Dispersion of Pigments by Ball
and Pebble Mills EARL K. FISCHER Research Laboratories, Interchemical Corporation, New York, N. Y.
Scope and Limitation of Optimum conditions for the operation of pebble in the steel ball mills for pigment dispersion have Project f a c t u r e Of been investigated experimentally in mills I n connection with a larger h i s h e s may be considered in connection with other milling ranging from laboratory to Production program on dispersion, the ball sizes. Microscopic examination of the dismill has been considered as a equipment. The two-roll mill persions and the rate of strength developunit operation, and an attempt and the Banbury mill are best merit were used as criteria for evaluation. has been made to isolate cerutilized with mixes of such high tain important variables and apparent viscosity that the The dispersion rates were correlated with to study the effect of each materials may be considered ball size, volume Of and mix upon the rate and quality of plastic solids. The three-roll charges, and mill diameter. The charge the dispersion and the physical mill is probablythe most versaratios usually recommended depart conproperties of the compositions, tile of all instruments, working &derably from optimum milling as well as to obtain the rudiover a wide range of viscosities ments of the economics of ball though reaching the high tions. By adjustment of the formulation mill of the two-roll mill; the quality of the dispersion, too,is excelto allow proper cascading of the balls and Specific formulation probby maintaining the charge only slightly in lems as related to the use of lent on a mill in good condition excess of the voids in the ball mass, milling the ball mill, such as synthetic and properly operated. At the resin finishes, news inks, and low extreme of the viscosity times can be shortened. Operating in this other compositions, have not range in this classification are way, laboratory mills provide a close indicabeen overlooked. Indeed it the colloid and the ball tion of the results obtainable with Producwas in connection with the mill. The colloid mill is notable for high production rates, tion size mills. proper grinding of these products that the need for specific but the quality of the dispersion is usually well below that information became evident. However, these special problems often involve a choice of inof other types of milling, especially for pigments with hard gredients for the formula and the sequence of their combination, aggregates. so that the primary factors of dispersion may be obscured if The steel ball mill, on the other hand, may be operated such formulations are employed exclusivelyin the experimental without attention until any desired dispersion quality is obwork. Accordingly, relatively simple pigment-vehicle systained; volatile solvents in the vehicle are retained by the tems have been used in the experiments reported here. closed drum; preliminary mixing is unnecessary, for all the ingredients may be introduced at one time; the mechanism is simple and low in both maintenance and operation costs. Definitions These advantages are offset to a degree by the high initial I n this discussion, terms which may have ambiguous meancost of the mills and the contamination introduced in light ings are used with the connotations indicated below. colors by the minute amounts of steel abraded from the grind“DispersionJJis a generic term which includes all phases ing interfaces. The pebble mill is generally less efficient than the ball mill, but contamination is different in character. of the preparation of binary systems of immiscible liquids and solids. Pigment powders are considered as “aggregates” or Descriptions of commercial mills were given recently (16, 18). Ball mill operation for the manufacture of dispersions of the masses of homogeneous particles which require an appreciable coating type has been based largely on empirical observation, amount of mechanical work to separate the component particles. This is distinct from “flocculation” where the combinaand discussion on the subject is given in the trade journals tion is disrupted by very weak mechanical forces and by a (2, 6,9, 10). Mill operation for ore dressing processes has, change in the interfacial chemical forces. “Comminution” is however, been studied in considerable detail by a group of investigators, and this work is of assistance (3, 4, 8, 7, 11). considered as the process whereby aggregates are broken down The requirements for ore processing differ notably in that the to smaller and smaller entities. “Grinding” is used synonysize reduction of minerals is from pieces several inches in a mously with comminution, with the implied reference to some device for the application of mechanical work. given dimension to coarse powders; in pigment grinding, on Used more specifically for ball mill work are the following the other hand, a fine pulverized pigment powder is reduced terms: “Cascading” refers to the motion of balls in a rotating to sizes approximating those of the primary partic!es which range from submicroscopic dimensions to several microns. mill in which the balls are rolling over each other in a conContinuous milling is common in ore grinding but has not tinuous, coherent, mobile mass. This is distinct from “catarbeen applied t o pigment dispersion. acting” in which individual balls are thrown clear of the ball
T
HE place of ball and
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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2 hours
7 hours
Vol. 33, No. 12
46 hours
( x 150) SHOWING PROGRESSIVE REDUCTIOX IN PARTICLE SIZEWITH MILLING FIGURE 1. PHOTOMICROGRAPHS (IRON BLUEIN LINSEED OIL) mass and move independently, and “slipping” in which the ball mass moves as a static unit in relation to the walls of the rotating mill. The term “critical speed” was defined by Herman Fischer (7) as the number of revolutions per minute a t which the balls are held by centrifugal force a t the interior periphery of the drum without motion relative to the drum; it has been defined also by the equation. C. S. where C. S.
