Cork and its uses

CORK AND ITS USES GILESB. COOKB.ARMSTRONG CORK COMPANY, LANCASTER, PENNSYLVANIA

Cork i s the bark of the cork oak which i s found along the shores of the Western Mediterranean. When the tree is twenty years old the cork i s stripped from the trunk and at nine-year intewals thereafter. Cork i s composed of tiny air-filled cells held together by a natural resinous binder. Because of this structure cork i s light, compressible, resilient, and waterproof. Cork has a low thermal conductivity and a high coefficient of friction. Cork has b e a shown to contain tannins, phlobaphaes, lignin, and cerin. Glycerin and several acids h o e been obtained by saponification. Natural cork is used to manufacture stoppers, life presewers, and many other articles. Scraps of cork are ground and used i n the manufacture of cork composition from which gaskets, polishing wheels, crown seels, and numerous other articles are made. Corkboard for insulation and cork tile, cork carpet, and linoleum for floors, are manufactured from cork.

. . . . . . History

The beginning of the cork industry can be traced back to more than two thousand years ago. The cork oak and its products were well known to the ancient Greeks and Romans. Theophrastus describes cork in his famous work on botany, "Historia Plantarum." Pliny tells of its use for stoppers, floats, and shoes in his "Naturalis Historia." Horace refers to cork as a closure for wine vases, and according to Virgil cork slabs were used for roofs of houses. Plutarch gives the following account of its use by a messenger sent to Rome a t that time besieged by the Gauls: Pontius Comminius, having dressed himself in mean attire, under which he concealed some pieces of cork could not pass the river by the bridge, therefore he took off his clothes which he fastened upon his head, and having laid himself upon the pieces of cork swam aver and reached the city.

In later days cork was used to make barricades on English naval vessels. From Samuel Pepys' diary, we read: 14 July, 1666-After having written to the Duke of York for money for the fleet,

I went down to Thames Street and there agreed for from four to five tons of cork to be sent to the fleet, being a new device to make barricades with instead of junts (old cables).

Notwithstanding these applications of cork its general use was limited and remained so until the middle of the eighteenth century. The first traces of special culture of the cork oak began in Spain in 1760. A resident German leased several cork forests, put them under regular cultivation, and exported the cork to Germany. This example was soon followed by 1463

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, ,

THE DISTRIBUTION OF THE C ORK OAK I N WESTERN EUROPE

AND

N ORTHERN AFRIW

others and the cultivation of cork by 1830 had spread into France, Portugal, Italy, and northern Africa. Growth and Cultivation The widespread use of cork stoppers today has made almost every one acquainted with cork. Yet there is no natural product so universally used about which so little is known. This interesting material has many applications in its natural condition, and it is compounded with other materials to make many valuable products. Cork comes from the cork oak, being a thick protective bark for this species of the oak tree. This oak grows only in certain regions and is found chiefly around the Mediterranean Sea. Portugal is the largest producer of cork and Spain ranks second. Large quantities of cork are also produced in northern Africa, in Algeria, Tunisia, and Morocco. Some cork is also produced in southern France, Italy, and the island of Corsica.

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LEAVES O P THE CORK OAK Similar in shape to the holly leaf, but soft as velvet to the touch

There are two species of cork o a k s t h e Quercus Suber and the Quercus Occideutalis-the chief difference being in the duration of their foliage and the structure and ripening season of their fruits. Although both species belong to the evergreen oaks and have perennial foliage, the Quercus Occidental;~loses its leaves in the early spring of the second year while those of the Quercus Suber remain on the tree for two or three seasons. The cork oaks grow to a height of thirty-five to forty-five feet, and measure from nine to fifteen feet around the trunk. They live for about one hundred years and produce eight or nine crops of cork. The cork acorns ripen in the fall and are usually fed to swine. The piquant flavor of Spanish hams is said to be due to these acorns. Hogs range through the cork forests rooting out underbrush, thus reducing fire hazard. The hog grower pays for this privilege. The hogs are weighed before and after the season, and the owner of the cork forest is paid according to the gain in weight. Cork requires from eight to ten years for growth. If a large cork stopper be carefully examined, the annual rings showing the yearly growth can be seen distinctly. Seasonal differences affect the growth of cork and these variations can be seen in the varying widths of the annual rings. Cork is the outer bark and is only a protective coating for the oak tree. When about twenty years old the cork oak is ready for its first stripping.

