CHEMICAL PROCESSING OF TIMBER

Treatability

The ease with which a timber can be impregnated with liquids, especially wood preservatives, is generally referred to as its treatability.

Treatability is related directly to the permeability of timber. The permeability is a function not only of moisture content and temperature, but also of grain direction, sapwood/heartwood, earlywood/latewood and species.

Longitudinal permeability is usually about 104 × transverse permeability owing principally to the orientation of the cells in the longitudinal direction. Heartwood, owing to the deposition of both gums and encrusting materials, is generally much less permeable than sapwood, while earlywood of the sapwood in the dry condition has a much lower permeability than the same tissue in the green state, owing to aspiration of the bordered pits in the dry state.

Perhaps the greatest variability in ease of impregnation occurs between species. Within the softwoods this can be related to the number and distribution of the bordered pits and to the efficiency of the residual flow paths, which utilise both the latewood bordered pits and the semi-bordered ray pits. Within the hardwoods variability in impregnation is related to the size and distribution of the vessels and to the degree of dissolution of the end walls of the vessel members. Four arbitrary classes of treatability are recognised in BS EN 350-2. These are:

  1. Easy to treat.
  2. Moderately easy to treat.
  3. Difficult to treat.
  4. Extremely difficult to treat.

Different timbers are assigned to these classes according to the depth and pattern of penetration.

The treatability classification for selected hardwoods and softwoods of importance in Europe are also given in BS EN 350-2 and illustrated for a selection of British-grown timbers in Table 1.

Table 1 Treatability of the heartwood
and sapwood of certain British-grown species
(BS EN 350-2).

This classification is derived primarily for preservatives, but is equally applicable to impregnation by flame retardants or dimensional stabilisers, for although differences in viscosity will influence the degree of penetration, the treatability of the different species will remain in the same relative order.

Preservatives and preservation: Except where the heartwood of a naturally durable timber is being used, timber should always be treated with a wood preservative if there is any significant risk that its moisture content will rise above 20% during its service life. At and above this moisture content, wood-destroying fungi can attack.

The relationship between service environment and risk of attack by wood-destroying organisms is defined in BS EN 335-1, 335-2 and 335-3 using the ‘use’ (formerly ‘hazard’) classification of biological attack, while BS EN 460 sets out the durability requirements for wood to be used in these use classes.

The natural durability and treatability of certain timbers is given in BS EN 350-2, and clearly those timbers of greater permeability will take up preservatives more easily and are to be preferred over those that are more difficult to treat. It is normally not necessary to protect internal woodwork, which should remain dry (use class 1). However, where the risk of water spillage, leakage from pipes or from the roof, or where condensation is seen as likely or significant (use class 2), application of wood preservative becomes necessary for most timbers.

A variety of methods for the application of wood preservatives are available. Short-term dipping and surface treatments by brush or spray are the least effective ways of applying a preservative because of the small loading and poor penetration achieved. In these treatments only the surface layers are penetrated and there is a risk of splits occurring during service that will expose untreated timber to the risk of attack by wood-destroying organisms.

Such treatments are usually confined to do-it-yourself treatments, or treatments carried out during remediation or maintenance of existing woodwork.

The most effective methods of timber impregnation are industrial methods in which changes in applied pressure ensure controlled, more uniform penetration and retention of preservative. The magnitude of the pressure difference depends on the type of preservative being used. Essentially, the timber to be treated is sealed in a pressure vessel and a vacuum drawn. While under vacuum, the vessel is filled with the preservative and then returned to atmospheric pressure, during which some preservative enters the wood.

At this point, an over-pressure of between zero and 13 bar is applied, depending mainly on the preservative being used, but also on the treatability of the timber. This can be held for between several minutes and many hours, after which the vessel is drained of preservative.

A final vacuum is often applied to recover some of the preservative and to ensure that the treated timber is free of excess fluid. The degree of penetration and retention of the preservative in solid wood is classified according to BS EN 351-1 in one of nine penetration classes; these should be used as a basis for specifying preservative treatments for particular products. The procedures to be used in the prepara‑ tion of samples for the determination of the penetra‑ tion and retention of preservative are described in BS EN 351-2.