=
=
54.19/Rlf2
critical speed, r. p. m.; R
=
radius, feet
Experimental Equipment Four mills were used in obtaining the data reported here. The laboratory mill used in the majority of the tests is a five-jar unit equipped with both porcelain and steel jars and driven through a variable-speed transmission. A unit 24 inches in diameter and 6 inches in width was constructed to provide a mill of larger effective diameter with a minimum batch volume. A mill of this type may be of wider interest for factory development tests and is suggested for the reasons of low cost and relatively small mix volume in comparison with an equivalent commercial mill which is 24 X 36 inches in size with a maximum working capacity of 48 gallons. Factory runs were made on a 5 X 5 foot production mill. The dimensional data of these mills are summarized in Table I.
AND CAPACITIES TABLEI. MILLDIMENSIONS
Mill
Lab. (commercial tvwe)
h 6 . - (commercial
type) Lab. (ssecial) Pilot (Commercial type) Production
Construction
Dimensions Ft Diam. Lehgtd
Mill Vol.,
Gal.
Allov steel
0.49
0.46
0.64
Porcelain Steel
0.43
0.40
2
0.5
0.41 11.0
Alloy steel Alloy steel
1.25 5
1.75 5
17.0 734
may be noted from the curves of strength us. hours. Isolation of the effect of various changes in ball size, charge, viscosity, yield value, and other factors may thus be made. The form of the curves obtained may appear on casual examination to be hyperbolic in type, and this may actually be the case. The method for determining the points, however, is such that the curves intersect both axes. I n other words, simple hand stirring of a pigment in oil develops.’a small but finite strength; this would correspond with zero grinding time. The determination of the strength a t the termination of the grind is subject to an error of at least one per cent, so that infinitely small increments of strength gain cannot be detected, and the curve as drawn will meet the 100 per cent limit. The use of bleach tests for determining the relative degree of dispersion is not without objection. Frequently a rapid change in the apparent strength of a bleach has been noted. For carbon black this change is usually less than 2 per cent but in some cases may be as large as 5 per cent. In part the cause has been traced to flocculation, either of the carbon black or the white pigment in the bleaching ink. For example, flocculation of the black reduces the relative tinting effect of the black, and the bleach on drying will appear weak. On the other hand, flocculation of the white pigment will make the black appear stronger. It has been found desirable to use a white ink formulated with essentially the same vehicle as the tinting composition. Error is minimized also by basing strength estimates on the wet drawdown immediately after it has been made. Visual microscopic examination of the grind samples was made for a qualitative check of the dispersion rate as obtained from strength measurements. The increase in strength parallels the reduction in the aggregate size as the milling continues. This is illustrated with photomicrographs in Figure 1 for a grind of iron blue in linseed oil.
Relative Charge Volumes of Balls and Mix EVALUATION OF DISPERSION.A charge of pigment and oil without preliminary mixing was placed in the mill, and samples were taken at intervals during the operation. In the early stages of grinding, samples were taken at frequent intervals; 2Phour samples were found suitable toward the end. A portion of the mix at the termination of the grind was given five passes over a three-roll mill. This was taken as the maximum available strength. Bleach tests were made from each sample and expressed as a percentage of the three-roll mill grind taken as 100 per cent, and from these data the rate of strength development
From theoretical grounds the optimum grinding conditions are obtained when the pigment aggregates are exposed to the maximum number of contacting ball surfaces, and this result is achieved when the mix volume does not exceed the volume of the interstices or voids in the ball mass. The crushing force applied to a fine particle trapped a t the interface of two balls is of the order of several tons per square inch, on the average, in a typical production mill, while the portion of the
December, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