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The cork removed a t this time is called virgin cork. It is of an inferior quality and the second stripping furnishes a much b e t t e r grade. However, i t is not until the third stripping that a really fine quality of cork is obtained. Stripping is carried out during the summer months. T h e cork must be very carefully removed, for if the inner bark is damaged no more cork will grow. Cuts are made around the tree, a t the ground and a t the top of the trunk. Then the bark is split vertically in two placesmaking use of the natural crevicesand the cork is forced off the tree. In some places cork is also removed from the larger limbs, but in North Africa t h e French government prohibits the stripping of t h e branches. When the cork is removed from the trees it is stacked in piles and dried in the air for several days. The cork is thenweighed on the type of scale-the Romanowhichwasintroducedby the Romans almost two thousand years ago.

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Preliminary Treatment The cork is then ready for boiling and is conveyed to the boiling station. Boiling is carried out in large tanks over open fires. Large bundles of cork are lowered into the tanks by means of a block and tackle. The tanks are about nine feet in length and depth and about six feet wide. They are half-filled with water, which is kept boiling and the cork is kept submerged by weights for half an hour. This boiling process removes tannins and sap and softens the cork. Also the hard coating on the outer surface of the cork, "hardback," is loosened by the hot water and this is later scraped off. By this treatment the cork bark loses about fifteen per cent of its original weight. After the cork slabs have been boiled and scraped they are trimmed, graded, a n d baled. Each bale weighs about one hundred f i f t y pounds. There a r e many grades of cork and so slight is the difference between some of the grades that only an expert can notice it. Before shipment to the United States cork is sorted into a dozen or more grades of different qualities a n d thicknesses by expertsorters. Againinthiscountry the cork is graded by skilled sorters into approximSECTION OF C O RK S HOWING THE ANNUAL RINGS ately a hundred classes. The importance of this classification cannot be over-emphasized as the economical manufacture of all cork products depends upon utilizing the proper quality of cork for each process. The very finest quality of cork is needed for high-grade stoppers, while stoppers for certain uses can be cut from less perfect cork. Bark that is very porous, or very thin, is ground and used in composition products. Microscopic Structure When a microscopic study of the structure of cork is made, the reasons for its characteristic properties can he more clearly seen. Under the micro-

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STRIPPING CORK FROM A TREE I N SOUII~ICRN F K A N C E Care must be taken not to injure the inner bark.

scope cork is shown to be composed of very small cells, having many sides. It has been shown that fourteen cork cells touch each individual cell ( I ) . The cells are closely packed, and when individual cells are isolated they rearrange t o six-sided figures. The exact size of cork cells depends on the time and the conditions of growth. They average about two-ten-thousandths of an inch in diameter. The cell walls are very thin and their volume foms but a small fraction of the total volume of the cells. Between the walls of the cells can be seen a thread-like layer of darker material. This substance has resin-like properties and binds the cork cells together. The cells are apparently filled with air. (See page 1476 for a photomicrograph showing this characteristic cellular structure.)

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Bunno LADEN WITH C O RK This little animal is not overloaded, for cork is very light.

Physical Properties The conditions under which cork is formed in nature are responsible for its unique and valuable physical properties. Through a ten-year period of very slow growth, and in an arid, windy climate, exposed to abundance of sunlight, cork develops as a protective coating for the oak tree. It is natural then to expect cork to have a t least some of the properties of wood, and other properties which have resulted from changes due to sunlight and heat in a dry atmosphere over a long period of time. Cork is light, compressible, resilient, waterproof, and a non-conductor of heat. It possesses a high coefficient of friction, and has a relatively high tensile strength. These properties are responsible for the varied and indispensable applications of cork. By referring to the photomicrograph (page 1475) of cork, the reasons for these characteristic properties can be more clearly seen.

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CORK AND ITS USES

CORK BARK I'REI'ARATORY TO SCRAPING A N D I%ALINI; A batch is being liftcd from the vat of hot water.

