The general method of making firebricks has altered little over a hundred years although the grinding, extruding and pressing equipment has become much more sophisticated. Clays are still ground and mixed with crushed fired aggregate called grog, to stop excessive shrinkage in the material as it is mixed with water and extruded into a soft lump to be pressed into the desired shape. Right up to the 1960’s, many of the larger and special brick shapes were hand moulded. This involved throwing hand sized soft pieces of clay mixture into wooden moulds which had detachable bottom and side inserts shaped to suit the characteristics of the firebrick. A typical full gasworks setting would contain up to 1500 tonnes of firebricks with over 100 different shapes. Many of these contained tongues and grooves to provide a gas tight seal and were hand moulded, as were large “D” shaped pipe retorts up to 5 meters in length also used in these settings.