SYNTHETIC HYDROCOLLOIDS AND DENTIFRICES 465 a few parts per million of cellulase. The enzyme is produced by moulds and bacteria, and may originate in the water, on the factory floor, or within the bags of SCMC if this becomes damp enough to support growth. Killing the organism will not, of course, remove the enzyme. The more thixotropic grades of SCMC can cause difficulties in filling due to the very thin consistency of the toothpastes in the worked state (Fig. 1). It has also been our experience that toothpastes containing the more thixotropic grades retain occluded air quite tenaciously in the vacuum mixer, and more care is required in their manufacture. These manufacturing difficulties can be, and clearly are, overcome in the many factories successfully using SCMC for dentifrices. Selection of SCMC Grade Degree of substitution (D.S.) For toothpastes containing dicalcium phosphate dihydrate or calcium carbonate polishing agents, it is preferable to select a D.S. of about 0.7-0.8, because of better compatibility with calcium ions. These SCMCs tend to give products which are gel-like, and disperse very readily in the mouth. The extruded ribbon retains its shape but is often not completely smooth in texture when first extruded, due to disturbance of the gel surface. Materials with a D.S. of above 1.0, tend to give products of very shiny and attractive appearance but of lower dispersibility. They' are less thixo- tropic, and ribbon flow may occur. This behaviour may be the result of the better solubility of the high D.S. materials, which tend to give relatively less thixotropic toothpastes. The formulafor must therefore choose the D.S. most suited to his needs and usually there has to be some sacrifice o• one attribute or another. Viscosity There is a wide range of matehals available at the present time, although SCMCs with very high D.S. are restricted to rather low viscosity types. It is obviously more economical to use high viscosity thickeners, but on the other hand, it becomes difficult to manufacture uniformly when small weighing errors, or variations in SCMC quality, on a low level ingredient can have a very profound effect on final consistency. It is convenient to choose a grade which can be added at a level of approximately 0.5% to 2.0% to give the required consistency.

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Stability in the presence of electrolytes In general, SCMCs with very high D.S. values show better fluoride or monofluorophosphate tolerance than those with D.S. 0.7-0.8. In the case of toothpaste containing calcium ions, all SCMC toothpastes which are alkaline tend to harden due to the formation of the calcium carboxymethyl cellulose which is insoluble. The calcium salt does not form so readily in neutral pastes and in practice only chalk pastes of pH above 8.5 give any difficulty. Glycerine compatibility The well-known dehydrating effect of glycerine on SCMC and other mucilages results in a large change in viscosity which is reflected in the consistency of the finished toothpaste. Fig. 2 shows the effect of replacing water with glycerine in 1% SCMC slurries. Quality control is easier, and •00.. 0 6000, •ooo, Glycerinel Water IOO 90 lb 2'0 30 '40 50 6i0" 70 80 90 I00 BO 70 60 50 40 30 20 IO O Figure paste consistency/dry-out characteristics are improved if ratios on the steep parts of the curve are avoided.