The performance of different pre‑ servatives according to use class is determined by biological testing as set out in BS EN 599-1, while 599-2 specifies for each of the five use classes (defined in BS EN 335-1) requirements for classifying and marking wood preservative products according to their performance and suitability for use. The UK has an interpretive document (BS 8417) which smoothes the transition from the previous ‘processbased’ British specifications to these new ‘resultsbased’ European specifications described above. There are three main types of preservative.

The first group comprises the tar oils, of which coal tar creosote is the most important. Its specification is given in BS 144. Its efficacy as a preservative lies not only in its natural toxicity, but also in its water repellency properties. It has a very distinctive and heavy odour, and treated timber cannot be painted unless first coated with a metallic primer.

Creosote, however, was withdrawn from use in the DIY market by EU member states on 30 November 2003, though it can still be used for industrial applications such as telegraph poles, sleepers, bridges and piles. The second group comprises the water‑borne preservatives. In the past, the most common formulation was that containing copper, chromium and arsenic compounds (CCA preservative).

However, since September 2006 approval for the use of CCA preservative has been withdrawn in Europe, though for an interim period it is possible to import into Europe timber treated with CCA for use only in accordance with a restricted list of applications. Combinations of copper/chromium and copper/ chromium/boron have also been withdrawn. A number of new water-borne preservatives have appeared on the market, most of them based on copper/organic compounds such as the copper azoles, copper HDO and ammoniacal quaternary compounds either with or without boric acid (Reynolds et al., 2007). All these preservatives are usually applied by a vacuum–high-pressure treat‑ ment. The chemicals react once in the wood and become fixed, i.e. they are not leached out in service and can be used for ground-contact conditions.

Inorganic boron compounds are also used as water-borne preservatives, but have the disadvantage that they do not become fixed within the wood and therefore can be leached from the wood during service. Their use is therefore confined to environments where leaching cannot take place. The third group comprises the solvent‑type preservatives, which currently represent about five per‑ cent of preservative-treated wood.

These preservatives tend to be more expensive than those of the first two groups, but they have the advantage that machined timber can be treated without the grain being raised, as would be the case with aqueous solutions. The current formulations of the solvent type are based on a variety of compounds including copper and zinc naphthanates, copper and zinc versatate, zinc ocoate, and the metallic soaps known as acypetacs zinc and acypetacs copper.

It should be noted that pentachlorophenol has now been with‑ drawn from use and that tri-n-butyltin is now rarely used. Some organic solvent preservatives include insecticides and water repellents. These preservatives find uses in the DIY and industrial sectors. Industrial treatment processes include double vacuum and immersion techniques, while DIY and on-site treatment includes dipping and brushing. In looking at the application of these different types of preservative, creosote and the new water-borne preservatives based on copper/organic compounds are able to protect timber in high-hazard situations such as ground contact, while organic solvent pre‑ servatives are used in timber out of ground contact and preferably protected with a coat of paint.

It should be noted that the use of creosote is restricted to only industrial applications. Guidance on the selection of appropriate preservatives for use in external timber structures is given by Reynolds et al. (2007). Although it is not an impregnation process as defined above, it is convenient to examine here the diffusion process of preservation. The timber must be in the green state and the preservative must be water-soluble.

Timber is immersed for a short period in a concentrated (sometimes hot) solution of a boron compound, usually disodium octoborate tetrahydrate, and then close-stacked under cover for several weeks to allow the preservative to diffuse into the timber.

Although colourless, odourless and low in cost, boron preservatives can be leached out of the timber if it is wetted in service and their use must be restricted to use classes where there is no or little risk of wetting. For many years there was some difference of opinion as to whether preservatives merely lined the walls of the cell cavity or actually entered the cell wall.

However, it has been demonstrated by electron microanalysis that whereas creosote only coats the cell walls, the water‑borne preservatives do impreg‑ nate the cell wall. It is doubtful whether solvent-type preservatives in general penetrate the wall owing to their large molecular size and by being carried in a non-polar solvent. There is considerable variation in preservative distribution in treated dry timber.

In softwoods only the latewood tends to be treated owing to aspiration of the earlywood bordered pits. In hardwoods, treatment is usually restricted to the vessels and tissue in close proximity to the vessels. In those timbers that can be impregnated it is likely that the durability of the sapwood after pressure impregnation will be greater than the natural durability of the heartwood, and it is not unknown to find telegraph and transmission poles the heart‑ wood of which is decayed while the treated sapwood is perfectly sound.

