I found this Chinese copper coin, which I believe is a 4/2 characters type. One side is almost legible, the other not due to sea corrosion. I am reluctant to clean it without knowing what or from which period it may be. Any advice as to cleaning or interpretation will be appreciated.
This looks like a Chinese square hole coin; the four characters identify its type and era. They are read in the order North-South-East-West, and reads:
咸豐通寶 Xianfeng Tongbao
This page has a lot of information on how to interpret those characters. The first two characters give the era, and the second two the type of coin.
This coin is a 通寶 tongbao.
The Tongbao means "circulating currency", and is the most basic unit. It almost always denotes a value of 1 cash, unless a mint mark at the back states otherwise.
The era is 咸豐 Xianfeng, minted at 1850–1861. This will be during the reign of the eponymous Qing Xianfeng Emperor.
The reverse side of this coin will show you its face value. Even if the reverse side is unreadable, you may be able to determine the value based on the coin dimensions, because they did vary based on value:
As you can see here, the coins are 10, 20, 50 and 100, and dimensions 37mm, 41.5mm, 58mm and 73mm respectively. The last one is pretty big - bigger than the palm. There are other denominations not shown in that picture, going all the way down to 1.