Cork: Its Origin and Industrial Uses

by Gilbert Erwin Stecher

[BOTANY AND CHEMISTRY]

IN considering “cork” for the purpose of ascertaining its characteristics, texture and composition we will, instead of analyzing the material after it has reached the market, look at it from the standpoint of botany and learn of its formation upon the tree, from which it is procured. It appears that the word “cork”[21] in botany signifies a growth peculiar to all plants and pertaining to none in particular, being described as “a peculiar tissue in the higher plants forming the division of the bark (which name is sometimes restricted to the dead tissues lying outside the cork); consisting of closely packed air-cells nearly impervious to air and water and protects the underlying tissues.”[22] Again, “It is produced by the activity and division of certain merismatic cells known as phellogen or cork cambium which are situated immediately within the epidermal covering of the young growth. As the cork cells grow older, their protoplasmic contents disappear and are replaced by air. In order that this formation may be clearly understood, I will quote from a paragraph entitled “Cork and Epidermal Formations Produced by It” contained in “A Text Book on Botany,” by Sacks.

“When succulent organs of the higher plants, no longer in the bud condition, are injured, the wound generally becomes closed up by cork tissue, i.e., new cells arise near the wounded surface by repeated division of those which are yet sound, and these forming a firm skin separate the inner tissue from the outer injured layers of cells. The walls of this tissue offer the strongest resistance to the most various agencies, similar to the cuticular layers of the epidermis in their physical behavior, flexible and elastic, permeable only with difficulty by air and water, they for the most part soon lose their contents and become filled with air. They are arranged in rows lying at right angles to the surface or parallelopipedal form, and form a close tissue without intercellular spaces. These are the general distinguishing features of cork tissue. It is formed not only on wounded surfaces, but arises in much greater mass where succulent organs require an effectual protection (e.g., potato tubers) or where the epidermis is unable to keep up with the increase of circumference where growth in thickness continues for a long period. In these cases the cork tissue is formed even before the destruction of the epidermis, and when this splits under the action of the weather and falls off, the new envelope formed by the cork is already present. The cork tissue is the result of repeated bipartitions of the cells by partition walls, rarely in the epidermis cells themselves, more often in the subjacent tissue. The partition walls lie parallel to the surface of the organ, divisions also taking place in a vertical direction, by which the number of the rows of cells is increased. From the two newly formed thin-walled cells of each radial row one remains thin walled and rich in protoplasm, and in a condition capable of division; the other becomes transformed into a permanent cork cell. Thus arises, usually parallel to the surface of the organ, a layer of cells capable of division, which continues to form new cork cells, the cork cambium or layer of phellogen. In general this is the innermost layer of the whole cork tissue, so that the production of cork advances outwardly and new layers of cork are constantly formed out of the phellogen on the inner surface of those already in existence. When in this manner a continuous layer of cork arises, steadily increasing from the inside, it is termed “periderm.” As the epidermis is at first replaced by the periderm, so in turn is this replaced by cork (the dead tissue). The development and configuration of the cork cells may change periodically during the formation of periderm. Alternate layers of narrow, thick-walled and broad, thin-walled cork cells are formed; the periderm then appearing stratified, like wood, showing annual rings as in the periderm of the Quercus Suber, Betula Alba, etc.”

