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.
[21] See “Etymology of Word” in preceding chapter.
[22] “New English Dictionary,” Murray.
“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.
[23] Showing a permanent set of 12.5 per cent.
[Illustration: VOLUME OF CORK]
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:
Cerin, a soft fragrant resin.
Yellow and red coloring matter.
Quercitannic acid.
Gallic acid.
A brown nitrogenous substance.
Salts of vegetable acids.
Calcium.
Water.
Suberic acid.
Suberin (cellulose).
I am inclined to think that Chevruel selected a poor grade of cork, full
of stone cells and Jasperado, as his findings include much that would
indicate that was the case.
In further defining the various substances which go to make up the body
of corkwood the one that is most impressive is that substance that is
peculiar to cork itself, the others being readily known, but suberic
acid is the one of interest, and this is described by Fownes as a
product of the oxidation of cork by nitric acid; is a white crystalline
powder, sparingly soluble in cold water, fusible and volatile by heat,
the chemical formula given being (C_{4}H_{14}O_{4}). Suberic acid is
also described as a dibasic acid which forms small granular crystals
very soluble in boiling water, alcohol and ether. It fuses at 300
degrees Fahrenheit and sublimes in acidular crystals. It is also
produced when nitric acid acts on stearic, margaric or oleic acid. The
chemical analysis is given as (C_{8}H_{14}O_{4}) and I am inclined to
believe it is the truer one, as it is much later than Fownes’.
This suberic acid has been further broken up to ascertain its
fundamental characteristics and it was found to partake of the two
compounds suberone and suberate.
Suberone (C_{14}H_{24}O_{2}) being regarded as the ketone of suberic
acid, an aromatic liquid compound obtained when suberic acid is
distilled with an excess of lime.[24] Also described as a colorless oil
with an odor of peppermint and a boiling point of 179 to 181 degrees
Centigrade, chemical formula, Suberone—Cycloheptanone—
CH_{2} . CH_{2} . CH_{2}
/
CO
\
CH_{2} . CH_{2} . CH_{2}
[24] “Standard Dictionary.”
Suberate (C_{8}H_{12}M_{2}O_{4}) is known as the salt of suberic acid
having a metal cast,[25] and Suberin or cellulose[26]—is that portion
remaining after nitration and is chemically expressed by the formula
(C_{6}H_{10}O_{5}). Dr. Robert K. Duncan, Prof. of Industrial Chemistry
in the University of Kansas, informs us that this material is the
commonest of common things[27] and when dry, forms one third of all the
vegetable matter in the world. This mysterious substance is the
structural basis of the wood, but with all its prominence and use, we
know nothing more about it than that which is expressed in the formula.
[25] “Watt’s Dictionary” (“M” signifying metal).
[26] “Century Dictionary.”
[27] _Review of Reviews_, September, 1906.
The presence of this cellulose is only a natural fact, as the greater
part of plant life is cellulose; nor is the list of elements that go to
make up the solid matter so strange and unaccountable, but the quality
that makes it a wonderful growth and so popular above its fellows is its
lightness—this is its commendable feature and it is light indeed.
Ure puts the specific gravity at .24 and this is concurred in by
Brisbane.
TEST OF CORKWOOD FOR ASCERTAINING THE POSSIBLE PRESENCE OF AN ESSENTIAL
OIL, BY STEAM DISTILLATION
Two tests were made on this material to ascertain the presence of an
essential oil. The first showed the presence of an oily film,
resplendent in colorings, opalescent, variegated and beautiful, but
odorless and of such small quantity that it may safely be said “No Oil.”
The second proved the same as the first, and although the strong odor of
cork or suberic acid was present, no oil appeared.
The results of these tests indicate that there is no essential oil in
corkwood obtainable by steam distillation.