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Natural Durahility of Wood 



Durability is a measure of the time during wliich wood remains 

 sound. There are many different factors affecting the durability 

 so that in order to compare different woods in this respect it is 

 necessary to know just what is embraced in the term. The period 

 of soundness is commonly spoken of as the life of a material. 



As a rule, when durability is considered it is with reference to 

 the decay-resisting powers of a wood when in contact with the 

 ground. Decay or rot is the decomposition of wood due to the 

 action of fungi and, to a slight extent, bacteria. Fungi are low 

 forms of plants belonging to the toadstool family. Some of them 

 produce the shelf-like punks so common on old stumps and logs. 

 These punks are the fruiting bodies of the fungus. 



There are a great many different kinds of these fungi. Some 

 tittaek green trees, sometimes killing them or making them hollow, 

 or, as in the case of pecky cypress, filling the wood with galleries 

 like those of boring insects. A much larger number work only in 

 dead wood. Fungi are spread in two ways — by the tiny dust-like 

 seeds (more correctly spores) and by transfer of parts of the 

 living plant itself. Perhaps everyone has squeezed a puffball 

 (which is the fruit of a certain fungus) and observed the issuing 

 "smoke." This "smoke" is in reality composed of thousands of 

 spores, each too small to be seen with the naked eye, but under 

 proper conditions capable of germinating and producing a new 

 plant. Similar spores are produced by other fungi and are shed 

 from the gills of a toadstool or from the underside of a shelf 

 punk. They are scattered far and wide by the wind and other 

 agencies, which explains why they are so widely distributed. 



When a spore finds lodgment in a place where conditions are 

 favorable, it begins to grow. Slender, thread-like structures 

 (mycelium) are produced, which ramify through the wood in search 

 of food which they find in the cell contents or more often in the 

 cell walls. When the desired elements are dissolved out of the 

 ■walls the eventual product is a rotten, moist, pale or dark brown 

 substance crumbling readily under the slightest pressure. 



In order for any fungus to develop, it is necessary that the 

 conditions of heat, moisture and air be favorable. The most 

 conducive temperatures lie between 60 and 100 degrees Fahrenheit, 

 though some growth may take place beyond these limits. It fol- 

 lows that a wood will not rot if its temperature is kept high 

 enough or low enough. Frozen wood will last indefinitely. 



The presence of moisture is absolutely essential to rot produc- 

 tion. Wood containing not more than ten per cent of moisture is 

 imnunie from decay ;ip most fungi require .a larger amount than 

 this for their development. In very humid climates wood is much 

 more liable to decay than in drier regions. Wood has the power 

 of absorbing water from tlie air, the amount depending upon the 

 humidity, so that to preserve wood it is not sufficient that it may 

 have been dried — it must be maintained in tliat condition. Even 

 "dry rot" will not attack dry timber. This fungus grows in 

 humid places where the air is stagnant, so that a free circulation 

 of air under a house is usually sufficient to prevent or check the 

 destructive action. 



In addition to favorable temperature and sufficient moisture, 

 fungi require oxygen (air). If any one of these factors is absent, 

 the fungi cannot live. .lust as a wood, that is kept at a sufficiently 

 high or sufficiently low temperature or in a thoroughly dry condi- 

 tion, will never rot, so will the same effect be secured by 

 excluding oxygen. Thus wood buried in wet clay, in peat bogs, 

 or even immersed in water may last for centuries. Specimens of 

 wood in almost perfect condition have been taken from beneath 

 glacial deposits where they must have lain for thousands of years. 



Peat possesses a remarkable antiseptic property. Large trunks 

 of trees in bogs are often so perfectly preserved that they are 

 used as timber (e. g., bog-oak o'f Ireland and white cedar of New 

 Jersey), and stumps similarly preserved are found with the roots 

 firmly fixed in the under soil as if they had grown there before 

 the bog accumulated. That wood should be durable under such 

 —28— 



conditions is not remarkable, since even human bodies may like- 

 wise be preserved. For instance, in 1747 in an English bog the 

 body of a woman was found, with skin, nails, and hair almost 

 perfect and with sandals on her feet. In Ireland, under eleven 

 feet of peat, the body of a man was found clothed in coarse 

 hair-cloth'. 



It is very evident then that durability is a relative term. 

 Material which may be used as furniture and last indefinitely 

 will, if brought in contact with the soil, rot in a very short time. 

 Woods which may be employed for posts and poles in an arid or 

 semi-arid country are wholly unsuited for such purposes in humid 

 climates. Timbers which can be used in exposed places in very 

 cold climates would be extremely short-lived in the tropics. 

 Moreover, the life of a post or pole may be materially affected by 

 the character of the soil in which it is set. 



It is a well-recognized fact that the sap-wood of no species of 

 timber is as durable under exposure as is the heart-wood. It is 

 evident, therefore, that in the change from sap to heart the wood 

 undergoes some process which makes it more resistant to fungous 

 action. It is well-known, too, that the variability in the decay- J 

 resisting properties of the heart-wood of different species is very 

 great. To what are these differences due ? 



To understand what happens it is necessary to consider the 

 nature of heart-wood and sap-wood. The outer layers of growth 

 of a tree, especially one of considerable size, contain the only 

 living elements of the wood and make up the sap-wood. It is 

 through these layers that the sap finds its way from the roots to 

 the leaves, taking its downward course through the inner bark. 

 There is usually a sharp line of demarcation between the living 

 elements of the sap-wood and the dead elements of the heart, 

 though the vigor of the living cells wanes as their distance from 

 the bark increases. 



When wood is first formed it is almost if not entirely colorless. 

 After a year or two it usually becomes yellowish and still later 

 when changed into heart-wood a decided deepening of the color 

 results. There are numerous partial exceptions to this rule — for 

 example, spruce, fir, hemlock, alder, holly and buckeye. The - 

 thickness of sap-wood varies widely in different species, in dif- | 

 fereut individuals, in different portions of a single tree, and even 

 on diffcrejit radii of any ]iarticular section. Thin sap-wood is 

 characteristic of many woods, such as catalpa, Osage orange, 

 sassafras, chestnut, mulberry, Kentucky coffee tree and red cedar, 

 while in others, for example hickory, maple, ash, hackliciry and 

 beech, thick sap-wood is the rule. 



In the same species there generally exists a constant relation 

 between the size of the crown of the tree and the cross-sectional 

 area of the sap-wood of "the stem. Eapid-growers and trees in the 

 open (second growth) have a larger proportion of sap-wood than 

 those of the same species growing in less open stanils. In the 

 latter ease the number of rings of growth in the sap-wood is 

 greater. Some species, like black walnut and persimmon, are at 

 first very slow in forming heart-wood, but in later life the sa]i-wood 

 becomes comparatively thin. 



These features are important in understanding the different 

 behavior of woods of the same species. For instance, the heart- 

 wood of old walnut makes a lasting post, while a post from a 

 young tree will rot very quickly. The same contrast obtains 

 between the heart of old pine and the sappy second growth. On 

 the other hand, a wood like Osage orange seldom has over two 

 or three growth rings of sap-wood even in young shoots. The 

 writer knows of a ease in Indiana where a stick of Osage orange 

 not over two inches in diameter was split into four pieces and 

 these stakes stuck into the ground about a small red cedar. They 

 remained there for seventeen years and when removed were not 

 'iompletely rotted off. 



Change from sap-wood to heart-wood is not accompanied by 

 increase of wood substance, as some people are inclined to believe. 