Mr. Sacks, as a botanist, has clearly set forth the explanation of the formation of the periderm of the Quercus Suber in the foregoing, and although the story of the life producing this formation would be an acceptable sequel to this explanation, it would in no wise assist in the ultimate findings, and therefore it is dispensed with. Mr. William Anderson, in a paper read at the Royal Institution of Great Britain in 1886, has the following to say on cork formation, which is very interesting: “In considering the properties of most substances, our search for the cause of their properties is baffled by our imperfect powers and the feeble instruments we possess for investigating molecular structure. With cork, happily, this is not the case; an examination of its structure is easy and perfectly explains the cause of its peculiar and valuable properties. All plants are built up of minute cells of various forms and dimensions. Their walls or sides are composed chiefly of a substance called cellulose, frequently associated with lignine, or woody matter, and with cork, which last is a nitrogenous substance found in many portions of plants, but is especially developed in the outer cork of exogenous trees, that is, belonging to an order, the stems of which grow by the addition of layers of fresh cellulose tissue outside the woody part and inside the bark. Between the bark and the wood is interposed a thin fibrous layer, which in some trees is very much developed. The corky part of the bark which is outside is composed of closed cells, exclusively, so built together that no connection of a tubular nature runs up and down the tree, although horizontal passages radiating toward the woody parts of the tree are numerous. In the woody part of the tree, on the contrary, and in the inner bark, vertical passages or tubes exist, while a connection is kept up with the pith of the tree by means of medullary rays. In one species of tree, known as the cork-oak, this is strongly developed.” It appears that Mr. Anderson enlivened his lecture by microscopic projections, for he goes on to say: “First I project on the screen a microscopic section of the wood of the cork tree. It is taken in a horizontal plane, and I ask you to notice the diversity of the structure and especially the presence of large tubes or pipes. I next exhibit a section taken in the same plane of the corky portion of the bark. You see 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 being disposed radically to the trunk. The walls of the cells are extremely thin and yet they are wonderfully impervious to liquids. Looked at by reflected light, 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 next exhibited shows a cross section of the cells like a minute honeycomb. In some specimens large crystals are found. These could not be distinguished from the detached elementary spindle-shaped cells, of which woody fiber is made up, were it not for the powerful means of analysis we have in polarized light. I need hardly explain that light passed through a Nicol’s prism becomes polarized, that is to say, the vibrations of the luminiferous ether are all reduced to vibrations in one plane and consequently if a second prism be interposed and placed at right angles to the first, the light will be unable to get through; but if we introduce between the crossed Nicol a substance capable of turning the plane of vibration again, then a certain light will pass. I have now projected on the screen the feeble light emerging from the crossed Nicol. I introduce the microscopic preparation of cork cells between them, and you see the crystals glowing with many colored lights on a dark ground. Minute though these cells are, they are very numerous 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.” In his research or experimentations Mr. Anderson was most deeply impressed with the elasticity of cork, and has the following to say upon his findings: “It would seem difficult to discover any new properties in a substance so familiar as cork, and yet it possesses qualities which distinguish it from all other solid or liquid bodies, namely, its power of altering its volume in a very marked degree in 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 .00005 part by the pressure of one 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, we find it really very rigid. Metals, when subjected to pressure 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. Not so with cork, its elasticity has not only a very considerable range, but it is very persistent. Thus in the better kind of corks used in bottling champagne and other effervescing wines, you are familiar with the extent to which the corks expand the instant they escape from the bottles. I have measured this expansion and find it to amount to an increase of volume of seventy-five per cent; even after the corks have been kept in a state of compression in the bottles for ten years.[23] 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. This property is common to all solid elastic substances when strained beyond their elastic limits, but with cork the limits are comparatively low.” 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, and for this purpose Mr. Anderson instituted a series of experiments of this kind. Into a strong iron vessel of five and one half gallons’ capacity he introduced a quantity of cork and filled the interstices with 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 the 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 which is discernible on the accompanying diagram.

The 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 side, 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 sixteen hundred pounds per square inch the volume was reduced to forty-four per cent, the yielding then becoming very little, showing that the solid parts of the cells had come together and formed a solid, compact mass, thus corroborating Mr. Ogston’s determination that the gaseous part of cork constitutes about fifty-three per cent of its bulk.

In further study it has been found that no matter what compression is used, providing there is no disintegration, the corkwood will retain just that slight spongy character that so marks its growth.

In analyzing this solid matter, Ure found by treating it with nitric acid the yielding was:

White fibrous matter (cellulose)	0.18	parts
Resin	14.72	“
Oxalic acid	16.00	“
Suberic acid	14.4	“
	45.30	parts

Chevruel in an analysis of corkwood states that he found the following constitutents, but he does not give percentages: