388 CHAPTER XIX 



tion to obtain solutions which in the case of sugar syrups may contain as much 

 as 50 per cent, more solid than corresponds with saturation. Such solutions 

 are, of course, in an unstable equilibrium. If S be the solubility of sugar in 

 saturated solution, and S^ be that in supersaturated solution, the ratio Sx/S 

 is termed by Claassen the coefficient of supersaturation. Claassen's system 

 of boiling consists in maintaining in the massecuite at various periods of the 

 cycle such definite coefficients of supersaturation as experience has shown to 

 be desirable, and actually this is what the capable sugar boiler unknowingly 

 does in the practice of his art. In the scheme of Claassen, however, definite 

 numerical values of the coefficient are connected with the different stages 

 of the operation, and these are obtained through the medium of an instru- 

 ment known as a brasmoscope or brixometer and described in a later section. 

 This instrument is based on a definite physical law, and its indications are 

 substituted for the sugar boiler's senses of sight and touch. 



Before sugar will crystallize from solution, the latter has to become 

 supersaturated, and for purer materials such as straight juice Claassen 

 states that the coefficient should be about 1-2, as at that condition crystals 

 will readily form with the devices indicated in the previous section. After 

 crystals have once been formed they themselves exercise an influence on the 

 deposit of sugar, and so high a supersaturation is not required. During the 

 growing of the grain the coefficient should be maintained between i-o and 

 I 2 that is to say, the valve should be opened to admit a charge of syrup 

 when the coefficient rises to 1-2 and closed when it falls to i-o. If a con- 

 tinuous feed system is followed the coefficient should be maintained at i I. 

 As the sugar crystallizes out the mother liquor becomes less and less pure, 

 so that the coefficient should be raised so as to maintain the rate of deposit, 

 and finally at striking it should reach 1-3. 



In boiling to grain products of lower purity, such as first molasses, 

 supersaturation coefficients considerably higher must be employed, rising 

 to 1-5 to 1-6 with material of 60 purity, whence exhausted molasses is 

 expected. This point is discussed more fully under the section " Crystal- 

 lization-in-Motion. ' ' 



Determination of the Supersaturation. The determination of the super- 

 saturation is based on the physical law which states that the elevation of the 

 boiling point (cf. Chapter XVIII) is independent of the temperature at which 

 ebullition occurs. Thus the boiling point of a 75 per cent, solution of cane 

 sugar at atmospheric pressure is 231-2 F. Under a pressure of 2-42 Ibs. 

 per sq. in. or a vacuum of 25 ins. water boils at 132 F., and a 75 per cent. 

 solution of cane sugar will boil at 132 +13-2 or 145 -2 F. Accordingly, when 

 the temperature of the boiling mass under which ebullition occurs is known, 

 the concentration can be obtained from reference to published tables, and, 

 when the concentration so found is higher than that which corresponds to 

 saturation at that temperature, the supersaturation can be obtained by 

 calculation. Thus, if at a 25 in. vacuum the boiling mass has a temperature 

 of 165-2 F., the elevation is 33-2 F., corresponding to 87-5 per cent, of 

 sugar in solution, and the coefficient of supersaturation is 87-5/75*0 or 

 1-075. 



The table below gives the elevation of the boiling point of sugar solutions. 

 It is worth while noting that Dutrone, in 1790, published the first table of 

 this nature, and advocated the use of the thermometer to determine the 

 strike point. 



