The Technique of FURNITURE MAKING

STAINS IN WOOD

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.

Stain identification

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.

Mineral stains

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.

Chemical stains

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.

Bleaching stains

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.