Popular Science Monthly/Volume 31/September 1887/Cork, its Manufacture and Properties
|CORK, ITS MANUFACTURE AND PROPERTIES.|
By ARTHUR GOOD and WILLIAM ANDERSON.
A CONSIDERABLE number of trees, including the cherry, birch, elm, plane-tree, and maple, produce a corky substance in their bark, but in too thin layers to admit of economical use. In Brazil, the bark of a tree of the family of Bignoniaceæ, and the pith of Pourretia tuberculata, of the family of Bromeliaceæ, furnish a kind of cork, as does also the Euphorbia alsaminifera of the Canary Islands. But none of these substances is capable of any important use.
Two varieties of oak, the cork-oak (Quercus suber), which grows in the Mediterranean basin, and the Western oak (Quercus occidentalis) of Gascony, share the monopoly of the production of cork in thick-enough sheets to be utilized. But the natural cork which they furnish, and which is called male or virgin cork, has, whatever its thickness, but slight commercial value; and it is not employed industrially till it has been improved by cultivation. An article of cork, a bottle-cork, for example, is therefore doubly an industrial product: first, as a substance, the qualities of which have been brought out by perfected processes of cultivation and harvesting; and, secondly, as an article that has been manufactured, either by the hand of man or by a machine. It will hence be profitable to study the processes of cultivating and gathering the cork, and the various industrial applications to which the substance lends itself—subjects concerning which no well-composed account has yet been given.
The bark of the cork-oak is composed of two distinct concentric layers: an inner sheet, which is the active part of the bark, and corresponds with the liber of other trees; and a thicker, outer zone, composed of light, compressible, spongy substance, only slightly permeable to liquids, and constituting the cork proper. Wherever upon the body of the tree the inner sheet, or "mother" bark, is destroyed, no further formation of bark or wood takes place; and even a narrow decortication clear around the tree would cause it most certainly to perish. The other coat, or cork, is inert, and does not contribute to the active functions of vegetation; and this explains how it is possible to strip the cork-oak of its corky envelope without endangering the existence of the tree. The inner bark, moreover, if left untouched, will form yearly new layers of cork which may ultimately, when they have become thick enough, be removed in their turn, and furnish the cork of commerce, also called female cork.
According to the most excellent account of the process, given by M. Matthieu, in his "Flore forestière," the demasclage, or removal of the cork, is done in July or August, when the condition of the sap-movement permits an easy separation from the "mother." The work must be suspended when the winds of the sirocco are blowing, for they would destroy the vitality of the inner bark by drying it up immediately; and about two per cent of the trees are likely to be lost, if the operations are crudely performed, by exposure to the glare of the sun. The renewed young bark, if it is permitted to form itself in contact with the air, is also exposed to the attacks of insects and liable to become cracked. To obviate these disadvantages, M. Capgrand Mothes, a French sylviculturist, has devised a method of clothing the stripped oak-trees, by replacing and leaving upon them for a while the cork-bark which has been taken from them. Having been removed in the shape of two half-cylinders, it is easily tied back upon the trunk with wires, while the joinings are covered with strips of paper. The dress is taken off at the end of three months, when the cork which has been utilized to compose it will be found to have become better seasoned than it would have done by the usual method of stacking it. The new bark, under this protection, will have formed only a thin, superficial crust, and that free from cracks and the marks of insect-stings. This process, which furthermore protects the trees against hot winds and insolations, has the additional advantage of expediting by a year the time when the next crop of cork will be fit to gather in.
