Wood which is deeply buried in the ground or completely submerged in water does not decay, as witness the highly prized black bog oak which has been buried for centuries under layers of wet peat, and the use of timber baulks for underwater piles and breakwaters. However, where there is free oxygen, living fungi will attack the wood, provided there is moisture present; and although most of these fungi are relatively harmless, causing little more than local staining, two main types of rot actively feed upon the wood substance and eventually destroy it.
These are usually outdoor types, commonly known as 'wet rots' (rotting logs, fence posts, etc.). They consume both the cellulose and lignin content of the wood, leaving only a white pithy or slimy residue.
These are mainly of the indoor type, but can also occur out of doors. They attack the cellulose content only, leaving the lignin content intact, and are commonly known as 'dry rots', with the wood residue usually brown, spongy, dry and deeply cracked with and across the grain, giving a characteristic charred appearance.
This is an outdoor type of brown rot known commercially as 'dote' or 'punk'. Living trees are affected, also felled logs full of sap and awaiting sawing, the infection spreading through the heartwood and finally emerging as a bracket-shaped fruit-body on the bark. Incipient dote is easily recognizable, as the wood is peppered with round or oval greyish-white spots which do not appear to affect the mechanical strength to any great extent, but in later stages the wood becomes brown, dry, with spongy white streaks. The fungus cannot exist in the dry state, therefore sound unaffected wood need not be treated. As some harmless fungi cause similar brown staining, doteiness can be tested for by lifting a small splinter of the wood surface with a knife; if dote is present the splinter will break easily with a brash fracture. Fruit woods such as pear, apple, cherry, plum, etc. are particularly susceptible to this form of rot.
It is dote that causes the now much sought after spalted timber. Favoured by wood turners and some furniture-makers, maple, beech and walnut produce the most highly prized markings.
True dry rot (Lacrymans merulius)
This is an indoor type, and of all the wood-destroying fungi the most serious as it will attack dry seasoned timber if the conditions are suitable. Initially, the infection needs damp,
badly ventilated surroundings, but once the fungus has taken hold it will carry its own water in wide thread-like hyphae which can penetrate thick brickwork in search of fresh material which it rapidly consumes, leaving the characteristic deeply cleft, cube-like charring, often hardly noticeable under painted surfaces except as local sinkages. In dark conditions the fungus may appear as a white woolly growth, or a smooth fan-shaped grey skin with ragged edges exuding moisture droplets, but if light is present then the typical fruit-bodies will appear. Those who have seen a fully formed fruit-body, sometimes many feet across, with its leprous-grey skin suffused with livid patches of white, yellow and lilac-blue, and peppered with rust-red spores are hardly likely to forget the experience.
Treatment for dry rot must be drastic: all infected woodwork cut away and burnt, infected brickwork and non-wood materials scorched with a blow torch, and treated with an efficient antiseptic. Sound wood in the vicinity should be thoroughly brushed over with either a water soluble preservative such as commercial sodium fluoride (4 per cent), zinc chlorate (5 per cent), applied hot if possible; or one of the commercial solvent-type preservatives recommended for the purpose. Care should be taken with all these chemicals in confined spaces, for although they are not intrinsically dangerous to handle they must necessarily be highly toxic if they are to be effective.
Several other dry rots, notably the cellar fungus (Coniophora cerebella), attack house woodwork, but they require continuous moisture and therefore present no hazard to dry timber.
Most important of these are the lyctus, death-watch and furniture beetles; and of secondary importance, in that they attack only unseasoned, sickly or softwood timber, the pinhole borers, longhorn beetles and giant wood-wasp. All these pests are winged and capable of flight in search of suitable breeding-places and in every case it is the larva or worm which actually attacks the wood.
Pinhole borers (ambrosia beetles)
These are confined to tropical countries and attack the sapwood and heartwood of freshly felled logs waiting for milling. Wood attacked by pinhole borers can be recognized immediately by the very small neat exit holes, usually stained with fungus, and the straight tunnels empty of dust running both with and across the grain. Structural damage is usually slight, and wood which has been attacked can safely be used in this country, although obviously there must be prejudice against it owing to the apparent similarity with the ravages of the true furniture beetle.
Lyctus (powder-post) beetle (21 :B)
Several species exist and are common to timber-yards where they attack the sapwood of hardwoods with large pores, notably oak, ash, elm and coarse-grained walnuts. Close-grained woods and softwoods are usually immune, as the large eggs are not laid in cracks or crevices but in the actual pores of the wood which must be wide enough to receive them. The attack usually dies out as the wood becomes seasoned and the available starch is exhausted, but in the meantime the entire sapwood can be reduced to a flour-like dust. The mature beetle is dun coloured, with a narrow body about 3/16 in (4.7 mm) long, slightly larger than the furniture beetle, and with a more pronounced head.
21 Wood-boring beetles
Common furniture beetle (Anobium punctatum) (21:A)
Commonly known as woodworm, this is the most serious pest, and commonly occurs in roof rafters, floors or general woodwork, including furniture. A few flight holes on the surface of polished timber may not appear serious, but the wood below may be so tunnelled as to reach the point of total collapse, therefore a constant watch should be kept on every item of woodwork.
