[A] Fig. 15 shows the arrangement for preventing dust at every point where it is produced in a factory where dry colours are ground, sifted, and packed on a large scale. On the upper floor, the chamber is shown in which the contents of a cask are tipped down a shoot leading in the one case to the burr stone mill on the left, and in the other into the Blackstone sifters. Exhaust is arranged at two levels to catch the dust arising from the displacement of air. After grinding in the closed-in burr stone mill, a hood and duct is arranged over the point where the material is discharged into the barrel. Similarly, the casing of the two Blackstone sifters is connected with the exhaust fan, and also the cover of the barrel into which the ground material falls. Inside the edge-runner (the door of which is shown open) a negative pressure is maintained, and one branch duct controls the dust in the scooping out of the material from the barrel, while another is connected to the cover of the receptacle into which the ground material is discharged.
Tapering of the ducts, tangential entry of branches, fan-box, and collecting filters, are all shown. In the factory in question there are four edge-runners, three burr stone mills, and two Blackstone sifters. Altogether exhaust ventilation is applied at twenty-five points. (Drawing kindly supplied by the Sturtevant Engineering Company, Limited, London.)
The lighter shades of yellow chrome are made by a cold precipitation process, or (as is usual for the deeper shades of chrome, orange, and red) by boiling the ingredients—lead acetate, pulp white lead, bichromate of potash and soda, and sulphate of soda—while barytes is added as the colour is being made. Danger in the first method does not arise (or only in minor degree when steam is injected to bring about more speedy solution) until drying and grinding (in edge-runners), sieving, and packing, are effected. The dust, when inhaled, is quickly absorbed, and in all these dry processes danger, in the absence of very carefully thought out exhaust ventilation, is great. In processes involving ebullition, danger is present in the steam which carries up with it chromate of lead in fine particulate state. Vats and vessels, therefore, in which the boiling is effected require partial hooding over and connection of the hood with an efficient exhaust. In subsequent wet processes of pressing the cakes of chromate of lead, the hands, arms, and overalls become thickly coated with pigment. Danger from chrome greens is practically limited to the dust created in dry grinding, usually effected in large edge-runners.
For references, see [end of Chapter XVII.]
CHAPTER XVII
DESCRIPTION OF PROCESSES—Continued
Coach-Painting.
[34]—Lead poisoning is peculiarly prevalent in this industry, and no corresponding reduction in the number reported can be observed from year to year (see the table on [p. 47]), or in the many industries grouped under the heading, “Paint used in Other Industries,” such as is noted for lead industries taken as a whole.
Of the 697 cases included in returns during the ten years 1900-1909, 352 were reported from railway carriage and waggon works, 299 from ordinary carriage works and wheelwrights’ shops, and 46 (separate tabulation was only commenced in 1905) in motor-car works. In the year 1903 inquiry was made in 603 factories and workshops, including all classes of coach and carriage building, railway carriage and engine works, and agricultural implement works. Information was asked (among other things) as to—(1) The number of persons employed in painting with lead paints; (2) description of the method adopted for smoothing the coats of paint; and (3) the substitutes tried for white-lead paint. Persons employed numbered 9,608. In 52 factories and workshops smoothing of the coats of paint was not practised, while in the remaining 551 it was affirmed that a wet method alone (pumicestone and water) was used in 178, a dry method alone (sandpaper) in 39, and both wet and dry methods at some stage or other of the work in 334. Substitutes were mentioned as having been tried in 94 instances, but this was almost exclusively for filling and jointing, and not for the first or priming coat.
The figure 178 (wet method alone) is probably much too high, because, while it is true that pumicestone and water alone are used for the flat surfaces of the body—the bulk of the work—dry sandpapering of the first two priming coats and of the final finishing coats (when of white, cream, or yellow colour), of the under parts of carriages, iron chassis of motor-cars, and of curved surfaces, such as the spokes of wheels, is almost universal. The reason for thus treating the priming coat dry is that a wet process would raise the grain of the wood. The 52 factories in which it was stated that no smoothing was done were nearly all premises for the repair or manufacture of railway trucks, requiring no special finish, and the 39 factories in which only sandpaper was said to be used in smoothing were premises in which rough, cheap, or common vehicles, such as carts, were made. Use of sandpaper is quicker and less expensive than use of pumicestone, and water and wet methods cannot be used very well on iron surfaces.
In ordinary coach and carriage painting, after the sandpapering of the first two priming coats, six or seven coats of “filling” (usually ground slate mixed with gold size and turpentine) are applied, and each coat is rubbed down wet. Joints and interstices of woodwork and irregularities in iron surfaces are generally filled in with a stopping or paste of white lead, in the smoothing of which sandpaper is used.