Before being put into the market, the bark has to be subjected to the operations of steeping, scraping, sorting, and packing. The object of the steeping, which is performed in large boilers of water, heated by means of chips of the bark, is to swell the cork and increase its elasticity, and it has the further effect of enabling curved pieces to be made straight. Scraping, for the removal of the woody parts, is done with iron scrapers, or with a machine in which the rotating chisels make nine hundred turns in a minute. It involves a loss of twenty-eight per cent of the crude bark, but is not so necessary when the formation of the young cork has been protected in the manner that we have described. In England, these two primary operations are replaced by a process of scorching and brushing the bark. The sorting is done with reference to five degrees of thickness, after which the bark is packed in bales containing about two hundred pounds each. At the market it undergoes another testing for quality, in which there is a wide range, and a corresponding diversity of prices. According to M. Lamey, in his book on "The Cork-Oak in Algeria," the bark should never be gathered till it is seven eighths of an inch thick, and that is preferred in commerce which is from one and an eighth to one and a quarter inch thick. To produce such thickness, from six to nine years of growth are needed.
The density of cork varies with its quality and age. Thin corks are usually heavier than those of the same volume that have grown more rapidly, and, in corks of the same class, the density increases with the age. M. Brisson gives 0·240 as an average maximum, and the ordinary density of a ten-years-old cork may be taken at 0·2. With extreme lightness are associated other valuable qualities: that of being a poor conductor of heat and sound; impermeability to liquids; imperfect combustibility, and non-liability to decay, by reason of which it is susceptible of very numerous applications in industry. The most important use of the substance is for bottle-corks. The bark which is intended to be used in this form is kept in a damp cellar. When taken to the shop, it is cut by the first workman into strips, the width of which corresponds with the length of the future cork. A second workman cuts these strips into squares suited in size to its diameter. The squares, strung, are plunged into boiling water to make them swell out. They are then stored in a cool place, and kept constantly moist by sprinkling, till they pass into the hands of the cork-maker. He applies them in succession, giving them a rotary motion, to the edge of a wide-bladed knife, drawing them at the same time slowly along its length, and by skillful manipulation transforms the square into a round cork. This is the method usually practiced in France. Workmen in other countries handle the knife in different manners. It is essential, to obtain a good and solid cork, to take care that its axis, as it is cut from the bark, be parallel with the axis of the tree on which the bark grew; but the broad, flat corks have to be cut perpendicular to the axis of the tree. Only the finest corks are now made by hand. A good workman can turn out, in the method described, about one thousand corks a day.
We give representations of three machines invented by Demuth, which are so simple in their operation that any one can make corks upon them at the first trial. The first machine cuts the cork into strips (Fig. 1); the second into squares—eight thousand a day with a
Fig. 1.—Machine for Cutting Cork-Strips.
woman or a child to work it (Fig. 2). This machine separates the squares automatically, according to their sizes, and by an ingenious arrangement the knives are made to sharpen themselves by passing over a whetstone-rubber at each forward and return motion. With the third machine, for shaping the corks, five thousand corks may be finished in a day (Fig. 3). It is so arranged that the square of cork, firmly fastened between two pointed jaws, turns with them in front of a knife-blade, which is managed with the hand after the fashion of a plane. This blade is so connected by a chain-gearing with the jaws holding the cork, that the movements of the two tools are in harmony with one another. The parts of this machine can be arranged to cut corks of any size, and of cylindrical or conical shape as may be desired. The conical corks are used when the corking is done by hand, the cylindrical ones when it is done by machinery. Other machines for punching corks into shape and for grinding them have been tried and discarded. M. Moreau, the inventor of the latter machine, combined with it the idea of cutting a portrait-face on the cork,
Fig. 2.—Cork-Square Cutting-Machine.
so that every house might send out the image of its chief on its wares; but the device has not advanced further than to the stage of a happy thought. Another machine has been invented in which the knife is a rotating disk, self-sharpening, and the cork also turns.
After having been shaped, the corks are washed in water containing oxalic acid or chloride of tin, or are sometimes treated with sulphuric acid. They thus acquire a characteristic salmon tint, and become velvety and soft to the touch. They are then sorted according to size, tested for quality, counted by hand or by the aid of special machines, and packed in sacks containing from fifteen to thirty thousand each.