mix in the interstices of the ball mass is subjected only to viscous shear from turbulent flow. Any excess of mix above the grinding media receives merely a tumbling action, quite incapable of disintegrating pigment aggregates. A consideration of the relation of the gross volume of the grinding media (balls or pebbles), the volume of the interstices of the charge of the grinding media, and the total mill volume will serve to clarify the point. Recommended charges of grinding media in relation to the mill volume vary (1, I d , 14), but fairly general practice is
1467
I- 100
6
u
ke.0 0 < I 6.0
I-
Lh
3 0
0
CHARGE F I L L I N G B A L L INTERSTICES
0
EXCESS C H A R G E
12 24 36 GRINDING TIME IN
A8 HOURS
60
72
FIQURE 2. RATESOF DISPERSION AND RELATIVE CHARGEVOLUME (IRONBLUE IN LINSEEDOIL)
based on a one-third charge for steel balls and up to a 50 per cent charge for pebbles. The maximum effective loading is of the order of 55 per cent (6, 7). Larger loadings result in a marked decrease in power consumption, which indicates that ball mobility is lessened and the work done on the pigment particles is correspondingly reduced. Experimental evidence at hand indicates that a ball charge of 45 to 50 per cent of the total mill volume is a practical optimum loading. The limiting packing volume for spheres of a single size has been calculated as 26 per cent voids and 74 per cent solids. This figure holds for spheres of any given, uniform diameter, but not for mixed sizes or shapes which depart from spherical. Actually, however, the volume of the interstices of balls and pebbles as used in milling operations varies generally between 38 and 42 per cent; 40 per cent may be taken as a representative average. The volume of the mix in commercial practice, however, usually exceeds the volume of ball voids by a large amount. Several explanations can be given for adherence to this loading procedure. Promnitz (IS), for example, suggests that the highest point of the ball mass in an operating mill should be immersed in the mix a t all times; hence a relatively large volume of fluid will lie above the balls. Such considerations imply that the mix drains rapidly from the ball voids, and while correct as an assumption for extremely low viscosity grinds (which are not usual), in general the mix follows the balls closely and only slowly drains from the ball interstices. A second explanation lies in the desire to grind as large a volume of product in one charging as possible, although over-all efficiency may be greatly reduced thereby. Experimental evidence for the advantage in charging the mill only slightly above the ball surfaces is given in Table I1 and Figure 2. I n the examples in Table 11,the production is
TABLE11. RELATIVEMIX VOLUMEAND GRINDINGRATE (Iron blue, 30%. b weight in linseed oil. laboratory ball mill, ball sine 0.6 incx, gross ball volume 56% of mill volume) Exoess over Fillin$ ball ball vol mill Relative mix vol. voids filled ib% Charge of mix, grams B t r m th,
Courtesy, -4bbe Engineering Company
(Above) PEBBLEMILLWITH DRIVE, (middle) LABORATORY JAR MILL,(below) BALLMILLWITH DRIVE
7.8%r. 24 hr. 48 hr. 102 hr. Output for equivalent
produat, grams/hr.
Basis for calculation of output.
510
1200
81
65
~ 0 5 100
100 21.2
83 89 995 11.8
I N D U S T R I A L A N D E N G 1-N E E R I N G C H E M I S T R Y
Vol. 33, No. 12
use has been advocated by Kendall (10) who gives a tabulation of recommended size mixtures for porcelain and steel balls and flint pebbles. Practical difficulties inherent in the use of very small balls for grinding media are not t o be overlooked. Discharging the mix, separation of balls accidentally carried into the product, greater heat development, rate of wear, and possibly other problems attend the application of such grinding media. It is postulated that rate of wear, however, is a constant for all ball sizes for a product carried to the same dispersion levels. Thus the longer grinding period with 0.5-inch balls
TABLE 111. CALCUL~TED NUMBER OF CONTACTS OF STEEL BALLS Calcd. No. Calcd. No. No. per Contacts .4v. Size, No. per Contacts Pound per Pounda Inch Pound per Pound0 17,040 2840 0.65 24 144 435 2,610 0.75 16 96 780 130 1.0 6.4 3s 0.50 53 318 2.0 0.8 4.8 I n close packing, each ball touches a total of twelve adjacent balls. Since each contact is formed by two balls, the theoretical number of grinding interfaces is half t h e number of contacts or six times t h e number of balls. Av. Size, Inch 0.131 0.250 0.375 @
TABLE
CouTtesy, Pattersoil
F o u n d r y & Machine Company
INTERIOR OF LINED MILL, SHOWKG CHARGEOF PORCELAIN BALLS
Iv.
RELATION O F
BALLSIZE
TO R.4TE
O F STRENGTH
DEVELOPMENT
(Iron blue, 3 0 % ; No. 0, Litho (linseed) varnish 70%; viscosity, 2.2 t o 3.9 poises; yield value oj
Ball Size, Inch 0.131
-,-Sretnght 2 hr. 78
0.250
0.375
65 46
0.50
55
8 hr. 98 86
71 71
as
7% of Maximum24 hr.
