haakma said:
I don't believe sulphur is the way these coins are made or any way written in this thread;
the quality is so consistent and the back is totally normal silver this must be prifessional done.
I'm looking for anybody who nows reaaly about how they are mader, but did not found any yet..
Still they are beautiful coins i think..
Source Coin Web:
OXYGEN COINS
In recent years, a number of altered Australian bullion silver coins have appeared on the market, particularly in Europe, sold as 'Oxygen' coins.
The coins, mostly one and two ounce kookaburra series specimen issues, exhibit a rainbow-like colouring (patina) of the field.
The effect is created by heating (roasting) the coin in an oxygen-rich enclosure. The oxygen reacts with the surface of the coin to create a variable depth layer of silver oxide (AgO). When light strikes this translucent layer, it penetrates to differing depths before refracting, depending on the amount of silver oxide laid down.
The varying depth of the silver oxide layer across a single coin is achieved by two factors working together - stress and heat. The more that the silver has been stressed, for example in the frosted field of specimen bullion coins, the more that the surface of the silver will oxidize. Uneven heating causes oxidation to proceed at differing rates across the coin, producing a patina with colour gradations.
The following 'Oxygen Coin' examples have been seen -
1991 1oz Kookaburra Near Tree Trunk.
1994 1oz Two Kookaburras.
1998 2oz Kookaburra on Fence.
2000 1oz Kookaburra on Branch.
2000 1oz Lunar Dragon.
2002 2oz Kookaburra over Map.
2003 1oz Kookaburra on Branch.
2003 2oz Kookaburra on Branch.