The malleability of gold permits it to be easily hammered into very thin sheets, and when pure, into extremely thin leaf. The beating of gold into foil was developed in the Bronze Age, the foil then being used to gild silver and copper by wrapping it around the base metal and folding over the edges. This technique was in use by about the beginning of the 3rd millennium BC. From about the middle of the 1st millennium BC, attachment of the foil was improved by inserting the edges into grooves cut into the surface of the base metal and then hammering them closed. Use of this technique continued for about five centuries. Techniques for purifying gold were developed around 2000 BC in Mesopotamia and somewhat later in Egypt. The more pure gold permitted preparation of thinner gold leaf, which was attached to surfaces probably by an organic adhesive. It may also be attached by burnishing. This process is especially effective on a hot metal surface, and is called hot clodding or diffusion bonding. Leaf may also be attached by punching it into the substrate. From about the beginning of the 1st millennium to the 2nd or 3rd century AD was a period of experimentation in which variations on essentially the same theme were employed. From about the 3rd century AD, fire-gilding, using an amalgam of mercury and gold, came into widespread use and was the method of choice in Europe and the East throughout the Middle Ages until it was finally replaced by the introduction of electroplating in the mid-19th century.