I have some questions. This lot is almost completly black/brown: But this one is red: Does anyone of you know why those two different lots above have different color/patina? What caused it? Is it clearning methods that determine the color? Or is the reason that certain soil(s) caused the specific color through hundreds of year when buried? And one more question: Can you see when a lot or coins were buried in the SEA instead of earthmud? I am asking for both copper coins and gold/silver coins. And one more thing: Byzantine Solidus or Hyperpyron are not 100% pure gold, but are respectively 98% and 90-94% pure. Will they still not be corroded if being in seawater for hundreds of year? I know gold/silver don't corrode, but what if it is 90-94%? Thanks
A coins colour today will be based on: Original metal alloy + burial condition (soil, water, adjacent chemicals) + cleaning methods used + (if you are looking at a photo) photography (method, lighting, after photo edits and adjustments). In short there is no simple answer. Your second batch has been cleaned chemically. The chemicals ate through much of the green (etc) corrosion products, though some remains as a faint green "shadow". Hard to tell if the brown coloured copper was first reduced to shiny copper and then re-toned or whether the chemical treatment left it as is. The upper lot is either a result of a few different cleaning methods or of a few different patina types cleaned by one method. SC
There are quite a few books written on the subject of bronze and patinas...my favorite (I only have two on the topic) is Copper and Bronze in Art: Corrosion, Colorants, Conservation by David Scott and a quote from Pliny the Elder about red (rust colored) coins "Things made of copper get covered with copper-rust more quickly when they are kept rubbed clean than when they are neglected, unless they are well greased with oil. It is said that the best way of preserving them is to give them a coating of liguid vegetable pitch."
