Wood Preserving Department 



By E. A. Sterling 



IN the gradual adoption of wood-preserving methods 

 by small consumers for farm and home use, the 

 decision to use treated timber is usually based on 

 the stated facts that decay of timber may be prevented 

 indefinitely by the application of a proper preservative. 

 Those most familiar with the theory and practice of wood 

 preservation are apt to forget that the average consumer 

 has had no occasion to investigate the reasons for the 

 decay of wood, or to ascertain why certain oils or mineral 

 salts will prevent or retard its action. It is, therefore, 

 interesting to review some of the essential features of tim- 

 ber decay and its prevention. 



Nature has provided for the return to the soil, or to 

 conditions resembling it, of all organic matter after it has 

 performed its life function. Otherwise the earth would 

 be encumbered with dead animal and vegetable growths. 

 The medium for this dissolution in the case of wood is 

 mainly a low form of vegetable growth known as fungi. 

 Bacteria may contribute to this result, but in a more 

 obscure and less understood way than fungi. Wood- 

 destroying fungi have their origin in microscopic spores 

 which correspond to the seeds of plants. These spores 

 are present everywhere in the air, and develop rapidly 

 when they lodge upon wood under conditions favorable to 

 their growth. The essential requirements for their growth 

 are moisture, air, moderate temperatures, and food supply. 

 These requirements obviously exist under most outdoor 

 conditions, especially where wood is in contact with the 

 ground, or at the ground line. The fact that all wood is 

 not immediately destroyed is due to its inherent resistance, 

 or to the absence of one or more of the essential factors, 

 usually that of moisture. Furniture and wood used else- 

 where indoors lasts indefinitely because it is dry, and is 

 usually protected to an extent by a surface coating of 

 paint or varnish. 



THERE are several kinds of wood-destroying fungi, 

 but all develop in practically the same way. The 

 tiny spores germinate and send out invisible 

 threads of growth which gradually extend through the 

 tissues of the wood, breaking it down through the process 

 of absorbing certain elements for food. This growth 

 may develop gradually or rapidly, according to conditions, 

 and frequently there is no external evidence until decay 

 is firmly established or widely extended. These hyphael 

 threads may extend and work independently, or become 

 massed together into what is known as mycelium, which 

 is the scientific name of the mass of white fibers often 

 seen between damp boards or under bark. Ultimately 

 these feeding threads produce fruiting bodies which 

 appear on the surface as shelf fungi, or what would be 

 popularly called toad-stool growths of various forms and 



colors. In these fruiting bodies are produced the spores 

 which reinfect other pieces of wood. 



The function of wood preservatives is to have a toxic 

 or poisonous effect on the fungi spores, so that they are 

 killed or cannot develop. Since all decay originates on 

 the outside, often a superficial coating of preservative 

 oil or paint prevents decay for a time. Surface checks 

 and other abrasions ultimately, however, give the spores 

 ingress, after which they develop in the interior of the 

 stick regardless of surface coating. It is for this reason 

 that the most effective preservative treatment actually 

 penetrates the tissues of the wood for a considerable dis- 

 tance, or, as in the pressure treatment, permeates either 

 the entire stick, or at least the sapvvood, which is the 

 most liable to decay. 



THE relative durability of different woods is an 

 interesting subject in itself. This natural resis- 

 tance to decay is considered in selecting woods for 

 treatment, and is really a phase of wood preservation, 

 although not concerned directly with the kind or appli- 

 cation of preservatives. The durability of such woods 

 as cedar, locust, osage orange, redwood and cypress is 

 well known. With the reason for decay in mind, one 

 naturally questions why these woods should remain 

 sound longer than others. It is easier to ask the question 

 than to answer it. 



A wood like locust and cypress, which resists decay, 

 must have some natural qualities which retard the growth 

 of wood-destroying fungi in much the same way as arti- 

 ficial preservatives. Just what these qualities are is not 

 clear. There must exist in these woods some natural oil 

 or substance which is unfavorable to fungus growth. The 

 dense heartwood of pine, since it is heavily impregnated 

 with resinous substances, offers physical resistance to 

 fungi by keeping out moisture and air. Cypress and 

 redwood, on the other hand, are comparatively light woods 

 without resinous contents. What retards decay in one 

 case, therefore, may not be present in some other wood. 

 In chestnut, which the farmers for years have used for 

 fence posts, because quite durable, tannic acid is present in 

 considerable quantity, and has been tried as a constituent 

 of some preservative compounds. Among the durable 

 woods, the sapwood, or outer portion, is less resistant to 

 decay than the heartwood. This is for the reason that 

 sapwood contains more starch and sugar which serve as 

 desirable food for the fungi. This is well illustrated in 

 the case of fallen trees of western red cedar which are 

 often sound after lying in the woods for a century or 

 more. The sap portion in all such cases, however, is 

 completely rotted away, and it is only the heart which 

 remains under the moist, decay-producing conditions of 

 the forest floor. 



561 



