But mass production required more than changes

 in technology; quite apart from the question of mental

 attitudes, it also required new materials, especially

 iron (because of its strength) and copper (because of

 its electrical properties).

 Until late in the eighteenth century, almost all

 machinery had been made primarily of wood. Metal was

 used chiefly in various steam engines and in machine

 parts which were most subject to wear or which required

 greater strength than could be afforded by wood. But

 metal was hard to work and finished metal lacked many

 desirable qualities. Cast iron, subject to breakage on

 impact or twisting, was too brittle for many purposes;

 and wrought iron, although tough and malleable, was

 relatively soft. Steel was extremely expensive and

 could be produced only in relatively small quantities.

 The quality and uniformity of each of these forms of

 iron was suspect as well.26

 Mass production, heavily dependent upon the

interchangeability of parts, and in turn upon fine

accuracy, could not be adopted on a wide scale without

the development of a suitable material from which to

fashion slow wearing parts with precision. The material

had to be tough, yet easily worked into various shapes,

 26
 Cast iron, wrought iron, and steel are all forms
of iron, each having a different carbon content. Wrought
iron contains the least carbon, cast iron the most, and
the carbon content of steel lies in between.