Flat sheets of wood of uniform thicknesses made of two or more thin layers of veneer bonded together with the grain of adjacent layers crosswise -- often also "cross-ply" --, a process that renders much greater strength to withstand swaying and/or to resist warping. Or, sometimes consisting of two series of such layers, where -- with a thicker central core of a different material -- sheets of plywood act as outside layers, thus making a "sandwich".
Compare laminated wood [not yet done]
1907 Timber Trades Journal 13 Apr. 818/2 (advt.)
Agents for Swedish‥wood goods‥ply wood (oak, birch, alder, etc.).
1919 A. W. Judge Handbook of Modern Aeronautics 4 page 234
The ply-woods chiefly employed in aeroplane work may vary from 1/ 8 to 1/ 4 in. in thickness, and are made of ash or birch with an intermediate layer of poplar‥or white~wood.
Historically, plywood appears in the 1920s, but not until the 1930s does plywood become a major component in furniture, especially from the design perspective.
Plywood has an earlier -- much earler -- history, however, under other names, as discussed below.
from John Gerald Shea, Plywood Working for Everyone, 1963:
What Is Plywood?
Wood of itself is a wonderful material. ft has served mankind in countless ways through the centuries. The texture, the mellow tones, the toughness of this natural building material is hard to beat. But wonderful as it is, wood in its native state is subject to deterioration. It cracks, warps, splits, shrinks and swells.
To make the most of the inherently fine qualities of wood for his long-term needs, man has been forced to add a few refinements and improvements. He has contrived methods to re-distribute the physical properties of wood and to control these properties to achieve greater structural strength and stability.
To attain an improved product, the tree was taken apart and its contents redistributed. Thin sheets, or plys, of wood were shaved from the tree. These plys were welded with glue into a sandwich of three or more layers. The grain of each ply was bonded crosswise to the grain of ply-veneers above and below it. The result: a composite, multi-deck sandwich consisting of an odd number of wood plys, or layers, with grain running alternately in both directions.
What are the advantages of this redistribution and reassembly of natural wood?
One decided advantage, because the strength of wood lies with the grain, is the added toughness achieved when plys of wood are bonded together in opposing grain direction. In this way, both length and breadth are strengthened. The resulting panel, made up of alternating wood plys, is twice as strong and rigid as natural, solid wood.
Other advantages are apparent in plywood's resistance to shrinkage. Solid woods tend to shrink across the grain. But in plywood this tendency is greatly minimized by the force of opposing longitudinal grain. In its overall aspects, you will find that plywood has better dimensional stability than wood and that it is less likely to alter its dimensions with each change of moisture content in the atmosphere.
Because of the balanced construction of plywood -- with the grain of one ply bonding and crossing the grain of another -- it has much less tendency to warp than solid wood. Solid wood also is naturally subject to cracking, checking and splitting. On the other hand plywood will resist destructive impact because of its cross-laminated construction. And since plywood has no plane of cleavage, it is virtually splitproof.
Origin of Plywood
In the historic sense, there is nothing new about plywood. Centuries ago Chinese carpenters used the shavings from blocks of wood for surfacing furniture. Antique Chinese wood-work was fabricated on the cross-laminated, plywood principle. Early Egyptian furniture built on the plywood principle has survived generations to become museum pieces. Egyptian mummies were entombed in cases fabricated of plywood and veneer.
In the 18th-century To strengthen the thin sheets used for fretwork on chairs -- and other peices that used fretwork -- cabinetmakers such as George Chippendale would glue thin veneer sheets together -- stringing the sheets in a "cross-ply" arrangement.
George Dodd, a prolific writer on manufacturing process in Victorian Britain tells us in his Dictionary of Manufacturers etc., in the entry on "Veneer", of "press-work", or, otherwise, "a peculiar kind of cabinet-work":
... A peculiar kind of cabinet-work called press-work, of recent introduction, consists in making the entire substance of the wood by means of several veneers placed one upon another. Five, seven, or even nine thicknesses are used; glue, heat, and pressure being the modes of insuring perfect adhesion. The grain of the veneer is made to cross in different directions. Being very strong and yet very light, this pressed work is used for chair-backs and other articles of furniture. The inner veneers need not be of such choice quality as the outer.