72 hr.
87 85
98 100 92 90
doubled for equivalent dispersion for a given milling time. Thus, a t 24 hours iron blue is carried to 99 per cent maximum strength with charges equal to the ball voids, whereas a t the same time with conventional loading the strength developed is of the order of 92 per cent. A milling time of over 100 hours is required in the latter case t o equal 99 per cent strength. I n Figure 2, which is based on a number of experiments, the rate of dispersion for conventional loading practice is several times slower.
Ball Size and Rate of Dispersion and Contamination The attrition produced by the balls increases as the number of contacts increases. For small balls, 0.131 inch in diameter, the number of points of contact in a given volume is fifty-six times that of balls 0.50 inch in diameter, and the rate of dispersion is proportionately more rapid. A calculation of the number of contacts of balls of different sizes, assuming the maximum theoretical packing of twelve points for each sphere, equivalent t o six grinding interfaces is given in Table 111. The increased rate of dispersion obtained with small ball sizes is shown in Table IV. The advisability of mixing balls of various sizes for batch milling of pigment dispersions is doubtful. Experiments indicate that a mixture of two or more sizes is additive in effect. Thus a mixture of 0.5-inch and 0.25-inch balls effects a rate of dispersion intermediate between that obtained when the same volume of each size is used independently. During grinding the balls classify partially according to size, and theoretical advantages of mixed sizes are not realized. I n unusual milling conditions, however, the addition of a relatively small number of larger balls may serve to prevent localized cementing of the grinding media and the mix. This
Courteey, Patterson Foundry & Machine C o m p a n y
BALLMILL SHOWING CHARGE O F BALLSAND LIFTERBARS ON MILL LINING
INTERIOR O F STEEL
STEEL
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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1469
lustration of the character of the contamination from the mills is given in Figure 3. Partial separation of the steel from the ball mill was accomplished by passing a representative dispersion through a magnetic separator. The product of the abrasion of flint pebbles in a porcelain mill was obtained by running the mill with Varnolene (a petroleum fraction boiling between 150"and210° C.). Courtesy, Patterson Foundry & Machine Company The steel is in the form of extremely fine splinters, CONTINUOUS PEEDAND DISCHARGE TUBBMILL clustered because of the magnetism induced bv the may introduce the same contamination as that of 0.25-inch separator. The wear of the pebbles produces particles which balls, provided the mill is operated only long enough to proare smaller than most pigment particles together with a reladuce the same required fineness. tively small number of chips several microns in a given dimension. I n normal operation the particle sizes of the abraded Data on the wear of grinding media of various types for materials ordinarily do not exceed the minimum required for current formulations are lacking. Kendall (10) gives estiuse in paints and inks. mates as shown in Table V. These figures appear high on the basis of preliminary tests and may be construed to approxiUnusual milling conditions sometimes result in abnormal wear. Excessive speed, resulting in cataracting, produces mate the wear obtained with pigments of the more abrasive chipped and scarred balls and pebbles. Slipping of the types. charge, particularly where the ball mass is cemented into a c solid unit, results in flat areas on the balls. Continued operation leads to orientation of such flat areas against the spherical TABLEV. ESTIMATED WEAR ON GRINDINGMEDIA PER 100 surfaces of adjacent balls, and eventually concavities are HOURS (IO) ground into the balls (Figure 4). Mills are usually cleaned by Porcelain balls 2.0-3.070 Granite balls 0.5-1 .O operation with solvents, and if the operation is unnecessarily High-rarbon for ed steel 0.3-0.7 French flint peb%les 0.3-0.5 prolonged, excessive and abnormal wear results. Danish flint pebbles Forged chrome steel Forged chrome manganese steel
0.2-0.3 0.2-0.5 0.1-0.3
Mill Diameter and Rates of Dispersion The contamination introduced by the wear of the grinding medium is a real problem. Porcelain balls and pebbles on disintegration ordinarily do not seriously affect the color of the grind but instead introduce hard chips or grit. Steel ball mills, while wearing away a t a lower rate, introduce marked color contamination in light colors. Removal of the abraded steel by magnetic Glters has not been entirely successful. 11-
FIGURE3. PHOTO-
(x
MICROGRAPHS 1400) OF STEEL FROM A BALL MILL (le t ) AND
d~-
ARRADED
TERIAL FROM
A
PEBBLE MILL (right) T h e translucent pigment in t h e left-hand picture is natural barytes.