The mature beetle is oblong shaped, from 1/10 to 1/5 in long (2.5 to 5 mm) and dusty dark brown in colour, with wing-cases covered with lines of small punctures (hence the specific name). It commonly breeds in the dead branches of living trees, fencing-posts, etc., to enter the house by any open window during the dry summer season. After the female beetle has mated about 20 eggs are laid in any suitable crack or crevice, and in particular the exit or flight holes of previous larvae, and the curved white grub, which is equipped with strong biting jaws, hatches in about six weeks to commence its tunnelling by consuming the wood and excreting small pellets of granular dust. When it has reached maturity, which can be anything from one to two years according to the starch content of the wood, the grub, which is now about 1/4 in (6.3 mm) long, moves to close under the surface, where it pupates and tunnels its way upwards to emerge finally as an adult beetle at any time from late May to August. It can then fly off to mate and renew the cycle.
While the sapwood of any wood is particularly liable to infestation (and it is for this reason that in some countries sapwood should never be used in furniture), the heartwoods of certain sweet woods, notably birch, beech, alder and some soft walnuts, are also attacked, while any wood affected by dampness or incipient decay is also prone. Alder, in particular, speedily becomes riddled with furniture beetle, hence the wiseacre's claim that it is better to have a plank or two of alder lying about the timber- or lumber-yard to attract any migrant beetles—a highly invidious contention, for the alder then becomes a first-class breeding-ground. Polished and painted surfaces do, of course, accord protection against the egg-laying female beetle, but invariably there is some hidden crack or crevice or an old flight hole which invites attack, therefore carcass backs and the underframing of furniture should be periodically examined for freshly expelled fine dust. Birch or alder plywoods glued with animal or vegetable glues are particularly susceptible, but resin-glued ply is normally resistant.
Death-watch beetle (Xestobium rufovillosum) (21 :C)
This is a larger cousin of the furniture beetle, about 1/4 in (6.3 mm) to 1/3 in (8.5 mm) long and without the characteristic punctures on the wing-cases. The mature beetle raps the wood with its head during the mating season in early summer, producing the familiar ticking sound which is magnified by the wood and can travel considerable distances—hence the specific name of 'death-watch' beetle.
The life-cycle closely resembles that of the furniture beetle, but can extend over anything from one and a half to several years, dependent on the condition of the wood; while after pupation the mature beetle does not emerge immediately, but usually waits until the following spring. The seasoned heartwood of most timbers can be attacked, but infestation is usually confined to old oak in which some subtle change, probably induced by damp conditions, has rendered the wood more digestible, hence its incidence in old buildings, church roofing, etc. where the stoutest structural timbers can be reduced to mere shells. Infestation is rare in furniture, and is immediately recognizable by the large exit holes and characteristic bun-shaped pellets of wood-dust.
House longhorn beetle (Hylotrupes)
The pest is extremely destructive on the continent of Europe, though in England virtually confined to certain pine-forest regions of Surrey. Softwood only is attacked, and infestation is usually limited to roof or attic. The mature beetle is from 1/4 in (6.3 mm) to 3/4 in (19 mm) in length, varying in colour from brown to iridescent green, with characteristic long curved feelers, while the oval exit holes can be very large and up to 3/8 in (9.5 mm) in size.
Certain other types of longhorn beetle infest the sapwood of both hardwoods and softwoods. They do not as a rule attack seasoned timber/ lumber, and are, therefore, confined to forest and timber-yard.
These only attack the living tree, and are, therefore, the foresters' concern. An occasional mature wasp of somewhat terrifying aspect. much bigger than a hornet and with a long egg-laying probe which could be mistaken for a sting, can sometimes be seen haunting the wood-pile in late summer, but can safely be disregarded.
No treatment is required for pinhole borer as the tunnels are always empty, but infestation by furniture and death-watch beetle should be taken in hand immediately. The entire surface should be cleaned with a powerful vacuum-cleaner to clear the flight holes of dust, and a proprietary woodworm killer of the solvent type, usually a metallic or chlorinated naphthalene, liberally brushed or swabbed over the surface. Each flight hole should also be injected with the fluid, using the cheap polystyrene squeeze bottles with nozzle stoppers available, so that the hidden tunnelling can carry the fluid into the body of the wood. The entire treatment should be repeated in the following spring just before the beetles emerge (for the insecticide will not necessarily reach all the hidden grubs which will continue to pupate), after which a yearly check should be made for any signs of fresh dust, and measures taken accordingly. Most proprietary woodworm destroyers do not stain the wood permanently or interfere with the existing polish, and should be applied as liberally as possible, for they are quite safe to handle. However, spraying in confined spaces should not be done unless efficient face masks are worn; and in any case liberal brushing is always preferable to spraying, as the oily fluid tends to bridge over existing flight holes; thus failing to penetrate.
Stains can be caused by harmless fungi as distinct from wood-rotting fungi, soil conditions, frost factors, etc., chemical contamination, and natural oxidization or weathering.
