The use of "splitting" in 17th century ironmaking?

I'm reading a book about spycraft in the 1650s. At that time Sweden was taking over the iron industry so England sent a spy to learn the new technique.


The technique is called "splitting" and it required the construction of a new machine. That is the entire detail the book provides.


Anyone have any idea what this might be?


**UPDATE:** The book is [A History of the British Secret Service](https://books.google.ca/books?id=g2whAQAAIAAJ&q=A%20History%20of%20the%20British%20Secret%20Service&dq=A%20History%20of%20the%20British%20Secret%20Service&hl=en&newbks=1&newbks_redir=0&sa=X&redir_esc=y). It is not very good, it basically talks about random events and provides few details or background. Its so poor, my copy even left the word "the" out of the title on the cover. Yet widely talked about.


