Growth is due to the transformation of pearlite into ferrite/graphite. Gray iron made for common heat resistance applications is by far the material that is most prone to growth because of its intentional pearlite matrix, by comparison to ductile iron, which often has an intentionally ferrite-predominant matrix. Results of experiments by BCIRA have revealed relative growth levels ranging from 1 to 4, depending on specific testing conditions. The progression of growth takes place gradually over time and is a result of a phase transformation in which the speed of the reaction is highly dependent of the temperature when it nears 720ºC, but becomes nearly insignificant at temperatures below 500°C.