Thus, until a more powerful blowing-furnace had been contrived, the production of pit-coal iron must necessarily have been limited.

Dudley himself does not seem to have been able to make more on an average than five tons a-week, and seven tons at the outside.
Nor was the iron so good as that made by charcoal; for it is admitted to have been especially liable to deterioration by the sulphureous fumes of the coal in the process of manufacture.
Dr.Plot, in his 'History of Staffordshire,' speaks of an experiment made by one Dr.Blewstone, a High German, as "the last effort" made in that county to smelt iron-ore with pit-coal.

He is said to have "built his furnace at Wednesbury, so ingeniously contrived (that only the flame of the coal should come to the ore, with several other conveniences), that many were of opinion he would succeed in it.

But experience, that great baffler of speculation, showed it would not be; the sulphureous vitriolic steams that issue from the pyrites, which frequently, if not always, accompanies pit-coal, ascending with the flame, and poisoning the ore sufficiently to make it render much worse iron than that made with charcoal, though not perhaps so much worse as the body of the coal itself would possibly do." [1] Dr.Plot does not give the year in which this "last effort" was made; but as we find that one Dr.Frederic de Blewston obtained a patent from Charles II.

on the 25th October, 1677, for "a new and effectual way of melting down, forging, extracting, and reducing of iron and all metals and minerals with pit-coal and sea-coal, as well and effectually as ever hath yet been done by charcoal, and with much less charge;" and as Dr.Plot's History, in which he makes mention of the experiment and its failure, was published in 1686, it is obvious that the trial must have been made between those years.
As the demand for iron steadily increased with the increasing population of the country, and as the supply of timber for smelting purposes was diminishing from year to year, England was compelled to rely more and more upon foreign countries for its supply of manufactured iron.