Second critical solid is the transition between the precipitation of burkeite and the precipitation of sodium carbonate. [5] When the second critical point event began, the heat transfer surface began to foul rapidly. The observation is that the total surface area of the suspended crystals did not increase during the second critical solid. [7] The lack of an increase in surface area during the second critical solid event may indicate that the fine crystals produced adhere rapidly to the surface leading to the rapid fouling observed. It is also possible that the second critical point may be brought on by a reduction in growth rate, and subsequent increase in supersaturation, due to diffusion limitations in high solids black liquor. [7] When black liquor transitions to precipitating Na2CO3, it leaves the dissolved phase at much higher rate. [5] And an analysis of the deposited scale on heating surface from the batch experimentation has been used as supporting evidence of the second critical solids point (56-58% Dry Solid).