m3!smmimili)i^'mimi!h^imsim:imimxmim^^ 



An Analysis of Durability 





Durability as applied to wood, broadlj- defined, means the ability 

 to resist decay or, more specifically, it is the length of time which 

 a piece of wood under given conditions resists the influences of 

 decay. Durability is frequently the determining factor in the value 

 of timbers for certain purposes and is also a prerequisite along with 

 other technical qualities for many uses. For instance, railway 

 sleepers must not only be durable but must be sufficiently hard, 

 tough and strong to resist the impact and wearing strain of the 

 heavy traffic on the rails. The length of life in the purpose which 

 they serve determines the relative value of timbers used for 

 telephone and telegraph poles, piling material for wharves and 

 foundations, posts, ties, shingles, sills, structural material and many 

 other uses to which wood is put. Not only does durability apply 

 to wood used in contact with the soil but also to practically every 

 way in which wood is used where it is subject to deteriorating 

 influences. 



All kinds of wood are durable under certain conditions. "When- 

 ever the three elements of heat, moisture and o.xygen are absent, 

 either collectively or individually, wood will last indefinitely. For 

 instance, if timbers are submerged underneath the surface of the 

 ground or water there will be no decay possible. Piles of the 

 ancient lake dwellers in parts of Europe that must have been 

 driven at least two thousand years ago have been unearthed and 

 found to be intact. Pieces of pine of unknown age in good condi- 

 tion have been dug up from holes sixty feet in the earth in the 

 Central West. It is reported that some old piling that Caesar used 

 in crossing certain rivers of France have been exposed and found 

 to be in splendid condition. 



On our lumber markets the durability of certain'Species is recog- 

 nized by the high prices which they command. There is seldom, 

 however, an understanding of the qualities which determine that 

 particular characteristic. Prices are usually directly dependent 

 upon the lasting qualities for certain purposes such as ties, wagon 

 bottoms, poles, structural timbers, etc. 



Whenever the influences of decay as mentioned are removed, the 

 duration of the life of wood is indefinite. Moisture and oxygen 

 may be present, but if heat is absent timbers will not decay. It 

 is the constant change of heat and moisture conditions in our 

 atmosphere that determines largely the life of most of our timbers. 

 For instance, furniture in a house will last indefinitely or until it 

 wears out or breaks apart — decay does not set in because of the 

 even temperature conditions and the absence of moisture. Wood 

 in damp places rots readily because of the presence of both moisture 

 and heat, which together with oxygen in the air are conducive to 

 the growth of minute organisms. This explains why beech furni- 

 ture will last indefinitely, while beech ties will only last from 

 three to five years. This also explains why timbers last a great 

 deal longer in dry, hot climates such as in the desert regions of 

 the Southwest than they do in the East, where there is greater 

 relative humidity in the air, greater precipitation and greater sea- 

 sonal changes of climate. 



In discussing the relative durabilitj' of timber, therefore, we 

 may divide the influences of decay into two broad divisions, — the 

 external and the internal influences. 



EXTERNAL INFLUENCES OF DECAY 



The principal external influence of decay consists of literally 

 hundreds of minute living organisms called bacteria and fungi. 



These cause not only the usual rotting of timbers before being 

 cut in the forest but also those after being manufactured and 

 utilized. The common names applied to the various forms of this 

 decay are "dry rot," "red rot, "• "punk," "peckiness," "blu- 

 ing," etc. The external evidence of these fungi are the fruiting 

 bodies, commonly occurring in shelflike form which scatter their 

 microscopical seeds, called spores, through the air to other wood. 

 These spores develop into living organisms, spread out and form 

 a perfect network in the wood, dissolving the woody structure 

 and cell contents for food and for their further propagation. In 



—22— . 



the case of "bluing" the fungus confines itself to consuming the- 

 starch in the cells and merely leaves a stain. Some fungi attack 

 only certain conifers, others confine themselves to hardwoods. 

 Occasionally some varieties confine their depredations to a single 

 species, but more usually propagate on a variety of woods. Some 

 work from the sap inwards and others attack the heartwood first 

 and cause the heart-rot so common in some of our trees. The tim- 

 bers that are most susceptible to attack are those that on account 

 of warping or checking or through some fire scar or mechanical 

 injury, have a fissure opened in the wood fibers for the entrance of 

 the fungi. This is the reason why it is well to apply an external 

 preventative or preservative such as corrosive sublimate (mer- 

 curic chloride) to prevent sap staining. 



Fungous growths are retarded by cold temperatures and by 

 lessening the moisture content. This is why timbers are more 

 susceptible to decay by rot in the South than in Minnesota, for 

 example. It also explains why seasoned lumber is less readily 

 attacked by fungi. If the moisture content can be maintained at 

 less than ten per cent, timbers are immune from decay. Extreme 

 heat or temperatures over 150 degrees F. also kill or prevent the 

 further development of fungous growths. It is said that tem- 

 peratures of from 65 degrees to 100 degrees F. are most conducive 

 and favorable to the spread and injurious effects of these organ- 

 isms. Wood, however, exhibits a strong absorptive ability and 

 after kiln-drying, lumber should be kept as dry as possible to 

 prevent the re-entrance of moisture in the fibers and a consequent 

 susceptibility to fungous attacks. 



Besides the external influence of living organisms, the presence 

 of water in the soil, the physical characteristics of the soil and 

 the condition of the atmosphere all have a strong bearing on the 

 durability of timbers. If a pole or post is kept in a constantly 

 moist or wet soil it will last much longer than if placed in an 

 alternating dry and wet soil. This is equally true of changing 

 atmosphere and explains why a tie, placed partly beneath "the 

 ground, will check badly. In a stiff clay soil a pole or post usually 

 rots at the ground line, whereas in a sandy or porous soil the wood 

 will be affected down to its base. Some fungi attack the heart 

 first and the wood is rotted gradually from the center out, and in 

 others the reverse is true. However, the sapwood generally decays 

 and rots away first, owing ,to the greater per cent of moisture 

 contained in it and therefore to the fact that it is more inviting 

 to the work of the decaying influence. Wood painted or tarred 

 l>ef ore proper seasoning is likely to be attacked by ' ' dry rot, ' ' 

 for the spores of the fungus are sure to be present and are pro- 

 tected from the drying out process so that they can do their work 

 quicker than if the wood were not painted. Charring posts pre- 

 vents decay because the charcoal resists the solvents of fungi and 

 prevents its spread into the wood. 



Durability is also determined to a certain extent by the method 

 of usage and by external wear and tear. For example, paving 

 blocks are laid with their transverse sections upwards because the 

 radial or tangential faces are more susceptible to wearing. 



Insects, beetles, etc., also have a certain relation to durability 

 but are a mechanical influence which presents a different problem. 

 The powder post beetle is the most prominent of the group and 

 works considerable damage through its attacks on the sapwood, 

 .particularly of such woods as hickory, oak, ash, etc. The tereda 

 or marine borer is also a powerful enemy of our wharf piling and 

 determines the life of timbers used in salt waters in the warmer 

 climates. 



INTERNAL INFLUENCES OF DECAY 



The internal influences which determine the rate of decay of 

 timbers are largely chemical rather than physical. It has been 

 demonstrated that there is no direct correlation between the prin- 

 cipal physical characteristics which go to make up the value of 

 woods used where durability is not a determinant factor. Thus 

 weight, strength, stiffness, hardness, toughness, elasticity, etc., do 



