Cleaning corroded metal objects is hard work. Think about it. Most metal objects (unless made of gold or silver) start out as ores, such as copper carbonate, or copper sulphate. In other words, objects made of metal start out as corrosion, and only after a lot of human intervention do they become metal objects. So, after several thousands of years of being buried in the soil, these metal objects want to revert back to corrosion. It goes without saying that, after discovered by an archaeologist, it usually takes a great deal of intervention on the part of the conservator to get to the metal surface. 
Yes, chemicals are one way to assist in changing the corrosion, so that it can be removed to possibly reveal a metal surface. Sometimes there is no metal core, below all of the corrosion. The object is completely mineralized. Mechanically removing the corrosion is also a safer, but often very time consuming and physically difficult process. At the site, this year we are trying a mild solution of EDTA (ethylene diamine tetra acetic acid) made basic with ammonia. It has proven to be helpful at softening the corrosion crusts, making the mechanical removal with a scalpel more effective, and safer. The solution turns blue as copper corrosion dissolves.
When the archaeologists register the finds, as Jaap is doing in this photo, it is much more useful to have an actual image on the coin, or other information that may help in dating a layer. This is why finding coins is often exciting, yet painful at the same time. Luckily however, unlike the corrosion that Jakki mentioned in her post on the Wadjet Figure, the corrosion crusts at the Mut Temple are mostly stable.
Lisa Bruno, Objects Conservator
