Tanning is the conversion of animal hides and skins into leather by soaking them in a liquid containing tannin. Tanning was carried out in tanyards - open sided buildings with many large pits in which the hides were treated. Hair and wool was cleaned by soaking the hides in milk of lime. The pelts were then soaked in tannin (a vegetable material found in the bark of birch, elm, hemlock, larch, oak , spruce and willow). The bark is ground to a powder in a bark mill, then soaked in water for six to eight weeks to extract the tannic acid (leaching). The pelts were soaked for up to ten months in 'layer pits' containing tannin of different strength, starting with the weakest solution first, with pieces of oak bark between each pelt. After soaking, the pelts, now leather were then hand scrubbed and rolled with a weighted hand roller. In C19 wood-lined tanning drums were introduced to replace hand scrubbing. At the end of the century chromium was used to produce chrome leathers. The leather could be dyed by a currier (William Jones, 1996, Dictionary of Industrial Archaeology).