These 5 sixpences I found with my metal detector, and they all came out the ground looking as they do in the picture. All I used to clean them with was a soft toothbrush and water. You can tell the one with no silver content.
A quick guide to the silver content of British coins.
Prior to 1920, British silver coins contained high purity, 92.5% Sterling silver.
From 1920 to 1946, British silver coins contained 50% silver.
From 1947 to 1971, British coins contained no silver, just silver coloured.
From 1971 - Present, British decimal coins were issued and contained no silver.