II0ILING

Air-filled cells with very thin walls make up the greater part of the volume of a piece of cork. This explains why cork is light and buoyant, and makes possible its use in life preservers, in floats for gasoline gages, and in many other places where a material of low density is needed. Cork is compressible and resilient, and tests made on one-inch cubes of cork have shown that 14,000 pounds of pressure per square inch could be applied to cork without breaking the cells or permanently affecting the strength of the cork cubes. After this enormous pressure had been removed the cork returned to ninety per cent of its original height, while the sides of the cube still measured one inch. The cork cells and the natural resinous binder which holds them together

\ ~ o R K E R S AT THE

E NTRANCE OF THE B ALING STATION, ALDEAGALLEDA, PORTUGAL Note the huee ~ i l of e cork incirle the vnrd

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CORK AS IT COMESFR OM THE TREE The piece on the left is virgin cork and is used for grinding only.

are impervious to water, giving cork an extensive use where a moisture-resistant material is needed. In a large cork stopper tiny pinholes can be seen and it is only through these openings that water can enter a piece of cork. The moisture-proof properties of cork protect the cork oak from the parching winds of southern Spain by preventing evaporation of the moisture in the tree. The low thermal conductivity of cork is due to the air-filled cells which number about one hundred million to the cubic inch. Heat conduction through a piece of cork must take place along the cell walls and therefore along a very curved path. And because the cell wall is but a very small fraction of the total volume of a cell, the number of such curved paths for heat transfer is relatively small. When one cuts through a piece of cork a great number of the hollow, air-filled cells are cut. These cut cells furnish innumerable cups which

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residual tannin and its presence can be shown by extracting a cork stopper with water, or alcohol, and precipitating blue ferric tannate by the addition ferric of a few d r o ~ sof ~- -----chloride solution. The darkening of cork stoppers in bottles containing iron salts is due to the reaction between tannins and iron. Chevreul (2) in 1807 reported that he had found gallic acid in cork. Drabble (3) in 1906, almost one h u n d r e d years later, extracted cork with water and o b t a i n e d gallic acid. No figures concerning the amount of gallic acid in cork are available, but the Dercentaee A PHOTOMICROGRAPH or C O K K S H O W I N G ITS CHARACis no doubt low. TEnrsTrc A ~ R - F I LCLELLULAR ~ STRUCTURE In connection with Maplification 100 X . the presence of tannins and gallic acid in cork it is interesting to note that Fisher (4)has shown eallic acid that the tannin molecule on hvdrolvsis . .yields ten molecules of " and one molecule of glucose.

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I s o r v ~ o u nCORK ~ CELL Magnification l500X

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Other acids have been obtained from cork but in just what form they are present is not definitely known. In 1877 Hohnel (5) discovered the existence of acid compounds in cork. He showed that cork can be saponified by the action of strong alkalies. Kugler (6) confirmed the observations of Hohnel and succeeded in isolating an acid melting a t 96 degrees. This compound corresponded to the empirical formula G2H,x03 and Kugler called i t phellonic acid. He showed that i t acts like a monobasic acid. Gilson (7), in 1890, found by extracting cork with alcoholic potassium hydroxide not only phellonic acid but also two other acids. One of these which he called was a semi-fluid substance having the formula C17H3003 snberinic acid, and to the other, of the formula CIIH~IO~, which melted a t 121 degrees he gave the name phloiouic acid. Although some structural formulas for these acids have been suggested, their exact structures are as yet unproved. Glycerin has been identified in the residues of cork extracts. Possibly the acids are present as glycerol esters, but these have not been obtained by exhaustive extraction of cork with fat solvents. Even in the finest quality of cork, spots of a reddish brown, p o ~ d e r ysubstance can be seen; this substance is composed of phlobaphenes. They occur in nature along with tannins and can also be prepared by treatin: tannins with a dehydrating agent, such as sulfuric acid. Therefore, phlobaphenes are considered as anhydrides of tannins. They are insoluble in water, but dissolve in alcohol and alkaline solutions. Practically nothiu: is known concerning the chemistry of the phlobaphenes. Cork also contains lignin and a substance which, although it is evidently not cellulose, is cellulose-like in some characteristics. The cork cell wall is made up of this last substance which is very resistant to chemical action. Cork cells may be boiled for an hour in fifty per cent potassium hydroxide, or in fifteen per cent hydrochloric acid, without any detectable change. Only strong oxidizing agents such as chlorine, nitrogen dioxide, or hydrogen peroxide attack the cork cell wall readily. Cerin, so called because it was first thought to be a wax, is also found in cork. This is an extremely unreactive substance and very little is known about it. Cerin was first extracted from cork almost a hundred years ago and since then chemists have endeavored to learn its structure but so far no one has succeeded. In fact, there have been no less than a half a dozen empirical formulas proposed, which indicates that cerin is either difficult to purify, or difficult to analyze, or both. That given by Siewart (S), Cl,H2s0, is probably the correct formula. Cerin is white, forms long needle-like crystals, and melts a t about 247 degrees forming a resinous substance. It is soluble in the common organic solvents. Kugler states that cork contains as high as 2.9 per cent of cerin. There are numerous contradictions in the literature in regard to the quantitative composition of cork. Age, growing conditions, quality of the

