Other lords had land with iron mining industries. Ore was dug from the ground and combined with wood charcoal in a shaft furnace to be smelted into liquid form. Wood charcoal was derived from controlled charring of the wood at high temperatures without using oxygen. This burned impurities from it and left a purer carbon, which burned better than wood. The pure iron was extracted from this liquid and formed into bars. To keep the fire hot, the furnaces were frequently placed at windswept crossings of valleys or on the tops of hills. Some lords had markets on their land, for which they charged a toll for participation. There were about fifty markets in the nation. Cattle and slaves (from the word "slav") were the usual medium of exchange. An ox was still worth about 30d. Shaking hands was symbolic of an agreement for a sale, which had to be carried out in front of witnesses at the market for any property worth over 20d. The higher the value of the property, the more witnesses were required. Witnesses were also required for the exchange of property and to vouch for cattle having being born on the property of a person claiming them. People traveled to markets on deep, sunken roads and narrow bridges kept in repair by certain men who did this work as their service to the King. The king's peace extended to a couple of high roads, i.e. highways, running the length of the country and a couple running its width. Salt was used throughout the nation to preserve meat over the winter. Inland saltworks had an elaborate and specialized organization. The chief one used saltpans and furnaces to extract salt from natural brine springs. They formed little manufacturing enclaves in the midst of agricultural land, and they were considered to be neither large private estates headed by a lord nor appurtenant to such. They belonged jointly to the king and the local earl, who shared, at a proportion of two to one, the proceeds of the tolls upon the sale of salt and methods of carriage on the ancient salt ways according to cartload, horse load, or man load. Sometimes there were investors in a portion of the works who lived quite at distance away. The sales of salt were mostly retail, but some bought to resell. Peddlers carried salt to sell from village to village. Some smiths traveled for their work, for instance, stonewrights building arches and windows in churches, and lead workers putting lead roofs on churches.