Fig. 75.—Bark-Breaker.
Oak-bark as it is taken off the trees is usually in lengths of perhaps three feet, and it is necessary to cut or break it into smaller fragments before it can be ground in most of the machines just described. This is frequently done by hand by chopping the bark into pieces about four inches long, and the operation is known as “hatching.” Machines on the principle of the chaff-cutter, consisting of a fly-wheel with curved blades radially attached to it, are sometimes used. Instead of “hatching” it, the bark is frequently broken by passing through toothed rollers fitting into each other, and often attached to the mill; the construction of this machine will be readily understood from [Fig. 75].
In Belgium, and some other bark-producing districts, the adhering moss and dead outside bark are usually removed before hatching, but apparently these impurities are frequently re-mixed with the bark after the hatching is completed! As such barks often also contain much clay and dirt, it is generally expedient to pass the hatched bark over a coarse screen before letting it enter the mill, so as to remove the greater part of such rubbish, since, if left in the bark, it produces black and unsatisfactory liquors.
In drawing up policies for fire insurance, it is usual to charge a higher rate where disintegrators are used to grind the tanning material, as owing to the amount of dust and the production of sparks by the striking of the steel parts of the machine on any chance piece of flint or metal which may get into it, there is a greater liability to fire than with toothed mills, although with proper precautions the risk is really small. (Cp. [p. 446].)
All disintegrators act like ventilating fans, and suck in air with the material, blowing it out again with great force at the periphery. This air is heavily laden with dust from the tanning material which is extremely irritating to the lungs. The difficulty is to some extent remedied by an air-channel or flue (generally cast in the casing of the machine) connecting the discharge with the feed-opening so as to convey the air back to the disintegrator. The air is thus circulated through the arrangement, but some is always drawn in from the external atmosphere and driven out with the ground material, and it is advisable that the chamber into which it is discharged should be provided with some means of filtering the air before it escapes. One convenient method is to have a large flannel bag which is blown out by the air like a balloon and out of which the dust can be shaken when the machinery has stopped. Another efficient method is to have one of the walls or the ceiling of the chamber made of canvas or of sacking; but in any case the air should be allowed an escape where a little dust will not cause annoyance.
Chain-Conveyors.—While, in England, the ground material is usually carried from the mill to the leaches in barrows or baskets, in America the use of conveyors is practically universal, and there is no doubt that they effect a great saving of labour at a comparatively small cost.
The most practical conveyor for tanning materials consists of a trough through which an endless chain passes, carrying scrapers. The chain generally used for this purpose is one consisting of square links fitting into each other and capable of running over toothed wheels. These chains are made by several firms in America, and in England by the Ewart Chain Conveyor Co., of Derby, who supply not only plain links but also those having projections to which buckets, scrapers and a variety of attachments may be fixed.
Fig. 76.—Chain-Conveyor.