Source: George Dodd, Dictionary of Manufactures, Mining, Machinery, and the Industrial Arts... New York: Virtue and Yorston 1869, page 414.
A peculiar kind of cabinet-work called press-work, of recent introduction, consists in making the entire substance of the wood by means of several veneers placed one upon another. Five, seven, or even nine thicknesses are used
In this country the plywood veneering technique was employed as early as the eighteenth century in fabrication of fine furniture. Daniel Webster's prayer desk, with a simulated ply-wood rack for holding the prayer book, is a notable American antique made of plywood-type materials.
But the first attempts at mechanical production of plywood are said to have been made in France. In the 1860's a Frenchman by the name of Garand invented a mechanical veneer slicer. The difficulty with this device was that the thickness of the veneer could not be regulated. However, in the 70's Garand replaced his original cutter with the first rotary veneering lathe. This machine laid the foundation for; the ply-wood industry as it is known today. Evolution in machines since that date has been rapid.
Douglas fir plywood, the original source of this nation's expanding plywood industry, is produced principally in the states of Oregon and Washington. It is here that the giant fir trees grow from which much softwood plywood is manufactured. This wood is ideally suited for peeling (veneering) on rotary lathes as well as for other processes of plywood fabrication.
The first panel of Douglas fir plywood was exhibited at the Lewis and Clark Exposition in Portland, Oregon, in 1905. From this panel has grown an industry producing enough fir ply-wood alone to make a four-foot wide ribbon ofplywood encircling the globe almost ten times! The output of this industry is measured in billions of square feet. This is augmented by massive production of hardwood plywoodsanother branch of the industry.
Evolution of Plywood: -- The Waterproof Breakthrough
Ever since its origin plenty of uses have been discovered for plywood. It found its way into the manufacture of numerous items for industry and the home. Because of its increased stability and ease of fabrication it was used extensively -- but only for construction that would not be exposed to moisture.
Douglas fir plywood, the original source of this nation's expanding plywood industry, is produced principally in the states of Oregon and Washington. It is here that the giant fir trees grow from which much softwood plywood is manufactured. This wood is ideally suited for peeling (veneering) on rotary lathes as well as for other processes of plywood fabrication.
The first panel of Douglas fir plywood was exhibited at the Lewis and Clark Exposition in Portland, Oregon, in 1905. From this panel has grown an industry producing enough fir ply-wood alone to make a four-foot wide ribbon ofplywood encircling the globe almost ten times! The output of this industry is measured in billions of square feet. This is augmented by massive production of hardwood plywoodsanother branch of the industry.
Ever since its origin plenty of uses have been discovered for plywood. It found its way into the manufacture of numerous items for industry and the home. Because of its increased stability and ease of fabrication it was used extensively-- but only for construction that would not be exposed to moisture.
In fact, this very restriction of protection against moisture was its Achilles' heel. For like the fabled Wax Man who performed great wonders until assailed by the Sun and thereby melted, early plywood could do most anything, unless it was exposed to dampness. Thus, for use outdoors, it was worse than worthless; the glueline melted and it simply flaked apart.
Among the first persons to observe this -- to their considerable regret -- were automobile manufacturers. Structurally, and because of its indoor endurance and stability, plywood was considered an ideal material for making floor boards and running boards as well as side panels for cars. The necessary surface protection was provided with numerous coats of paint and varnish. Even so, after a period of outdoor exposure surface plys commenced to bulge and flake apart. As the complaints of their customers ascended to an uproar, auto manufacturers had to forsake plywood. And thus the plywood industry not only lost one of its biggest customers but was also forced to reappraise the limitations of its product and to devise ways and means of doing something to improve it.
The trouble lurked in the glueline. Plywood was only as good and enduring as the glue which held it together. During the early years of its manufacture animal glues were brushed on each joining veneer and the assembled panel was then set up in presses for drying overnight a slow and costly operation resulting in a product which had to be kept dry and protected indoors.
Later, starch, casein and even vegetable glues were introduced. But far from being water-proof these were not even water resistant.
