Linoleum.—The larger proportion of the bulk of linoleum is pulverized cork (see Cork). By far the greater part of the cork used in this industry consists of the refuse cuttings from the manu facture of corks for bottles. In making these, there is a large amount of waste, amounting to about 40 per cent., for which there is hardly any other application. Thin sheets of cork, called " virgin cork," can also be procured at a low price ; but they are more difficult to break up, and are not so elastic as the older bark. The cuttings are emptied out of the sacks, in which they arrive, into a deep sieve, the quick lateral motion of which causes all pieces of stone, iron, &c., to fall to the bottom, the lighter cork passing on into the breaker. A considerable quantity of dust and dirt falls through this sieve, amounting to about 5 per cent. of the whole weight of the cork.
The working parts of the cork-breaker are shown in transverse section in Fig. 705, while Fig. 706 is a view from above ; a is a steel shaft, with a rectangular projection b, upon which are fitted the cast-steel discs c and (I. These discs, which are toothed like saws on their peripheries, are f in. thick, and 7 and 9 in. respectively in diameter. Closely adjoining, and partly fitting in between them, aro steel plates a f, firmly bolted together, end fixed to the framework of the machine by the bolts g h. Both discs and plates are kept in their places by nuts, which screw ou to the shaft and bolts.
The shaft should work at a rate of not less than 180 rev. a minute. As the hardest steel soon loses its edge when used for cutting cork, the teeth will require sharpening about once a week. There should, therefore, be dupli oates of both discs and plates, and the framework of the machine must be so arranged that all parts may be easily accessible. The quickest and indeed the only practicable method of sharpening the teeth is by means of a thin emery wheel, similar to those used for sharpening saws. The bearings of the shaft a must, of course, be so constructed as to admit of its being adjusted, as the discs and plates wear away. The cork passes into this machine through a hopper, fixed to the upper part of the sheet-iron casing, with which the whole machine is sur rounded.
On leaving the breaker, the cork appears in pieces about the size of peas, which are then reduced to powder by means of ordinary hori zontal mills, resembling those used for grinding corn. The best stones for the purpose are lava stones ; French burrs, although more dur able, do less work, owing to their smoother faces. The ground cork, on leaving the millstones, passes into a screw elevator, by which it is raised into a sieve. Great care must be taken to prevent an escape of the powdered cork into the atmo sphere of the mill, as a mixture of cork-dust and air in certain propor tions is highly explosive. This has been the cause of frequent accidents; in one instance, the roof of a cork-mill was blown off, and the building was set on fire. To diminish the risk from this source, it is advisable to construct the roof of iron, leaving several of the sheets loosely fastened at the lower end, so that any sudden pressure from within may find an outlet, without destroying the whole of the rooŁ The degree of fineness of the sieve depends upon the quality of the linoleum to he produced. Coarse cork renders the fabric more elastic ; but the surface is not susceptible of a high finish, and the printing is necessarily imperfect. On leaving the sieve, the finer particles fall into sacks, while the coarser residue returns to the mill. The sacks of powdered cork, having been brought to a uniform weight of 56 lb., are placed in a drying-stove, heated day and night to not less than 38° (100° F.). Here they must remain for at least 24 hours ; in damp weather, even longer. From the drying-stove, they are brought to the first mixing-machine, where they are roughly mixed with the cement which unites the particles of cork, forming crude linoleum.
It may here be remarked that some kinds of cork are much darker in colour than others ; on leaving the sieve, the light-coloured dust should be reserved for making brown linoleum, while the darker shades should be used for red. In buying the cuttings, it must be remembered that wet cork is dear at any price, as it takes a very long time to dry, and involves a considerable con sumption of fuel.