The quality most sought in a good cork is impermeability to gases and liquids. The bark may be tested for this quality before making up, by means of an apparatus invented by M. Salleron, in which it is subjected to the pressure of a liquid which has already been compressed in a hydraulic machine. A cork of the first quality should stand a pressure of several atmospheres without absorbing any of the liquid. The waste in cork-making amounts to about sixty per cent; but the chips can all be put to profitable use in making chalk-powders, linoleum, and feltings.
A large number of other substitutes for stoppers of cork have been tried. Those which have given the most satisfaction, in particular cases, are made of ground-glass and India-rubber; but no other device has come into a real competition with cork.
Fig. 3.—Machine for Trimming Corks.
Mr. William Anderson, describing some new applications of the mechanical properties of cork to the arts, insists, as the peculiar quality which distinguishes it from all other solid or liquid bodies, upon its power of altering its volume in a very marked degree m consequence of change of pressure. All liquids and solids are capable of cubical compression or extension, but to a very small extent; thus water is reduced in volume by only 1/2000 part by the pressure of an atmosphere. Liquid carbonic acid yields to pressure much more than any other fluid, but still the rate is very small. Solid substances, with the exception of cork, offer equally obstinate resistance to change of bulk, even India-rubber, which most people would suppose capable of very considerable change of volume, being really very rigid. The latter substance can be compressed indefinitely in one direction while it is left free to move and expand in other directions. When tightly inclosed so that it can not yield, it can not be compressed by any force that can be brought to bear upon it. In the former case all the volume which is lost by the pressure in one direction is regained by the expansion in other directions, and the whole is not changed; in the latter case there is no room for the compensatory expansion; so when India-rubber is stretched, it gains in volume in one direction at the expense of an equivalent loss in other directions.
Metals, when subjected to pressures which exceed their elastic limits, so that they are permanently deformed, as in forging or wire-drawing, remain practically unchanged in volume per unit of weight. Cork behaves in a very different manner. If a cylinder of cork is tightly inclosed in a tube in the same manner as the India-rubber which refused to yield to any force, and pressure is applied to it, it is readily and visibly compressed; and when released it expands back to its original volume. In this case a great change in the volume of the material is easily effected.
When cork is subjected to alternate applications and relaxations of pressure, it coincidently contracts and expands. It is this singular property which gives it its value as a means of closing the mouths of bottles. Its elasticity has not only a very considerable range, but it is very persistent. The extent to which the better class of corks used in bottling the effervescent wines will expand the instant they escape from the bottles, is well known. As measured by Mr. Anderson, this expansion amounts to an increase of seventy-five per cent in the volume, even after the corks have been kept under compression for ten years. If the cork be steeped in hot water, the volume will continue to increase till it becomes nearly three times that which the cork occupied in the neck of the bottle.
When cork is subjected to pressure, either in one direction or from every direction, a certain amount of permanent deformation or "permanent set" takes place very quickly. This property is common to all solid elastic substances when strained beyond their limits of elasticity, but with cork the limits are comparatively low; thus, in chemists' and other shops, when a cork is too large to fit a bottle, the shopkeeper gives it a few sharp bites, or squeezes it with pincers to beyond its elastic limits, and so makes it permanently smaller. Besides the permanent set, there is a certain amount of what might be called sluggish elasticity; that is, cork on being released from pressure, springs back a certain amount at once, but the complete recovery takes an appreciable time.
These peculiar and valuable properties of cork are easily explained after examining its structure. The corky part of bark is composed of closed cells exclusively, and this part is developed to a very unusual degree in the cork-oak. A section of cork, taken in the horizontal plane, such as is represented magnified in Fig. 4, microscopically examined, will show that the whole substance is made up of minute, many-sided cells about 1/750 of an inch in diameter, and about twice as long, the long way of the cells being disposed radially to the trunk. The walls of the cells are extremely thin, and yet they are wonderfully impervious to liquids. Looked at by reflected light, if
Fig. 4.—Cork. Horizontal Section, magnified about 300 Diameters.
