Jenny Bulstrode asserts that African iron was superior to European. I have found a paper which is helpful in explaining the difference.
‘Voyage Iron’: An Atlantic Slave Trade Currency, its European Origins, and West African Impact*
Chris Evans, Göran Rydén
Past & Present, Volume 239, Issue 1, May 2018, Pages 41–70,
Here is an extract which suggests why European iron was thought to be inferior.
“There is something else that requires further study: the relationship between imported iron and the native product. This remains obscure. That there was an underlying geographical complementarity is plain enough; voyage iron flooded the forest zones whilst indigenous smelting was practised most successfully in the semi-arid interior. In that sense, the role of voyage iron was simply to make good a historic deficit in the tropical forests of West Africa. But European iron and African irons were not exact substitutes. Voyage iron was a malleable material from which every particle of carbon had been expunged. Its malleability meant that voyage iron was not suitable for the manufacture of tools that needed a cutting edge — things that could bite into wood or flesh — for it was too easily deformed. Historically, European manufacturers overcame that difficulty by welding a thin edge of steel onto a body of malleable iron. Such composite tools, combining hard but expensive steel with softer but cheap iron, were standard across Eurasia. In Africa they were unknown, no doubt because West African smiths worked up blooms which, being heavy with carbon, could be made into effective sharp-edged implements without the addition of welded steel. In European iron, therefore, artisans in coastal West Africa encountered a material that was quite unsuited for making bladed tools in the traditional African manner. “