Artist's commentary
Hang in There, Kogasa-san 3319
The last batch of etching fluid took about six hours to dissolve things completely, maybe due to the concentration of the hydrogen peroxide, so I decided to use ferric chloride.
When you're done using the citric-acid-based etching fluid, you dissolve aluminum into it to precipitate out the copper, but it gets frickin' hot, so you're better off not adding it all at once. There were some comments about wanting to see the cleanup process, so I tried drawing it. You mustn't pour it down drain, now!
Touhou Ryuukyuu Fest is being held today in Naha. My regards to all the participants!