the specimen be turned, bands of silvery light alternate with bands of comparative darkness, showing that the cells are built on end to end in regular order. The vertical section (Fig. 5) shows a cross-section of the cells looking like a minute honey-comb. In some specimens large numbers of crystals are found, and are readily distinguished by the aid of polarized light. Minute though they are, they are very numerous
Fig. 5.—Vertical Section.
and hard, and it is partly to them that is due the extraordinary rapidity with which cork blunts the cutting-instruments used in shaping it. Cork-cutters always have beside them a sharpening-stone, on which they are obliged to restore the edges of their knives after a very few cuts; and the machines we have described are for the same reason provided with devices for the automatic sharpening of the knives. The cells of the cork are tilled with gaseous matter, which Mr. G. II. Ogston has proved by analysis to be common air, and to exist occluded in the cork, to the amount of about fifty-three per cent of its volume. The facility with which this air escapes when placed in an exhausted receiver, is very remarkable when compared with the impermeability of cork to liquids. It is the coexistence of these two properties—that of allowing gases to permeate while completely barring liquids, both of which are easily and clearly demonstrated by suitable experiments—that enables cork to be kept in compression under water or in contact with various liquids without the air-cells becoming water-logged; and it is the same properties that make cork so admirable an article for water-proof wear, such as boot-soles and hats. By virtue of the combination, it is superior to India-rubber, for it allows ventilation to go on while it keeps out the wet. The cell-walls are so strong, notwithstanding their extreme thinness, that they appear when empty to be able to resist the atmospheric pressure, for the volume of the cork does not sensibly diminish, even when all the air has been extracted. Viewed under very high power, cross-stays or struts of fibrous matter may be distinguished traversing the cells, which, no doubt, add to the strength and resistance of the structure.
We conclude, then, that cork consists practically of an aggregation of minute air-vessels, having very thin, very water-tight, and very strong walls, and hence, if compressed, we may expect the resistance to compression to rise more like the resistance of gases than the resistance of an elastic solid such as a spring. In a spring the pressure increases in proportion to the distance to which the spring is compressed, but with gases, the pressure increases in a much more rapid manner—that is, inversely as the volume which the gas is made to occupy. But, from the permeability of cork to air, it is evident that if subjected to pressure in one direction only, it will gradually part with its occluded air by effusion—that is by its passage through the porous walls of the cells in which it is contained.
On the other hand, if cork be subjected to pressure from all sides, such as operates when it is immersed in water under pressure, then the cells are supported in all directions, the air in them is reduced in volume, and there is no tendency to escape in one direction more than another. An India-rubber bag distended by air bursts if pressed between two surfaces, but if an India-rubber cell be placed in a glass tube and subjected to hydraulic pressure, it is merely shriveled up, the strain on its walls is actually reduced.
To take advantage of the peculiar properties of cork in mechanical applications, it is necessary to determine accurately the law of its resistance to compression. For this purpose, Mr. Anderson introduced a quantity of cork into a strong iron vessel of five and a half gallons capacity, and filled the interstices full of water, carefully getting out all the air. He then proceeded to pump in water, until definite pressures up to one thousand pounds per square inch had been reached, and, at every one hundred pounds, the weight of water pumped in was determined. In this way, after many repetitions, he obtained the decrease of volume, due to any given increase of pressure. The observations have been plotted into the form of a curve (Fig. 6). The
Fig. 6.—Volume of Cork.