The rate of dispersion is generally considered to be directly proportional to the mill diameter, and accordingly production mills are rated as of higher efficiency than small laboratory mills. Discussion relative to this point by paint technologists has been recorded (9). This differential in grinding time is a reflection of the charging volumes of balls and mix and the influence of the different radii of curvature of the large and small mills on the
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Viscosity of Charge The viscosity of a grind of pigment and vehicle generally increases in proportion as the degree of dispersion increases and may thus be considered a measure of the new surface exposed a t the oil-pigment interface. Yield value measurements in most cases show a rapid change to a constant value which remains essentially unchanged for the duration of the grind. Although these measurements might a t first be considered of value in iollowing the course of the dispersion, no primary reliance may be placed on them. The power consumed by the mill is a function of the viscosity of the mixture as shown in Figure 6. Yield value presumably is of some consequence in the estimate of power used, but inasmuch as it is nearly constant for the major portion of the grind, the relative effect is additive. There are, of course, maximum and minimum limits to the flow characteristics of mixes for best operation conditions in the WEAROF STEELBALLS(ORIGINALSIZE, FIGURE 4. ABNORMAL mill, but such limits cannot be precisely 0.50 TO 0.62 ISCH) placed. Hardinge (8) utilizes the noise of the operating mill to standardize conditions for optimum performance. A quiet mill is obviously mobility of the balls. It is reasonable t o assume that for lownot grinding, and one in which the balls are cascading with viscosity grinds differences in operation will be small. With abnormal velocity and consequent noise is operating with exrelatively high viscosity grinds the ball charge in the small cessive wear. The solution is difficult and necessarily based mill will to some extent pack together and result in a lessened on a measure of empirical judgment. However there appears grinding action; the mass of the balls in a large mill, however, to be no need for rigid control a t this point, for proper caswill retain a high mobility with undiminished grinding action. cading may be recognized by a continuous muffled roar. As a consequence, the rate of dispersion with equivalent ball It has been found convenient in preliminary tests t o start mobility (which may necessitate an adjustment in viscosity a mill with a charge slightly higher in viscosity than is judged of the grind) should be constant for ball mills of all sizes, prqto be desirable for maximum ball mobility and to examine the vided an excess of mix is not present. From present expericharge a t intervals. Additional vehicle or solvent can then mental work this deduction appears valid. be added in increments until optimum grinding conditions are Data on the dispersion of carbon black beads in mineral obtained. oils are given in Table VI and Figure 5 for four different mill The role of surface-active agents has been found in a number sizes. The strength developed after 4 hours of grinding of experiments to be almost exclusively that of reduction in (Table VI) varies from 93 to 96 per cent in mills that range from 6 inches to 5 feet in diameter. The range in viscosity yield value, and except as these materials permitted more effective ball or pebble mobility, there was no favorable effect was from 3.3 to 25 poises. on the dispersion rate.
Summary AND MILL DIAMETER TABLEVI. RATEOF DISPERSION
(CARBON BLACKBEADSIN MINERALOILS)
Mill Diam.,
Ft.
0.49
0.40 1.25 2.0
5.0 5.0
Strength at 4 Hr., % Pigment, % 96 95 94 96 95 93
15 15 15 15 20 25
Visoosity, Poises
3.3 19 15 15 8.3 23
Charging ratios of ball and mix are very important, in that the rate of comminution of aggregates is directly connected
Yield Value, Dynes/Sq.
Cm.
160 750 400 180 340 360
100
+
6U 90 !x
ao MILL D I A M E T E R
Shown graphically in Figure 5 are the data from a large series of experiments on various carbon blacks in the same mills. The points fall within the solid curves drawn as approximate limits which include various operation differences such as temperature, pigment content, viscosity, y.ield value. The significance of these experiments lies in the fact that mill diameter is not directly related to the rate of dispersion, and that for most purposes a laboratory mill will give a reasonably close approximation of the required grinding time for large mills.
