why use of ammonium print!

Generally,in a big construction firm or in a small private firm, after planning of concerned site,they usually draw that plan with multi drafter or by using autocad software. After plan ,section and elevation has been drawn,that sheet need to be enrolled in a blue rayed blueprint instrumemet along with a empty white sheet so that,the trace of that hand made drawing lines touches the empty sheet. After that trace sheet is ejected out, that sheet needs to be kept in ammonia box. Ammonia box in nothing but a empty box, where a cup of ammonia solution is kept. Then the trace sheet is rolled and kept in ammonia box. Due to some chemical actiom, the trace changes into blue which seems like a perfect blue print.
Why blue print ? And why not just plan printout in a4 sheet?
Answer for this question is just simple, a ordinary a4 sheet printout may not keep longer period as ink in a4 sheet disappear with respect to time, but for a blue print, it always there where it was even a generation. Government accepts only blue print plan and estimation as to keep the record for a long while registration and approval of a plan.