618 



TIMBER. 



seasons, if the trunk is wounded. In summer, on 

 the contrary, when the leaves are out, the sap is 

 rapidly expended ; and in winter, when the roots 

 are dormant, it is sparingly produced ; so that no 

 surplus of this fluid apparently exists. From rea- 

 soning a priori it would seem that no treatment 

 would be so effectual in getting rid of the greatest 

 quantity of sap as to girdle the tree, hy cutting 

 away a ring of alburnum, in the early part of sum- 

 mer, thus putting a stop to the further ascent of 

 the sap, and then to suffer it to stand until the 

 leaves should have expended, hy their growth, or 

 transpiration, all the fluid which could be extracted 

 by them, previously to the death of the tree.* The 

 wood would thus, probably, be found in the driest 

 state, to which any treatment could reduce it in 

 the living state. Buffon has recommended strip- 

 ping the trees of their hark in spring, and felling 

 them in the subsequent autumn. This method is 

 said to harden the alburnum ; but the cause is not 

 very apparent, nor is the success at all certain. 



Seasoning. At whatever period timber is felled, 

 it requires to be thoroughly seasoned before it is 

 fit for the purposes of carpentry. The object of 

 seasoning is partly to evaporate as much of the sap 

 as possible, and thus to prevent its influence in 

 rausing decomposition, and partly to reduce the di- 

 mensions of the wood, so that it may be used with- 

 out inconvenience from its further shrinking. Tim- 

 ber seasons best when placed in dry situations, 

 where the air has a free circulation round it. Gradual 

 drying is considered a better preservative of wood 

 than a sudden exposure to warmth, even of the sun ; 

 for warmth, abruptly applied, causes cracks and flaws, 

 from the sudden and unequal expansion produced in 

 different parts. Two or three years' seasoning is 

 requisite to produce tightness and durability in the 

 wood work of buildings. It must be observed that 

 seasoning in the common way only removes a por- 

 tion of the aqueous and volatile matter from the 

 wood. The extractive, and other soluble portions, 

 still remain, and are liable to ferment, though in a 

 less degree, whenever the wood reabsorbs moisture. 

 Such, indeed, is the force of capillary attraction, 

 that wood exposed to the atmosphere seldom gives 

 up all its moisture. 



Preservation of Timber. When wood is to be 

 kept in a dry situation, as in the interior of bouses, 

 no other preparation is necessary than that of tho- 

 rough seasoning. But when it is to be exposed to 

 the vicissitudes of weather, and still more when it 

 is to remain in a warm and moist atmosphere, its 

 preservation often becomes extremely difficult. Nu- 

 merous experiments have been made, and many 

 volumes written, upon the preservation of timber, 

 and the prevention of the dry rot ; but the subject 

 is not yet brought to a satisfactory conclusion. The 

 methods which have hitherto been found most suc- 

 cessful, consist in extracting the sap, in excluding 

 moisture, and in impregnating the vessels of the 

 wood with antiseptic substances. For extracting 

 the sap the process of water seasoning is recom- 

 mended. It consists in immersing the green timber 

 in clear water for about two weeks, after which it 

 is taken out, and seasoned in the usual manner. A 

 great part of the sap, together with the soluble and 

 fermentable matter, is said to be dissolved or re- 

 moved by this process. Running water is more 

 effectual than that which is stagnant. It is neces- 

 sary that the timber should be sunk, so as to be 



* Sec M'William on the Dry Rot, pp. 151 and 158. [ 



completely under water, since nothing is more de- 

 structive to wood than partial immersion. Mr 

 Langtonf has proposed to extract the sap by means 

 of an air-pump, the timber being enclosed in tight 

 cases, with a temperature somewhat elevated, and 

 the sap being discharged in vapour by the operation 

 of the pump. It appears extremely probable, that 

 if trees were felled in summer, and the buts imme- 

 diately placed in water, without removing the 

 branches, a great part of their sap would I. 

 pended by the vegetative process alone, and replaced 

 by water. It is well known that branches of plants 

 if inserted in water, continue for some (Jays to 

 grow, to transpire, and to perform their other func- 

 tions. This they probably do at the expense of 1 he 

 sap, or assimilated fluid, which was previously in 

 them, while they replace it by the water they con- 

 sume. This state of things continues until t la- 

 juices are too far diluted to be capable of any longer 

 sustaining life. 



The charring of timber, by scorching or burning 

 its outside, is commonly supposed to increase its 

 durability ; but on this subject, the results of ex- 

 periment do not agree. Charcoal is one of the 

 most durable of vegetable substances; but the con- 

 version of the surface of wood into charcoal does 

 not necessarily alter the character of the interior 

 part. As far, however, as it may operate in ex- 

 cluding worms, and arresting the spreading of an 

 infectious decay, like the dry rot, it is useful. 

 Probably, also, the pyroligneous acid, which is 

 generated when wood is burnt, may exert a pre- 

 servative influence. The exclusion of moisture, by 

 covering the surface with a coating of paint, varnish, 

 tar, &c., is a well-known preservative of wood 

 which is exposed to the weather. If care is taken 

 to renew the coat of paint as often as it decays, 

 wood, on the outside of buildings, is sometimes 

 made to last for centuries. But painting is no pre- 

 servative against the internal or dry rot. On the 

 contrary, when this disease is begun, the effect of 

 paint, by choking the pores of the wood, and pre- 

 venting the exhalation of vapours and gasses which 

 are formed, tends rather to expedite than prevent 

 the progress of decay. Paint itself is rendered 

 more durable by covering it with a coating of fine 

 sand. Wood should never be painted which is not 

 thoroughly seasoned. The impregnation of wood 

 with tar, bitumen, and other resinous substances, 

 undoubtedly promotes its preservation. It is the 

 opinion of some writers,^ that " woods abounding 

 in resinous matter cannot be more durable than 

 others;" but the reverse of this is proved, every 

 year, in the pine forests of America, where the 

 light-wood, as it is called, consisting of the knots 

 and other resinous parts of pine trees, remains entire, 

 and is collected for the purpose of affording tar, 

 long after the remaining wood of the tree has de- 

 cayed. A coating of tar or turpentine, externally 

 applied to seasoned timber, answers the same pur- 

 pose as paint in protecting the wood, if it is re- 

 newed with sufficient frequency. Wood impreg- 

 nated with drying oils, such as linseed oil, becomes 

 harder, and more capable of resisting moisture. It 

 is frequently the custom, to bore a perpendicular 

 hole in the top of a mast, and fill it with oil. This 

 fluid is gradually absorbed by the vessels of the 

 wood, and penetrates the mast to a great distance. 

 Animal oils, in general, are less proper for this 



t Repertory of Arts, 1826. Franklin Journal, ii. nml vi. 



j Tredgold's Elementary Principles of Carpentry, page 1G6. 