Mention has been made already of the difficulty of painting timber that has been treated with creo‑ sote. This disadvantage is not shared by the other preservatives and not only is it possible to paint the treated timber, but it is also possible to glue together treated components.

A considerable amount of attention over the last two decades has been focused on the application of artificial preservatives to wood-based panels, but with very limited success. The main difficulty lies in achieving efficacy of preservative treatment with‑ out loss in performance of the panel.

As a general rule, all types of preservative treatment of the manufactured panel result in considerable losses in its mechanical and physical properties together with a marked increase in its price. Some success has been achieved in reducing the degree of loss in panel properties by treating the chips or strands prior to resin application (e.g. Goroyias and Hale, 2004), the addition of powdered preservatives either in the resin or after resin application or using gaseous diffusion of trimethyl borate on actual panels (e.g. Turner and Murphy, 1998).

Flame retardants: Flame‑retardant chemicals may be applied as surface coatings or by vacuum‑pressure impregnation, thereby rendering the timber less easily ignitable and reducing the rate of flame spread. Intumescent coatings will be discussed later and this section is devoted to the application of flame retardants by impregnation. There is little evidence to suggest that when flame retardant-treated timber is subjected to fully developed fire conditions it makes the timber burn any slower. Indeed there is evidence to show that some of the treatments can lead to enhanced char rates owing to the nature of the action of the impregnated salts.

However, in all cases the treatment will suppress the tendency to ignite. If and when it does ignite, the combustion process will be accompanied by less flaming. The salts most commonly employed in the UK for the vacuum pressure impregnation process are mono-, di- and poly-ammonium phosphate and ammonium sulphate. These chemicals vary considerably in solubility, hygroscopicity and effective‑ ness against fire.

Most proprietary flame retardants are mixtures of such chemicals formulated to give the best performance at reasonable cost. The fact that these chemicals are applied as an aqueous solution means that a combined water‑borne pre‑ servative and fire retardant solution can be used, which has distinct economic implications. Quite frequently, corrosion inhibitors are incorporated when the timber is to be joined by metal connectors.

Treatment of the timber involves high pressure/ vacuum processes; as aqueous solutions are involved redrying of the timber is required. Considerable caution has to be exercised in deter‑ mining the level of heating to be used in drying the timber following impregnation. The ammonium phosphates and sulphate tend to break down on heating, giving off ammonia and leaving an acidic residue that can result in degradation of the wood substance.

Thus, it has been found that drying at 65ºC following impregnation by solutions of these salts results in a loss of bending strength of from 10 to 30%. Drying at 90ºC, which is adopted in certain kiln schedules, results in a loss of 50% of the strength and even higher losses are recorded for the impact resistance or toughness of the timber.

It is essential, therefore, to dry the timber at as low a temperature as possible and also to ensure that the timber in service is never subjected to elevated temperatures that would initiate or continue the process of acidic degradation.

Most certainly, timber that has to withstand suddenly applied loads should not be treated with this type of fire retardant, and care must also be exercised in the selection of glues for construction. The best overall performance from timber treated with these flame retardants is obtained when the component is installed and maintained under cool, dry conditions. Conscious of the limitations of flame retardants based on ammonium salts, a number of companies have developed effective retardants of very different chemical composition, many of which are polymerbased formulations.

These newer products are less likely to produce significant strength losses, are more leach resistant and are usually non-corrosive to metal fixings. However, they are considerably more expensive than those products based on ammonium salts.

Dimensional stabilisers and durability enhancers: Timber, because of its hygroscopic nature, changes in dimensions as its moisture content varied in order to come into equilibrium with the vapour pressure of the atmo‑ sphere. Because of the composite nature of timber such movement will differ in extent in the three principal axes. Movement is the result of water adsorption or desorption by the hydroxyl groups present in all the matrix constituents. Thus it should be possible to reduce movement (i.e. increase the dimensional stability) by eliminating or at least reducing the accessibility of these groups to water. This can be achieved either by chemical changes or by the intro‑ duction of physical bulking agents.

Dimensional stability can be imparted to wood by swelling of the substrate by means of chemical modification, since the bonded groups occupy space within the cell wall. At high levels of modification, wood is swollen to near its green volume, and anti-shrink efficiencies close to 100% are achieved. After ex‑ tended reaction, swelling in excess of the green volume can occur, which is accompanied by cell-wall splitting. Enhacement of durability appears to be an important side-effect.