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cork, and the amount of adsorbed moisture affect the results of quantitative work and no doubt account for some of the differences. G. Zemplin (9) gives the following analysis of cork: Moisture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alcohol extract.. . . . . . . . . . . . . . . . . . . . . . . Alcoholic KOH extract.. . . . . . . . . . . . . . . . Water extract of residue.. . . . . . . . . . . . . . . . Sulfuric acid (1.5%) extract.. . . . . . . . . . . Lignocellulose.......................... Residue resembling cellulose.. ............

The extractions were carried out in the order listed. Cork is very stable chemically as shown by its use as a bottle closure for thousands of chemicals and solutions, and this fact may be due to long exposure, eight to ten years, in an abundance of sunlight, and in an arid, windy region. Under these conditions polymerizations, dehydrations, oxidations, and condensations have no doubt occurred until the greater part of cork is extremely resistant to reactions of this type. However, there are certain reagents which have decided action on cork. Strong alkalies break down the cork structure, because the binding material between the cells is saponified and removed. Such action does not apparently affect the cell walls. The binding material is evidently a mixture of fatty acids, or their esters, together with lignin. It is well known that lignin readily forms halogen compounds and when cork is exposed to chlorine we get a reaction between the cork lignin and chlorine. If the concentration of chlorine is sufficient the lignin chloride can be removed by subsequent boiling, and the cork particles are reduced to cells. If the chlorine treatment is allowed to act too long, oxidation takes place and the cell walls are attacked. A large number of chemically pure organic solvents are kept in bottles stoppered with corks, the ends of which are covered with tin foil. These solvents do not destroy the properties of cork, but extract only small amounts of the cork constituents, such as tannins. The corks are coated to preserve the purity of the solvents.

Cork Stoppers One of the oldest and most widely known uses of cork is the manufacture of stoppers. The varied physical properties of cork make it an ideal material for bottle closures. When a cork stopper is pushed into the neck of a bottle, the cork is compressed, and because cork is resilient i t exerts a force against the neck of the bottle, making a tight closure. The high coefficient of friction of cork prevents stoppers from becoming loose and slipping out. It is difficult to estimate the number of cork stoppers manufactured


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annually. A conservative figure would be around 10,000,000,000. Corks of all sizes are manufactured, from the smallest medicine stoppers to the largest bungs for kegs. The very finest of all are L used for champagne STRIPS OF CORK CUT FROM A SLAB bottles and the inferior grades serve as closures for bottles containing solids. After the slabs of cork have been carefully graded they are given a steam bath. Exhaust steam is used for this purpose and the pieces of cork are fed into the steam oven a t such a rate as to allow from twenty to thirty minutes for steaming. This treatment softens the cork and makes it more pliable and easily cut. As the slabs are taken from the steam oven they are cut into strips, the width of the strips being determined by the size of the stoppers to he punched. The strips are passed before rotating tubular punches which cut out cylindrical pieces with straight sides. The strips are punched perpendicular to the annual rings. Therefore the diameter of a cork stopper is usually no larger than the thickness of cork bark. A limited quantity of large, thin stoppers are made by cutting the cork parallel to the annual rings. Also, laminated cork stoppers are manufactured for certain uses where very large corks are needed. There is a limited demand for straight-sided corks so most of them are tapered. The corks are fed onto an inclined conveyor which carries them against a revolving cylindrical knife. The conveyor also causes the corks to revolve and they emerge from this operation in the tapered form. The corks are then bleached and sterilized. This process is carried out in large tubs, the light corks being forced under the water by large revolving paddles. After washing the corks are dried. From the driers the corks pass into baskets and they are carefullv graded bv trained operators. Finally, the corks are

