188 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS SIEVES To come now to particle size measurement, the coatset fractions are dealt with by means of sieves. Here again the theory and technique of sieving has been the subject of a great amount of work. As most cosmetic materials are in the sub-sieve range I do not wish to consider sieves further here except for one or two comments. Their use for us will be confined to the natural minerals and to cases where accidental oversize particles are encountered. The practical lower limit of particle size measurement obtain- able by sieving is 40-50 microns. It is perhaps not too trite to point out that sieves, particularly the finer ones, are high-precision instruments and should be treated as such. The mesh should be checked occasionally with a measuring lens. There is no limit to the amount of sample which may be examined in usually 10-50 gm. batches. MICROSCOPE The optical microscope has been much used for particle size determination in the sub-sieve range and some authorities still consider it the best. How- ever, it is a very tedious time-consuming method and the amount of sample examined is extremely small. When it is realised that a 40-micron particle of, say, talc weighs about 10--7 gms. and a 2-micron particle about 10 -• grns., • count of 10,000 particles in this range may not amount to more than one- tenth of a milligram, and the reliability of sampling becomes of great importance. Much depends on the operator and the preparation of slides. ELUTRIATION AND SEDIMENTATION Many designs of apparatus have been based on the principle of vertical movement of solid particles in a fluid medium. Elutriation involves move- ment of particles in a rising column of fluid, usually water or air, and sedimentation the downward movement of the particles in a stationary column of liquid. The terminal velocity of fall v of a particle depends on the "diameter" d and density o- of the particle, the density p and viscosity n of the fluid, and on gravity g. The relationship is expressed by Stokes law (c.g.s. units). _ p)g V--- 18n Dealing first with elutriation in which separation of the powder is achieved by suspension in a rising column of fluid an upward speed of fluid is fixed according to the desired particle size cut. Particles coarser than this size will fall, and particles finer than this will rise to be collected and weighed. A series of such cuts must be made to give a measure of particle size range. Multiple instruments have been designed having a series of columns of suc- cessively larger diameters to give three or more fractionations in one operation. Examples of types:

FINE PARTICLES IN THE COSMETIC INDUSTRY 189 Single cut Gonell Air Elutriator. Double cut Andreasen Air Elutriator. Andrews Liquid Elutriator. Mul.tiple cut Haultain Infrasizer (Ai•). The size of sample used in such instruments is 10-50 grns. depending on density of particle, and the method is useful over the range of 10-50 microns. The main difficulty in such instruments is to maintain the fluid stream steady and even over the cross-section of the column. Further, and particularly with air elutriators, dispersion of the powder as single particles is almost impossible with some powders. By the same token the rising particles will adhere strongly to the surface of the column and re-agglomerate. Some form of vibration of the apparatus is often required. The time necessary for a determination by this method is very variable---from a few hours to a few days, depending on the nature and the particle range of the powder. Nevertheless it is very useful in some circumstances, particularly when close particle-range fractions are required for further examination. Looking back to the Stokes equation, it will be seen that increase of g would reduce the time of a determination. Consequently several methods have been devised using centrifuges, particularly in liquid media. A centri- fugal method of the air elutriation type has recently become available. The matehal is fed from the central point of a rotating chamber against a spiral air flow and the powder thereby separated into two fractions above and below the required particle size cut. The particle size cut is controlled by the speed of the rotor, which must be kept constant for each size limit, and by the spacing plate at the air induction point. The feed is of the vibrator type, closely controlled to a rate of 1-2 gm. per minute. When the finest fraction has been removed the residue is returned to the feeder and the whole repeated at a different setting. The instrument will deal with 10- 40 grn. sample. Sedimentation methods may be exemplified by the Andreasen pipette. A weighed amount of powder is suspended in a column of volume about 600 millilitres and height about 25 cm. The time of fall through a height of 20 cm. is calculated from Stokes Law for a series of particle sizes. At such times after the beginning of the operation a 10 mi. sample of the suspension is withdrawn from the level at 20 cm. below the top surface of the column. This is transferred to a dish, the suspension evaporated to dryness and the residue weighed. This residue is the weight fraction of the sample which is less than the particle size which had been used to calculate that particular period of settling. In this method 7-20 gm. of sample may be used, corre- sponding to 1 per cent by volume of the suspension. Many refinements of the sedimentation method have been attempted. One such incorporates a scale pan in the bottom of the suspension and gives a continuous register of the amount of deposition.