Visualizing the vast market awaiting ply-wood if it could only be used outdoors, the quest for a truly waterproof glue became a search for survival of the industry itself. In fact, during the depression years of the early thirties many plywood manufacturers went into bankruptcy and output of the entire industry declined alarmingly. This happened largely be-cause plywood was not qualified for exterior use.
Meanwhile, a chemical engineer by the name of Dr. James V. Nevin was working on formulae for development of truly waterproof glue. Nevin, an Irish scholar educated extensively both here and abroad, was acquainted with the early works of Dr. Leo Bakeland, the German chemist for whom Bakelite is named. Bakeland conducted experiments with phenolic resins as early as 1909and many of these experiments offered clues followed by Nevin in his later re-search.
There was great jubilation throughout the plywood industry when, late in 1934, Nevin announced that he had developed a completely reliable waterproof glue. Extensive tests were conducted, and in January 1935 commercial production of the product began. First it was called "SUPER" plywood by its original manufacturer -- the Harbor Plywood Corporation. Soon its manufacture became common to the industry, where it was classified as "Exterior Type" -- a revolutionary turning point in the progress of plywood.
Needless to say, Dr. Nevin's discovery of waterproof glue boosted the potential of ply-wood to hitherto unknown heights. All kinds of exterior construction which had previously been impossibleprefabricated housing, boat building, outdoor constructionall imaginable exterior applications of plywood, were now feasible as a result of Nevin's discovery.
Adapted from Walt Durbahn, "What You Should Know About Plywood", American Magazine 157 no 5 May 1954. pages 68-69;
Thin layers of alternating grain make this one of the strongest of building materials.
CAN you imagine wood being stronger than steel? It's a fact. At least, it is in the case of plywood. Pound for pound, plywood is one of the strongest of all building materials. Its strength is in its structure, which consists of alternating thin layers called veneers.Source: Walt Durbahn, "What You Should Know About Plywood", American Magazine 157 no 5 May 1954. pp 68-69
today's plywood sheets -- normally sold in dimensions 8 foot by 4 foot -- are composed of layers Veneer, bonded together with special Adhesives/Glue, under great pressure and heat, so that the Grain or fibers of the wood in each layer--run in opposite directions. Plywood sheets are plywood assembled in odd numbers of layers, or plies (from which it derives its name): 3, 5, 7, etc. The outside layers are called Face Veneers, while the layers in between are known as the core stock. This core stock usually is thicker, softer, and of a cheaper grade of wood. Face veneers, which give plywood its grain pattern, are of a higher-grade wood.
Back in 1200 B.C. the Egyptians built mummy cases for their departed kings by gluing thin layers of wood to a thicker base. In a sense, this was plywood. The veneering of furniture was begun in the Middle Ages, and reached its peak around 1760, when it was featured by Chippendale, one of the most famous of all furniture designers. So, you see, the idea of plywood goes back a long time.
In the manufacture of plywood using wood such as mahogany, walnut, rosewood, teak, the logs are sliced lengthwise, to take maximum advantage of natural grain figuration
Both crotch and fork sections of trees produce curly and/or flowerlike figure, often reminiscent of well-formed feathers.
The burl, a wart-like projection on a tree trunk, has a large number of dark pith centers of undeveloped buds which appear like "eyes" or knots on the surface of veneers. These are what make burl maple, for example, such a desirable furniture wood.
In addition, there are the striped, fiddleback, mottled, and several other designsall produced by slicing the log in different ways.
Source: Walt Durbahn, "What You Should Know About Plywood", American Magazine 157 no 5 May 1954. pp 68-69
Sources: E. Vernon Knight and Meinard Wulpi, eds., Veneers and Plywood: Their Craftsmanship and Artistry, Modern Production Methods and Present-Day Utility New York: Ronald Press, 1927; Percy A Wells and John Hooper Modern Cabinetwork Furniture and Fitments six editions, 1910 -1952, 2d 1923 ed reprinted 21st century; Rodney Hooper, Woodwcraft in Design and Practice 1937; Rodney Hooper, Modern Furniture Making and Design, 1939. (For an account of a major change in the content of the Wells and Hooper book reflective of the emergence of plywood as a component of furniture construction, click here. )