May 1, 1913.] 



THE INDIA RUBBER WORLD 



415 



Commercial zinc oxide consists of extremely small particles, 

 so small in fact, that it is doubtful if they can be seen under the 

 ordinary microscope. These small particles always form more 

 or less coherent aprgregates, which are apparently somewhat of 

 the nature of a slightly clastic sponge entangling considerable 

 air. These aggregates are what we see and speak of as the par- 

 ticles of zinc oxide. The tendency of an oxide to cohere in this 

 way can be to a considerable extent regulated during the manu- 

 facture. In White Seal oxide, for example, the coherence is 

 much slighter than in any of the other brands, which makes it 

 necessary to pack only 150 pounds to the barrel. By forcing in 

 a larger quantity this property can be partially destroyed. 



It is necessary to distinguish clearly between the degree of 

 coherence of an oxide and the true specific gravity, because 

 the two properties are not infrequently confounded under the 

 term "density." Broadly speaking, all amorphous oxides have 

 the same specific gravity, the variation being fractional and the 

 mean figure in the neighborhood of 5.6. This means that the 

 zinc oxide itself, disregarding altogether its state of aggrega- 

 tion, is 5.6 times heavier than an equal volume of water. If, 

 how'ever, we take a box having a capacity of 1 cubic foot and 

 weigh it full of White Seal oxide, and afterwards full of Red 

 Seal Oxide, being careful not to compress the oxide in the box, 

 we shall obtain a greater weight in the latter case than in the 

 former. This means merely that there is more air entangled 

 in the White Seal than in the Red Seal. It must be borne in 

 mind that this property- of entangling and holding a larger quan- 

 tity of air is associated with the character of aggrcgatiin and 

 compactness of the oxide groups, and that this latter condition 

 has a decided bearing in the application of the oxide for some 

 purposes. 



It is possible to make a series of zinc oxides varying from 

 solid crystals large enough for the form to be clearly visible 

 to the naked eye. and W'hich sink in water like stones, to the 

 very light form known as "philosophers' wool," which floats in 

 the air like thistle-down. All, so far as tested, have the same 

 specific gravity, but the great variations in the amount of en- 

 tangled air causes an enormous variation in the apparent 

 densities. 



The determination of the actual specific gravity is not an 

 easy matter, and considerable care is necessary-. The oxide 

 tends to retain its entangled air very obstinately, and the use of 

 a good vacuum pump is necessary to insure that the air has been 

 completely expelled. 



Owing to its extreme fineness it is useless to attempt to size 

 oxide of zinc by any system of screens. Screening is, however, 

 often useful to break up the aggregates formed. This can best 

 be done in a bolting-machine of the type known as a centrifugal 

 reel, in which a set of beaters revolve at a high speed inside a 

 cylindrical screen which revolves more slowly. The beaters 

 throw- the oxide against the screen and force it through the 

 meshes, w-hich do not need to be finer than 20 to the inch. 

 Shaking screens are unsuitable, as they tend to make the oxide 

 cohere in small balls. Revolving screens without beaters tend 

 to form balls, and also to coat themselves with an impervious 

 layer of oxide, and if brushes are used to clean the screen they 

 only make this coating harder. At intervals the coating drops ofif 

 the screens and. with the balls, is discharged as rejections. This 

 makes the operation of such screens very slow and wasteful. 



Oxide of zinc is one of the most important ingredients of manu- 

 factured rubber, and its use is increasing very rapidly. For- 

 tunately for the user it is remarkably constant in chemical com- 

 position and physical properties. The latter vary in the different 

 grades, but the makers use every endeavor to keep them as 

 nearly uniform as possible in each. Luckily, also for the user, 

 it is rarely adulterated, and the adulterations are easily detected. 

 The price, too. is stable, and any one who is misled by false 

 ideas of economy into buying material as oxide of zinc at much 



less than tlie market price can hardly blame any one but him- 

 self if it proves unsatisfactory. If purchased from a reputable 

 manufacturer and properly applied, it is certain to give satis- 

 faction. 



Lithopone: Lithopone is an amorphous white pigment 

 made by mixing solutions of barium sulphide and zinc sulphate, 

 which causes a transfer of the acids and produces an intimate 

 mixture of zinc sulphide and barium sulphate. The properties 

 of the mixture so formed are quite different from those of a 

 mixture of the two dry salts. Barium sulphate has very little 

 opacity, or hiding power, while zinc sulphide has a great deal. 

 A mixture of the two dry has a hiding power intermediate 

 between the two, and it varies almost directly with the pro- 

 portion of zinc sulphide present. In properly made lithopone 

 every particle of the barium sulphate appears to be coated with 

 zinc sulphide and the opacity is much higher than that of a 

 mixture of the dry salts in the same propoition. 



The process of manufacture is complicated. The carefully 

 purified solutions of the two salts are mixed; the crude litho- 

 pone filtered out. dried, heated to redness and plunged in water. 

 The mixture of lithopone and water is then ground, filtered, 

 dried and re-ground; it is then ready for packing. Every step 

 of the process requires great care and careful regulation of con- 

 ditions. If properly carried out it gives an excellent white pig- 

 ment of great "strength," or hiding power. It is largely used 

 in the manufacture of rubber goods, but the general opinion is 

 that it does not give as strong, or resilient, a product as oxide 

 of zinc. Care must be taken that no lead is present in the com- 

 pound or it w'ill darken seriously. 



A good sample of lithopone should contain not less than 28 

 per cent, of zinc sulphide. It should be a good w-hite, be smooth 

 and free from coarse particles and have good hiding power. 

 For use in rubber manufacture, however, probably the most 

 useful test is to make up a small quantity of the compound. 



NEW YORK FIRE RECORD FOR 1912. 



The report of Fire Commissioner Joseph Johnson for 1912, 

 recently issued, is an unusually interesting one, as it shows a 

 marked increase in the efficiency of the New York Fire Depart- 

 ment : the particularly interesting feature being that this increase 

 of efficiency occurs simultaneously with the increased use of 

 motor vcliicles in the place of horse-drawn vehicles in the de- 

 partment. 



The number of fires during 1912 w-as over 1,000 greater than 

 during the preceding year, being 15,633 against 14,547 for 1911; 

 but the fire loss was not only less per fire, but was over $3,000,000 

 less for the total number of fires. The total fire loss for 1911 

 was $12,470,806, or an average loss of $855, while the aggregate 

 loss for 1912 was $9,0f9.580. or an average loss of $580. the loss 

 of each fire being nearly one-third less than during the preceding 

 year. The commissioner has contracted for 75 more motor 

 vehicles for the department, as follows: 



2 hose wagons $8,326 



16 combination chemical and hose wagons.... 105,170 



3 high-pressure hose w-agons 13,875 



28 second-size steam fire engines 248,920 



1 ladder wagon with 85-foot ladder 8,210 



8 ladder wagons w-ith 75-foot ladders 63,152 



17 ladder wagons with 6S-foot ladders 125,716 



Total $573,369 



The high winds that prevailed during the early part of April 

 had tlic effect of taking the roof off the plant of the New York 

 Rubber Co., at Matteawan. New York, but no very serious harm 

 was done. 



