Colorless glass was difficult to produce because it required the use of impurity free materials. England starting making colorless glass in the 18^th century through the use of pure quartz rock (also known as calcined flint). However, through improved methods during the 19^th century, colorless glass was made more successfully and at a cheaper price. It is challenging to precisely date glass, but there are diagnostic tests that archeologists can perform to determine if the glass piece was machine made or mouth-blown, which can help to provide an approximate year.