What are indentures or indenture papers in "Great Expectations"?

In England, the method a young man could use to learn a trade was called indenture. He would promise to work for free or a very limited salary and in return, the person to whom he was indentured promised to teach him a viable trade. In other words, a young man who wanted to be a blacksmith would indenture himself to an older blacksmith. He would work for that blacksmith for a period of time ( usually 7 years) earning little but room and board. In return, the older blacksmith would teach the young man his trade. At the end of the period, the young man was free to set up his own blacksmith shop. Since there were no vocational schools at the time, this was one way a person could learn how to do a job and then be able to earn a living once the indentured time was up. In order to make the agreement legal, the indentured servant carried papers signed by both the indenturer ( the employer) and indentured ( the trainee) indicating how long he would serve, what his payment would be, and what he would learn.