First of all i would like to say that we are here to solve the dental problems ,not to teach you about dentistry. The type of questons you are asking are for academic interest and need elaborate answers wich is not pssible to answer. You can browse our web site for that as ample info is available on that. still in short i am answering your queries about plaque which is a tenacious layer of dead cell, bacteria and food particles . It is yellowish white till it calcifies and turn browish black. You can see it on the teeth, After how much time it creates a cavity depends upon many factors like the hereditary, saliva, type of food and frequency of food taken.