$-I d I 40I
I
IN F E E T
0 '0.A9 x 1.25 A
n
2.0 5.0
I
1
I
1
2
3
4
5
GRINDING
I
1
1
7
8 1 0 3 0 5 0 T I M E IN H O U R S
6
FIGURE 5. RAT^ OF DISPERSIONIN MILLS OF DIFFERENT DIAMETERS (CARBON BLACKIN MINERALOILS)
December, 1941
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1471
Courtesy, Patterson Foundry & Machine Company
LABORATORY PEBBLE MILL
with the number of ball contacts, and if the charge does not exceed the volume of the voids of the ball mass, optimum milling conditions are obtained. Grinding charges in excess of this volume markedly decrease the efficiencv of the mill; a c h a r g e smaller than the 100 volume of the int e r s t i c e s of t h e 90 grinding medium is of slight advantage, 5 if any. W STRENGTH The total charge, a balls or pebbles and 70 mix, in relation to the volume of the 60 0 1 2 3 4 5 jar is important j 4 0 0 chiefly as it affects d the motion of the M charge. Thus a mill g= W jar completely filled with balls is static m300 W and no grinding rez sults. Freeboard & 250 0 1 2 3 4 5 above the mix and 25 ball charge sufficient hB L , to allow maximum 20 cascading of the / Y, b charge provide the w best operation conI§VISCOSITY ditions. a Ball charges conIO -J rr l A sisting of mixtures -dl of two or more sizes 5are to a large extent purely additivdn effect. Duringgrinding the balls classify partially according t o size. The rate of grinding is proportional t o the number of contacts of the ball surfaces and is greatest with v e r y s m a l l balls, provided the visFIGURE 6. DISPERSION OF CARcosity of the mix is BON BLACK BE~ADS IN MINE~RAL sufficiently low to OILS (PRODUCTION MILL, DIAMBpermit cascading of TEIR 5.0 FEET)
;-
;
1
the grinding medium. The wear of balls is also in proportion to the number of contacts, and it is postulated that the wear per unit output of product is a constant value. For a soft pigment requiring chiefly turbulent mixing to obtain requisite dispersion quality, ball size is much less important. Viscosity and yield value limits for effective operation can be given only approximately. I n general, the viscosity should be such that the balls are able to cascade freely. I n a small laboratory mill the viscosity must be considerably lower than in a production mill, for the mass of balls in the large mill is far greater and is not a p t to pack into a statio mass. The rate a t which pigments are dispersed in mills of sizes ranging from the small laboratory units to factory production mills is approximately equal provided (a) the balls are cascading freely, (b) the mix volume does not exceed the volume of the interstices of the grinding medium, (c) the mill speed is approximately 50 per cent of critical, and (d) ball sizes and conditions are equal. I n other words, if the conditions of milling follow these requirements, a charge in a small mill will reach a given state of dispersion in the same time as a n equivalent charge in a large mill. This observation, which is a t variance with the usual formulation for grinding times generally accepted, suggests that the transition from laboratory experiments to plant practice may be made with fewer discrepancies if the mill is properly charged.
Literature Cited (1) Abbe, P. O.,and Sellman, Henry, Bull. of Paul 0. Abbe Co., 1937. (2) Anonymous, OfficialDigest Federation Paint & Varnish Production Chb8, NO. 181, 556-7 (1938). (3) Coghill, W.H., and De Vaney, F. D., U. S. Bur. Mines, Tech. Paper 581 (1937). (4) Fahrenwald, A. W., and Lee, H. E., Am. Inst. Mining Mat. Engrs., Tech. Pub. 375 (1931). (5) Garlick, 0. H., Oflcial Digest Federation Paint & Varnish Production Clubs, No. 146,200-6 (1936). (6) Gow, A. M., Campbell, A. B., and Coghill, W. H., Am. Inst. Mining Met. Enprs., Tech. Pub. 326 (1929). (7) Gow, A. M.,Guggenheim, M., Campbell, A. B.,and Coghill, W.H., Ibid., 517 (1934). (8) Hardinge, Harlow, IND. ENQ.CHIOM,, Nsws ED., 17,408 (1939). (9) Henssey, W. H.,Official Digest Federation Paint & Varnish Production Clubs, No. 148,360-4 (1935). (10) Rendall, S. W.,J. Oil Colour Chem. Assoc., 15,66-96 (1932). (11) Mary, J. D.,Paint Tech., 1, 290,298 (1936). (12) Patterson Foundry and Machine Co., Catalog 372 (1937). (13) Promnits, O.,Paint Oil Chem. Rm.,99,No. 6,22 (1937). (14) Redd, 0.F., Bull. Am. Ceramic Soc., 19,253-5 (1940). (15) Underwood, E. M., IND.ENO.CHEM.,30,905-8 (1938). Ibid., 30, 897-904 (1938). (16) Withington, W.H., PRESBXTED before the Division of Paint, Varnish, and Plastics Chemistry at the 102nd Meeting of the Amerioan Chemical Society. Atlantic City, N. J,