base-line represents a cylinder containing one cubic foot of cork, divided by the vertical lines into ten parts; the black horizontal lines according to the scale on the left hand represent the pressures in pounds per square inch which were necessary to compress the cork to the corresponding volume. Thus, to reduce the volume to one half, required a pressure of two hundred and fifty pounds per square inch. At one thousand pounds per square inch the volume was reduced to forty-four per cent; the yielding then became very little, showing that the solid parts of the cells had nearly come together, and this corroborates Mr. Ogston's determination, that the gaseous part of cork constitutes fifty-three per cent of its bulk. The engineer, in dealing with a compressible substance, requires to know not only the pressure which a given change of volume produces, but also the work which has to be expended in producing the change of volume. The work is calculated by multiplying the decrease of volume by the mean pressure per unit of area which produced it. The ordinates of the dotted curve on the diagram with the corresponding scale of foot-pounds on the right-hand side are drawn equal to the work done in compressing a cubic foot of cork to the several volumes marked on the base-line. The author has not been able to find an equation to the pressure-curve; it seems to be quite irregular, and hence the only way of calculating the effects of any given change of volume is to measure the ordinates of the curve constructed by actual experiment. As may be supposed, the pressures indicated by experiment are not nearly so regular and steady as corresponding experiments in a gas would be, and the actual form of the curves will depend on the quality of the cork experimented on.
So far as preservation of elasticity during years of compression is concerned, we have the evidence of wine-corks to show that a considerable range of elasticity is retained for a very long time. With respect to cork subjected to repeated compression and extension, there is very little evidence to be offered beyond this, that cork which had been compressed and released in water many thousand times, had not changed its molecular structure in the least, and had continued perfectly serviceable. Cork which has been kept under a pressure of three atmospheres for many weeks, appears to have shrunk to from eighty to eighty-five per cent of its original volume.
Mr. Anderson has brought under notice two novel applications of cork to the arts:
One is in the water-raising apparatus called a hydraulic ram, the structure of which is shown by Fig. 7. The ram consists of an inclined pipe, A, which leads the water from a reservoir into a chamber, B, which terminates in a valve, C, opening inward. Branching up
Fig. 7.—Cork in a Hydraulic Ram.
from the chamber is a passage leading to a valve, D, opening outward and communicating with a regulating-vessel, E, which is usually tilled with air, but which the author prefers to fill with cork and water. Immediately beyond the inner valve is inserted a delivery-pipe, F, which is laid to the spot to which the water has to be pumped, in this case to the fountain-jet in the middle of this pan.
The action of the ram is as follows: The outer valve C, which opens inward, is, in the first instance, held open, and a flow of water is allowed to take place through it down the pipe and chamber. The valve is then released, and is instantly shut by the current of water which is thus suddenly stopped, and, in consequence, delivers a blow similar to that produced by the fall of a hammer on an anvil, and, just as the hammer jumps back from the anvil, so does the water recoil back to a small extent along the pipe.
During this action, first, a certain portion of water is forced by virtue of the blow through the inner valve D opening outward into the cork vessel, and so to the delivery-pipe, and instantly afterward the recoil causes a partial vacuum to form in the body of the ram and permits the atmospheric pressure to open the outer valve C and reestablish a rush of water as soon as the recoil has expended itself. In the little ram here represented, this action, which it has taken so long to describe, is repeated one hundred and forty times in a minute. The practical action of the ram, as modified by Mr. Anderson, demonstrates that the elasticity of cork is competent to regulate the flow of water. When air is used for this purpose, the air-vessel has to be filled, and, with most kinds of water, the supply has to be kept up while the ram is working, because water under pressure absorbs air. For this purpose a "sniff-valve," G, is a necessary part of all rams. It is a minute valve opening inward, placed just below the inner valve; at each recoil a small bubble of air is drawn in and passed into the air-vessel. This "sniff-valve," is a fruitful source of trouble. Its minuteness renders it liable to get stopped up by dirt; it must not, of course, be submerged, and, if too large, it seriously affects the duty performed by the ram. The use of cork gets rid of all these difficulties, no sniff-valve is needed, the ram will work deeply submerged, and there is no fear of the cork vessel ever getting empty.
The second novel application of cork is for storing a portion of the energy of the recoil of cannon, for the purpose of expending it afterward in running them out.