Next in bulk to the cork, as a component part of linoleum, comes oxidized linseed oil (see Oils —Linseed), which is produced by exposing boiled linseed oil in a thin film to the action of the air, at a temperature of not less than 21° (70° F.). The most suitable quality of linseed oil for the manufacture of both floorcloth and linoleum is that known as " Baltic." The best way to test it is by boiling about 2 gal. in an iron pot, with the addition of per cent, of ground litharge and per cent. of red-lead. The temperature, controlled by a thermometer immersed in the oil, should not be allowed to rise above (500° F.). In order to accelerate the process of oxidation, air is blown into the hot oil, by moans of a pair of ordinary bellows. Small samples aro taken from time to time, and cooled upon an iron plate. As soon as they appear " stringy " when cool, the pot is
removed from the fire, and its contents are stirred till cold. If the sample in the pot become solid, theail ie suitable for oxidation ; bad oil remains sticky and semi-liquid. The time occupied in testing a sample of oil in the above manner is 2-4 hours, according to quality. Where gas is accessible, it is to be preferred for heating the sample, on account of the greater facility it affords for regulating the temperature. Some oils are liable to froth over the edge of the pot ; this may be prevented by continually raising the contents in a ladle, and letting them fall back into the pot. No reliable chemical test has yet been found to determine the quality of linseed oil, the sulphurio acid test being the only one which gives even approximate results. If the oil be at all fresh on arrival at the works, it should be stored in tanks for at least a month, to allow the mucilage and water contained in it to settle down. It is then run as required into the boiling pane, which are nsually made to contain 10-15 cwt. In order to avoid accidents, these pane should be constructed of wrought iron, and be provided with a hood to carry off the gases evolved. As these gases are extremely offensive, they should be destroyed by passing them through a coke fire. It is not advisable to lead them into a boiler flue, as they contain acrylic and other acids, which speedily corrode the boiler plates. It should also be borne in mind that a mixture of these vapours with air in certain proportions Is explosive ; the fire into which they are conduoted must not, therefore, be too near, nor the pipe leading into it of too great a diameter. The pane are heated by a fire undetneath, and are built in such a way that the oil cannot reach the fire, even should it boil over. Each pan is provided with a stirrer, which should be kept in motion during the whole time of boiling, in order to prevent the driers from caking on the bottom of the pan. From the lower arms of the stirrer, hang pieces of heavy chain, which, by sweeping the bottom of the pan, keep it free from impurities. The tem perature of the oil having been raised to about 177° (350° F.), the driers are stirred in. This should be clone gradually, as the moisture contained in them causes tho oil to froth. The driers used are the same as for testing tho oil ; it is not, how ever, necessary that the lithargo should be ground. The tempera ture ie kept at 149°-177° (300° 350° F.) for about 5 hours ; the fires are then drawn, and the oil ie allowed to cool. A higher temperature than 350° F. during boiling does not improve the drying properties of the oil, and careful experiments have shown that it ie actually injurious. The colour of the oil is darkened, it acquires a strong smell, and the loss of weight by volatiliza tion ie considerable. The tem perature of the boiling oil should therefore be carefully checked by means of a thermometer in serted through a hole in the hood covering the pane. When the oil is sufficiently cool, which will be the ease in about 12 hours, it is pumped or run into the tanks of the oxidizing-sheds. The arrangement of these is given in Figs. 707, 708, and 709 : a are pieces of thin cotton fabric, technically known as " scrim." Their width is either 6 ft., in which case, one piece only is required, or 3 ft , when two pieces are fixed side by side. The length of each piece should not exceed 25 ft., or the fabric will be unable to carry the weight of the oil accumulating upon it. The ends of these pieces of ecrim are pasted or glued over bars of iron b c, and allowed to dry upon them before hanging np. The top bars b are fixed into notches in iron castings, which, in their turn, are screwed to the upper beam h. The lower bars c, are kept in their place by inserting them between iron rods, fixed perpendicularly between the beams d e. Space must be left between d e, to allow for the stretching of the fabric, which sometimes takes place very unevenly. A few inches below e, is a smooth concrete floor, off which the oil runs into a gutter f, leading to the oil-reservoir. Fig. 707 gives a side view ; Fig. 708, a section,tnd Fig. 709, a view from above of a bay in an oxidizing shed. Each building usually contains two of these bays ; it is not advantageous to make the buildings of a larger capacity, on account of the difficulty of heating them uniformly. The interior of the building should be maintained at 21° 38° (70°-100° F.), except when the workmen are obliged to enter. Very efficient means of ventilation must be provided, in order to purify the atmosphere of the building before the entrance of the workmen, as the fumes of acroleln, generated during the oxidation of the oil, are so irritating as to render access difficult until they have been removed. It is chiefly these vapours from the oxidizing buildings that give rise to the nuisance complained of in the neighbourhood of linoleum works. Attempts have been made to destroy them by passing them through a fire ; but the great bulk of air which has to be treated with them, and in a short space of time, has rendered these endeavours fruitless.