English: The coin had been immersed in an approximately 3% sodium sesquicarbonate/2% sodium bicarbonate w/v solution for four weeks to treat bronze disease.

It had been rinsed four times previously and surface water patted off with the same crystallisation observed due to carbonates having infiltrated the coin's surface. While unsightly it is not harmful to the coin. It can be brushed off but will reform over time. With continued soaking in water the carbonate will eventually be removed and the coin can be displayed or be coated with a surface sealant.

The green mass on the coin's reverse is a copper carbonate that formed in the region where the bronze disease was most prevalent. It will be physically removed to ensure that the chlorides have also been removed prior to further conservation efforts.

The photos were taken some 20 minutes after rinsing and surface drying. Video footage of the crystallisation is also on wikipedia: Carbonate crystals forming on ancient bronze coin.ogg