Positive identification of any particular type of stain can be difficult, but as a rule fungal and mineral stains are of variable intensity, fading slightly at the edges and following the grain. Stains with a hard edge which do not follow the grain are usually chemical in origin, while overall discoloration is probably due to weathering or incorrect seasoning.
Mould- and sap-staining fungi, etc.
Certain harmless moulds feed on the carbohydrates in the wood cells and can produce grey, grey-blue, pink, white and black stains. Usually this type of staining is confined to the surfaces of the wood, and can, therefore, be eradicated by brushing or sanding down; but several types, of which the best known is Ceratostomella pint, attack the sapwood, producing the characteristic bluing of badly seasoned softwoods. This discoloration cannot be satisfactorily bleached out, and is due almost entirely to an optical effect, similar to the apparent blueness of watered milk. All these fungi have little if any mechanical effect on the wood, as the hyphae or mould threads constrict considerably where they pierce the cell walls, whereas wood-destroying fungi bore large apertures, and consume the cellulose.
Some fungi actually increase the value of the wood, as in the case of the beef-steak fungus [Fistulina hepatica) which can sometimes be seen growing on old, standing oak trees, the early stages of which produce a rich golden brown; and the much rarer Chlorosplenium sp. which stains oak a vivid green. Both have some mechanical effect on the timber, but not enough to impair its usefulness in decorative-work. All species of fungi require a certain minimum moisture content, usually well over 20 per cent, and therefore dry, seasoned timber is not affected.
Certain soil conditions can cause pronounced staining, usually in brown or greyish streaks, as, for example, black-hearted ash, brown-hearted beech, and the reddish-brown mineral streak in the heartwood of Parana pine. As these are natural pigment-stains they do not affect the mechanical properties of the wood. They cannot be satisfactorily bleached out. Some timbers show actual deposits of white and grey crystalline mineral salts in the grain, and at times actual calcium stones which penetrate some distance into the wood. The difficult working properties of some exotic woods is often due to the dulling action of these mineral salts on cutting edges.
Practically all woods contain sufficient colour-forming compounds to produce staining when brought into contact with acidic or alkaline chemicals, and in this respect behave much like litmus paper, producing reds, pinks and browns with acid reactions (french polish, urea glues, etc.), and blues and greens with alkaline reactions (soap, detergents, etc.). Additionally, most woods contain appreciable amounts of tannic acid which, if wetted, will react in greater or lesser degree with iron in any form, producing the characteristic blue-black staining of iron nails in exterior oak-work. Other woods besides oak showing strong reactions are afromosia, camphorwood, Douglas fir and sweet chestnut; while African mahogany, coigue, idigbo, gedunohor, sapele and utile react slightly to produce minor staining. Wood for scrubbed table-tops, draining-boards, etc. should be as neutral as possible. Teak, maple, sycamore, oak and deal are usually unaffected by soap and water, but agba, ayan, idigbo and afzelia are known to be unsuitable. Where there is any doubt a thorough test should be made beforehand.
Natural oxidization, weathering, etc.
Freshly converted green timber will oxidize rapidly if the wood surfaces remain wet for any length of time, due to the formation of chemical compounds which act on the colour-forming properties of the wood, producing brown or grey staining. This chemical effect is most marked in light-coloured woods, such as sycamore, which must be surface dried immediately it is converted if the overall whiteness is to be preserved. Moreover, sticker marks can penetrate quite deeply, particularly where there is chemical action between the actual sticker and the wood surface, and for this reason stickers should always be neutral woods—deal, horse-chestnut or poplar—which have been thoroughly dried.
Additional to seasoning discolorations is the effect of weather and sunlight on wood which can either bleach or darken surfaces very appreciably. Cherry, for instance, will darken considerably if exposed to strong sunlight for a few hours, so also will mahogany, while freshly sawn dry teak will rapidly oxidize from light green to dark brown. On the other hand, direct sunlight will bleach dark rosewood to a medium walnut colour, and walnut itself will lighten considerably, while kingwood will rapidly fade from deep purple to dark red-brown. The general rule would appear to be that dark woods lighten and light woods darken, but it is hardly safe to assume that the rule applies automatically, and tests should always be made. Some synthetic lacquers contain ultra-violet light barriers which to some extent inhibit the bleaching action of strong sunlight, but only certain woods are protected and the results are never altogether permanent.
Iron stains can be satisfactorily bleached out with either a saturated solution of oxalic acid in water or methylated spirit, hydrogen peroxide, some household bleaches, ink-eradicators, etc., and light chemical staining from other sources may also respond to several applications. The bleaching out of fungal stains, mineral discolorations and natural oxidization is never really feasible, although proprietary two-solution bleaches can assist in masking the discoloration by rendering the surfaces of the wood more opaque. Bleaching of entire surfaces, often—and to the writer's way of thinking, horribly—perpetrated on oak and pine, owes much to the lavish use of white grain fillers which choke the wood surface. The action of most bleaches is, of course, the release of free oxygen as the active bleaching agent.
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