The result of the explosion of gunpowder in a gun is to drive the shot out in one direction, and to cause the gun to recoil with equal energy the opposite way. To restrain the motion of the gun, "compressors" of various kinds are used, and in this country, for modern guns, they are generally hydraulic, that is to say the force of recoil is expended in causing the gun to mount an inclined plane, and, at the same time, in driving a piston into a cylinder full of water, the latter being allowed to squeeze past the piston through apertures, the areas of which are either fixed, or capable of being automatically varied as the gun recedes; or else the water is driven out of the cylinder through loaded valves. As a rule, the gun is moved out again into its firing position by its weight causing it to run down the inclined plane, up which it had previously" recoiled. For naval purposes, however, this plan is inconvenient, because the gun will not run out to windward if the vessel is heeling over, on account of the inclined plane becoming more horizontal, or even inclined in the reverse direction; and should the ship take a permanent list, from a compartment getting full of water, the inconvenience might be very considerable.
In land-service guns, when mounted in barbette, the rising of the gun exposes it and the loading detachment more to the enemy's fire; and in both cases, when placed in ports or embrasures, the ports must be higher than if the gun recoiled horizontally, and will therefore offer a better mark to the enemy's fire, especially that of machine-guns, while the sudden rise of the gun in recoiling imposes a severe downward pressure on the deck or on the platform.
To obviate these disadvantages, the author has contrived the gun-carriage, of which Fig. 8 illustrates the internal construction. The
Fig. 8.—Cork Buffer for a Gun.
gun is mounted on a carriage composed of two hydraulic cylinders, A, united so as to form one piece. This carriage slides on a pair of hollow ways, B, and also on to a pair of fixed rams, C, the rear ends of which are attached to the piece D forming the rear of the mounting. There are water-passages down the axes of the rams, and these communicate through an automatic recoil-valve, E, opening from the cylinders, with the two hollow sides B. There is a second communication between the cylinders and slides by means of a cock, F, which can be opened or shut at pleasure. The hollow slides are packed full of cork and water, the latter also completely filling the cylinders, rams, and various connecting passages. By means of a small force-pump, enough water can be injected to give the cork so much initial compression as will suffice to run the gun out when the slides are inclined under any angle which may be found convenient. When the gun is fired, the cylinders A are driven on to the rams C, and the water in the cylinders is forced through the hollow rams into the cork and water vessels formed by the slides B, and the cork is compressed still further. When the recoil is over, the automatic recoil-valve E closes, and the gun remains in its rearward position ready for loading. As soon as loaded, the running-out cock F is opened, the expansion of the cork drives the water from around it into the cylinders, and so forces the gun out. If it be desired to let the gun run out automatically immeniately after recoil, it is only necessary to leave the running-out cock F open, and then the water forced among the cork by recoil returns instantly to the cylinders, and runs the gun out quicker than the eye can follow the motion.
The arrangement adopted may be made by using air instead of cork, but air is a troublesome substance to deal with; it leaks out very easily and without showing any signs of having done so, which might readily lead to serious consequences. A special pump is required to make up loss by leakage.
The merit of cork is its extreme simplicity and trustworthiness. By mixing a certain proportion of glycerine with the water it will not freeze in any ordinary cold weather.
Each of the applications of cork is based on some of the physical or chemical properties of the substance. In bottle-corks its impermeability, elasticity, and imputrescibility are brought into service. Its lightness, the first quality that strikes us, on a superficial view, is not considered.
Cork is used for a variety of other purposes than those which have been mentioned, which, while not so economically important as these, still deserve attention. The male cork, while not well adapted for stoppers, has been made available in the decoration of parks and gardens. Rice-hulling mills have been made from it, but not with much success; small corks can be got out of it. Water-conduits and bee-hives have been constructed from it. It furnishes excellent damp-proof shelves and stands. The Kabyles employ it, mixed with a mortar of mud, in building the walls of their houses, and shingle their roofs with it. It is used for the floats of fish-nets.
