The U2 has an argon filled cavity with copper sulfate water absorbing capsule.
With copper finish (I use weak copper sulfate ), apply without cleaning off flux.
Copper copper compounds are used in fungicides and pesticides e.g. a traditional recipe is copper sulfate solution plus lime is used to kill greenfly.
You might like to try growing sugar crystals, copper sulfate crystals, sodium silicate crystals, potassium permanganate crystals or iodine crystals.
Only the most sharp-eyed observers will notice even the copper sulfate.