[4]: 7  Alongside his bookkeeping duties, he read extensively on the subject of iron making and, after a staff reduction was made in 1793, he began a series of experimental researches.

At first, these were encouraged by his employers, and he even taught assaying to the manager's son; but later, and without reason given, he was prohibited from experimenting during work hours.

It was here he made his second great discovery; in 1801, he demonstrated that 'Black-band Ironstone,' (typically found with a thin seam of inferior 'wild Coal'[6]) could be used to economically produce iron.

Previously, this abundant resource had been viewed as a useless form of coal, and while the discovery brought little financial reward to Mushet personally, the use of Black-band Ironstone was to lead to a remarkable expansion of the Scottish iron industry and eventually brought millions of pounds profit to Scottish industrialists.

Not unnaturally, perhaps, he was becoming dissatisfied with the limitations imposed by the Calder Iron Works and, restive for wider experience and greater opportunities, decided to move on".

While working there, he joined the Geological Society and wrote several authoritative articles on iron which were published in magazines and encyclopaedias, including the Encyclopædia Britannica.

[3]: 29 In 1808 Mushet was approached by Thomas Halford, a wealthy investor from London, who owned two furnaces at Whitecliff, in the Forest of Dean, Gloucestershire.

Halford was having difficulty with the quality and quantity of iron being produced at his works and offered to pay for Mushet's help in overcoming the production problems he was encountering.

Mushet designed and supervised a major rebuilding of the Whitecliff Ironworks, but the time he was spending in the Forest of Dean became instrumental in the complete breakdown of relations with his colleagues at the Alfreton works.

In February 1810 he moved to Coleford to take up full-time management of the Whitecliff works; buying a quarter share of the business.

He spent his formative years studying metallurgy with his father and was also to become a noted metallurgist, though was never to receive full recognition, financial or personal, for his achievements.

[9] David Mushet died at Monmouth on 7 June 1847,[5] but even in death was surrounded by bitter and public family feuding.