These various applications were known, as we learn from expressions of Theophrastus and Pliny, to the Greeks and Romans. Pliny says: "Only that bark is used that is thick and springs back when it is pulled. It is sometimes employed for the buoys of ships' anchors, fishermen's nets, barrel-bungs, and women's winter sandals. The Greeks felicitously called the cork-oak the bark-tree. Cork is used for the covering of roofs." The chips form a good non-conducting material for keeping ice, and reduced to fragments make excellent stable-floors and race-course tracks.
The real, or female cork, has a more homogeneous grain and works much better than male cork. It is a very poor conductor of heat and sound, and has been found valuable to protect hot surfaces against cooling, and to keep frigid substances from melting. It is the basis of several non-conducting mastics and coverings, which are used for protecting pipes, steam-boilers, hot-water reservoirs, etc. Three methods of applying the cork-covering are employed in France. Strips of cork touching at their edges may be laid along the pipes and cylinders and drawn together by wire as in No. 1, Fig. 9. The pipe clothed in this way is a tangent to the inner surface of all the strips, and presents in section a circumference inscribed within a polygon. In the second method, No. 2, thin strips of cork, glued to cloth, by a kind of India-rubber cement, are wrapped spirally round the pipe. The third method consists in employing two hollowed half-cylinders (No. 3) exactly fitting the surface of the pipe. These cylinders, which can be
Fig. 9.—Pipes Covered with Non-conducting Cork. No. 1, bands laid side by side, No. 2, band of cork pasted on cloth and rolled in spiral shape. No. 3, artificial cork.
made of any length, are composed of cork-powder mixed with starch, and are covered with strips of cotton cloth rolled spirally over them, which can be painted with coal-tar or any other suitable paint. Either of the methods will effect a great economy of fuel.
Cork, being also a very poor conductor of sound, is employed successfully in finishing the interior of telephone-cells. It may be put over the doors of consulting-offices; floors made of it are very acceptable in cure-houses and sick-chambers; and it has been adopted in some stringed musical instruments to prevent waste of sound.
Cork has superior buoyant qualities, which are sufficient not only to keep it on the surface, but also to enable it to support tolerably heavy bodies. It is thus employed as a float for night-lamps, for bath-thermometers, and for fish-lines. It is excellently adapted to use in swimming-jackets and life-saving apparatus, in the construction of which inventors have exercised their genius industriously. Many ships carry cork mattresses, which have proved of great service in cases of shipwreck. Life-saving buoys are composed of pieces of cork, are usually in the form of rings, and are furnished with knotted pieces of rope, permitting them to be easily taken hold of. They are also usually covered with painted sail-cloth, to insure their preservation. By the aid of the weighted floating stakes represented in Fig. 10, it is practicable to rescue a person who has fallen into the water at a short distance from the quay on the shore. The slide allows the cord to be taken between two fingers, and the apparatus to be thrown like a sling. This instrument is composed of a rattan or stick of Malacca-wood, having projecting points, around which lead is melted; the whole is then surrounded with cork chips and covered with cloth and
Fig. 10.—Floating Leaded-Stake buoyed with Cork(life-preserver). The apparatus is represented with the line, which is uncoiled when it is thrown into the water.
outside with a network to protect it against friction. Other forms of life-saving apparatus are exemplified in the cork jacket and life-buoy represented in Fig. 11. Rubbers are made of cork inclosed in canvas sacks, and placed along the sides of ships to lessen the shock of their friction against the pier.
The Roman women's custom of wearing cork soles, mentioned by Pliny, has not yet died out, for cork soles are common in the wardrobes of the present day. Cork heels were invented in the time of Louis XV, to be worn inside of the shoe, so as to increase the apparent height of the wearer, without displaying an outer heel. Cork is also useful at the other extremity of the body, shaped into helmets, or as a kind of lining for high hats, or in ventilating-bands, for the protection of the head in hot countries against insolation. Women in the barbarous days, when dead birds were worn in hats, used cork bodies, to which eves beaks, and features Mere added, as the molds for their ornithological structures. Trimming-makers use cork molds or bodies, which they cover with silk or cotton to form elaborate ornaments for mantles and cloaks. Cravats and babies' bibs have been made of cork; and in water-proof garments this material is preferable to India-rubber, in that it allows a freer passage to the air.
