is full of mercury, which is very much not good for the body if absorbed in any way (including through the skin! Senku’s not being very safe here).
There are a few more steps involved in fire gilding, like preparing the object to be gilded by cleaning it in diluted nitric acid and then “quicking” the surface with mercury before applying the amalgam, but Dr. Stone just covers the basics. You can read more about the process in the sources I’ll list below.
Any questions, corrections, or requests can be dropped in my askbox! I’ll be working on sulfa drugs next.
Sources