<P> Cort's process (as patented) only worked for white cast iron, not grey cast iron, which was the usual feedstock for forges of the period . This problem was resolved probably at Merthyr Tydfil by combining puddling with one element of a slightly earlier process . This involved another kind of hearth known as a' refinery' or' running out fire' . The pig iron was melted in this and run out into a trough . The slag separated, and floated on the molten iron, and was removed by lowering a dam at the end of the trough . The effect of this process was to desiliconise the metal, leaving a white brittle metal, known as' finers metal' . This was the ideal material to charge to the puddling furnace . This version of the process was known as' dry puddling' and continued in use in some places as late as 1890 . </P> <P> The alternative to refining gray iron was known as' wet puddling', also known as' boiling' or' pig boiling' . This was invented by a puddler named Joseph Hall at Tipton . He began adding scrap iron to the charge . Later he tried adding iron scale (in effect, rust). The result was spectacular in that the furnace boiled violently . This was a chemical reaction between the oxidised iron in the scale and the carbon dissolved in the pig iron . To his surprise, the resultant puddle ball produced good iron . </P> <P> One big problem with puddling was that almost 50% of the iron was drawn off with the slag because sand was used for the bed . Hall substituted roasted tap cinder for the bed, which cut this waste to 8%, declining to 5% by the end of the century . </P> <P> Hall subsequently became a partner in establishing the Bloomfield Iron Works at Tipton in 1830, the firm becoming Bradley, Barrows and Hall from 1834 . This is the version of the process most commonly used in the mid to late 19th century . Wet puddling had the advantage that it was much more efficient than dry puddling (or any earlier process). The best yield of iron achievable from dry puddling is a ton of iron from 1.3 tons of pig iron (a yield of 77%), but the yield from wet puddling was nearly 100% . </P>

Where was the rolling and puddling process invented