Among other miscellaneous applications, may be named those for prosthesis in surgery, naturalists' blocks, rolling-pins for pastry, bath-landings, and wine-labels. The facility with which it is cut, makes cork available for fanciful works of art, as in landscape combinations,
Fig. 11.—Cork Jacket and Life-Buoy.
models of monuments, cases for inclosing bottles to be mailed, spools for silk, the inkstands of our fathers' childhood; pen-holders, which being large and light, do not cramp the fingers, and hundreds of other articles of the kind. There is hardly a profession that does not make more or less use of cork. Gold-burnishers make their rubbers from it, and crystal-polishers their wheel-surfaces. It forms a very light and convenient mounting for watch-makers' lenses, which is used with a minimum amount of fatigue to the muscles of the eye. Applied as a tire to pulley-wheels, it secures a firmer adhesion of the bands. The stoppers of sucking-bottles have been replaced by cork tips which, being very cheap, can be renewed when the presence of a ferment in them is suspected. Cork is also used in a great many children's toys and plays; in fixing wigs on the heads of dolls; in toy guns and pistols; in shuttlecocks and skittles to be played in rooms. In fact, one is almost tempted to inquire to what use it can not be put. The manufacture of all these various articles naturally involves the production of considerable quantities of cork-clippings; these, together with the waste incurred in gathering the crop, and with old cast-away corks, constitute the raw material with which a number of important industries are fed. The coarser chips are sought for as packing material for fragile articles, in which their elasticity gives them a peculiar value. The finer particles constitute suberine powders, the balsamic properties of which are well known to hygienists. In treating the rashes of new-born children, they take the place of lycopodium and starch-powders. An insecticide, which is offered under the name of Zifa powder, is composed of cork mixed with phenol. Fire-kindlings have been made of cork-powder, but they do not seem to have given any grand results. The most important application which has been made of cork-refuse is in the manufacture of linoleum. For this fabric, cork-powder is mixed with oxidated linseed-oil. The resultant paste is then spread upon cloth if a carpeting is to be made, or on paper, if hangings are in view. The color, which is a little darker than that of cork, may be enlivened by colored designs. Applied to moist walls as a foundation, or in hangings, linoleum will receive more substantial paintings than wood which warps, or plaster, or other materials, which are liable to crack. Ceilings may be made from it, which can be washed whenever they become soiled or smoked. "When used for carpets, linoleum makes the floors quite insonorous, and transforms damp and unhealthy rooms into warm and salubrious habitations. It has the advantage, for kitchens and offices, of not being stained by grease. A new decorative product, lino-burgau, is obtained by fluting linoleum, and by the application of colored varnishes and the metalization of some parts giving beautiful iridescent effects.
A composition of coarse powder of cork and milk of lime, pressed into bricks or tiles, forms an excellent material for the construction of conduits, the lining of damp walls, and for roofs. Lining of the cellars of breweries with these tiles diminishes the melting of the ice; in gunpowder-factories it prevents the deterioration of the powder by damp, and, by virtue of its levity and friability, helps to decrease the damage in case of explosion. Employed as pugging for floors, they destroy the disagreeable resonance. In the spinneries of Alsace and Eastern France, the bricks have proved effective to resist the passage of sound, of heat and cold, and are economical withal.
When distilled in a close vessel, chips and waste of cork give off an illuminating gas, which is capable of shedding a brighter light than coal-gas, and is free from the sulphurous emanations which are so objectionable in that illuminant. When tried for lighting the city of Nérac, the difficulty of providing storage for the immense bulk of chips, needed to furnish the required amount of gas, proved so formidable an obstacle to its general use, that the system had to be given up.
Lastly, the extremely fine and durable paint, cork-black, or Spanish-black, is made from carbonized chips and waste of cork.