SUGAR BOILING AND CRYSTALLIZATION-IN-MOTION 387 



In so doing, and in controlling the rate of deposit or of evaporation the operator 

 is guided by the senses of sight and touch, especially as indicated by the vis- 

 cosity of the mother-liquor, in small samples withdrawn from the pan by the 

 proof -stick. Of all operations in the sugar-house this is one that has to be 

 learnt by experience, and which cannot be described. 



The rate at which the sugar deposits on the crystal is a function of the purity, 

 increasing as that increases. Under average raw sugar conditions four hours 

 is the over-all average time necessary for a strike, this period falling to two hours 

 in a refinery where a material of 98 purity is worked, and increasing to six 

 hours with material of lower purity. 



The one danger against which the boiler has to guard is the formation of 

 " false grain " or of a second independent granulation. This may occur 

 through a sudden fall in the steam pressure or by a sudden increase in the 

 vacuum, both causes acting through a fall in temperature of the contents 

 of the pan. It may again be caused by the introduction of too large a charge 

 which does not mix well with the material already in the pan, and is always 

 more likely to occur when the circulation is faulty due to bad design. A cause 

 of another nature happens when making sugar of large grain, as there is a limit 

 to the size to which the crystal can be grown, unless the rate of deposit is pro- 

 portionately decreased. When false grain does occur the boiler has two means 

 of removing the objectionable small crystals. He may raise the temperature 

 in the pan or introduce an excessive charge of syrup or even juice. Both 

 of these devices are intended to wash out or dissolve the false grain. The 

 trouble due to false grain presents itself in the drying of the massecuite and is 

 further discussed in the next chapter. 



After all the syrup is introduced into the pan, the mass is concentrated to 

 the striking point, where again the operator is guided by his sense of sight 

 and touch. On discharge from the pan, massecuites of whatever purity will 

 be found to have a density neighbouring on 93 Brix, but the actual water 

 content will be found, of course, to be very far from constant, as must be the 

 case if materials of different purities" are of the same degree Brix. 



It is a common belief that massecuites boiled hot give a hard grain. Although 

 the hardness may possibly be controlled by physical treatment, hardness in this 

 connection is psychological rather than actual, since with massecuites boiled 

 at a low temperature the crystals tend to stick together, and what the brain 

 registers as softness from the sense of touch is rather friability. 



In place of forming grain from syrup, crystals of sugar may be taken into 

 the pan. This process, known as seeding, is due to Lebaudy (patent 42 of 



1865). 



In the raw sugar industry seeding is mainly used as a means of utilizing and 

 obtaining as first product without remelting the small grained sugars that result 

 from low grade massecuites boiled blank. About I ton of this material is 

 used per 20 tons of massecuite in the pan. 



Supersaturation. In the section immediately preceding a sketch of the 

 operation of sugar boiling is given from the craftsman's point of view. The 

 establishment of a definite theory of the operation based on the conception 

 of supersaturation is due to Claassen. 1 By a saturated solution is meant 

 one that will neither deposit nor yet dissolve the solid which is in solution, 

 a position of equilibrium between solvent and solid being obtained. The pro- 

 cess of deposition of a solid from solution after the saturation point is passed 

 is not however instantaneous, and it is possible by means of continued evapora- 