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.\ILSC~!LLANEOUSARTICLES OR N A TJRAL ~ CORK AN D CORK COMPOSIT~ON Cork balls, used commonly for baseball Foundation layer of cork beneath the centrrs and surf balls outside wrapping on golf club handles Composition cork rings for laboratory Assortment of shell corks, jar corks, and use mustard corks Various types of cork floats for fisherCork rolls on various types of machinery men's nets and seines to feed and draw cloth, paper, etc. Shoe polish swabs with wood-topped Cork penholder tip cork Cork ribbon from which discs far tube Decoy fashioned from natural cork and bottle caps are punched Novelty pipe. with clay bowl covered with cork composition Composition and natural cork bobbers Cork insoles, fabric covered Natural cork floats used by plasterers

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Life Preservers Another very interesting and indispensable use of natural cork is in the manufacture of life preservers. There are two types, those which are strapped about the body and the circular or "doughnut" type which are known as ring buoys. In the manufacture of ring buoys dry pieces of cork are trimmed to fit into a ring-shaped mold. A biding material is spread between the pieces of cork and wooden pegs are used to hold them together. The top is placed on the mold, compressing the cork as i t is forced down, and made fast with clamps. The mold is then transferred to an oven. This treatment causes the cork to expand and fill in the open spaces between the large pieces, and the binder sets. Afterward, the mold is cooled and the cork ring is removed. Specially cut canvas is sewed around the buoy and rope is attached. Ring buoys must comply with government specifications as to size, weight, and other details. Life preservers must also meet government regulations. The canvas covers are made first and cork is placed in the pockets and sewed in. The cork must be dry and of good quality. Inferior grades of cork are too heavy, or they may be very porous and too light. The cork may be of one piece, or of several pieces skewered together, and is caref u l l y trimmed and shaped to fit the canvas pockets. Other Applications of Natural Cork There are many uses of natural cork which are more or less familiar to every one. Cork CORK A N D CORK COMPOSITION LINERS I N CROWN SEALS is used in the production of sporting goods. Baseballs of the finest grade have cork centers. Fishing rod handles and bobbers for fishing lines are made of cork. Hunters use duck decoys of cork and beach sandals are made with soles of natural cork. Whistles contain small cork balls. Highly carbonated beverages are sealed with a metal cap, lined with natural cork. Cork liners are used in the caps of containers of many toilet articles. For certain uses corks have a wood top upon which a trade-mark may be embossed. Cork rings reinforce wooden spigots in kegs. Fishermen employ cork floats for their seines and nets and large mooring buoys are made of cork. Cork floats are used for gasoline gages. Han-

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Assortment of natural cork handles Ring buoy of cork covered with heavy canvas Natural cork block, used to build up soles of shoes for lame people

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Cork table mats Cork paper shaved from bloeks of natural cork and used principally on tips of cigarettes Heel pads and half insoles from cork

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dles of tools and penholder tips are made of natural cork. Cork is cut into sheets as thin as tissue paper and is used in the manufacture of corktip cigarettes. Insoles for shoes are made of cork and cork is used to build up shoes for the lame. While there are many other applications of natural cork, these are some of the most important and will serve to show the extensive use of this interesting natural product. Cork Composition In the manufacture of cork stoppers a large quantity of the cork, about fifty into scrap. This scrap cork is of high quality and is . .per cent, goes clean and free from h a r d particles. T o waste this scrap would make the price of cork stoppers very high. But the cork industry has found a most important use of this natural cork s c r a p t h e manufacture of cork composition. The scrap formed in punching and beveling cork stoppers is ground to different degrees of fineness and dried. The dry cork granules are mixed with an adhesive material which binds S ~ c n

Cork and its uses

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