Coal replaced coke as the principle locomotive fuel in the UK upon the introduction of the brick arch and firehole deflector plate about 1860. It soon became obvious that some varieties of coal were better than others. The so-called UK hard steam coals from Durham, Northumberland and South Wales were considered better, with the South Wales coal being the best. These coals had a high carbon content and a volatile content below 14%. When burnt, little clinker was formed and a good flame was produced. Yorkshire coal was also used by some railways but it was not such high quality as the South Wales type. Poor coal produces clinker and does not allow good draught. Coal size is also important, in that larger pieces needed to be broken down to about the size of a man's fist to allow the spread of even flame over the whole grate. Larger pieces would cause 'black spots' in the fire, a sure sign of lower temperatures and less effective combustion. Coal burning locomotives were tried by Joseph Beattie in 1854 on the London & South Western Railway. He designed a double firebox and combustion chamber to this end. Various other devices were tried over the next few years in an attempt to improve combustion, including the use, by some engineers, of centrally divided fireboxes fitted with a mid feather to improve circulation.