In the early 17th century, ironworkers in Western Europe had developed the cementation process for carburizing wrought iron. Wrought iron bars and charcoal were packed into stone boxes, then sealed with clay to be held at a red heat continually tended in an oxygen-free state immersed in nearly pure carbon (charcoal) for up to a week.  During this time, carbon diffused into the surface layers of the iron, producing cement steel or blister steel&mdash;also known as case hardened, where the portions wrapped in iron (the pick or axe blade) became harder, than say an axe hammer-head or shaft socket which might be insulated by clay to keep them from the carbon source.  The earliest place where this process was used in England was at Coalbrookdale from 1619, where Sir Basil Brooke had two cementation furnaces (recently excavated in 2001&ndash;2005).  For a time in the 1610s, he owned a patent on the process, but had to surrender this in 1619.  He probably used Forest of Dean iron as his raw material, but it was soon found that oregrounds iron was more suitable.  The quality of the steel could be improved by faggoting, producing the so-called shear steel.