Various attempts have been made to substitute the hydroxyl groups chemically by less polar groups, the most successful of which has been by acetyla‑ tion (Rowell, 1984). In this process acetic anhydride is used as a source of acetyl groups. A very marked improvement in dimensional stability is achieved with only a marginal loss in strength. Using car‑ boxylic acid anhydrides of varying chain length, Hill and Jones (1996) obtained good dimensional stabilisation that was attributed solely to the bulk‑ ing effect, a conlusion that has been supported by further investigation (Papadopoulos and Hill, 2003). One commercial product based on acetylation came on the European market in 2007. It is claimed that the swelling and shrinkage of this product is reduced by at least 75% compared to untreated timber, while the durability is increased to Class 1 (BS EN 350-2). The product can be obtained in the UK and an example of its use in the UK is given in Suttie (2007).

Another chemical modification reagent is furfuryl alcohol. In a commercial operation that started in 2003, this reagent was prepared from plant waste and reacted with the timber. Not only are the dimensional stability and durability markedly improved, but the hardess of the timber is also increased. The product is also available in the UK (Suttie, 2007). Good stabilisation can also be achieved by reacting the wood with formaldehyde, which forms methylene bridges between adjacent hydroxyl groups.

However, the acid catalyst necessary for the process causes acidic degradation of the timber. You can find more information on chemical modification in the review by Rowell (1984) and the textbook edited by Hon (1996). In contrast to the above means of chemical modification, a variety of chemicals have been used to physically stabilise the cell wall. These impreg‑ nants act as bulking agents and hold the timber in a swollen condition even after water is removed, thus minimising dimensional movement.

Starting in the mid‑forties and continuing on a modest scale to the present time, some solid timber, but more usually wood veneers, are impregnated with solutions of phenol–formaldehyde. The veneers are stacked, heated and compressed to form a high‑ density material with good dimensional stability, which still finds wide usage as a heavy-duty insulant in the electrical distribution industry. Considerable success has also been achieved using polyethylene glycol (PEG), a wax‑like solid that is soluble in water. Under controlled conditions, it is possible to replace all the water in timber by PEG by a diffusion process, thereby maintaining it in a swollen condition.

The technique has found application, among other things, in the preservation of waterlogged objects of archaeological interest, the best-known examples of which are the Swedish war‑ ship Wasa and Henry VIII’s Mary Rose. The Wasa was raised from the depths of Stockholm Harbour in 1961, having foundered in 1628. From 1961 the timber was sprayed continuously for over a decade with an aqueous solution of PEG, which diffused into the wet timber, gradually replacing the bound water in the cell wall without causing any dimen‑ sional changes. The Mary Rose was launched in 1511 and sank in 1545; its exact location was dis‑ covered in 1971 and a large section of the hull was recovered in 1982.

chemical processing of timber
Fig. 1 The comparative rates of swelling in water of untreated pine timber and timber impregnated with a 50% (by mass) solution of polyethylene-glycol (PEG). This is equivalent to a 22% loading on a dry-wood basis.

It was sprayed continuously with fresh water until 1994 when the spray was changed to an aqueous solution of PEG, a process that will continue for up to 25 years. PEG may also be applied to dry timber by standard vacuum impregnation using solution strengths of from 5 to 30%. Frequently, preservative and/or fire‑retardant chemicals are also incorporated in the impregnating solution. It will be noted from Fig. 1 that following impregnation with PEG, the amount of swelling has been reduced to one third that of the untreated timber.

Developments in the production and use of waterrepellent preservatives based on resins dissolved in low‑viscosity organic solvents have resulted in the ability to confer on timber a low, but none the less important, level of dimensional stability. Their application is of considerable proven practical significance in the protection of joinery out‑of‑doors.

Chemical Pulping

The size of the pulping industry has already been discussed, as has the production of mechanical pulp. Where paper of a higher quality than newsprint or corrugated paper is required, a pulp must be pro‑ duced consisting of individual cells rather than fibre bundles. To obtain this type of pulp the middle lamella has to be removed, which can be achieved only by chemical means. A number of chemical processes are described in detail in the literature. All are concerned with the removal of lignin, which is the principal constituent of the middle lamella.

However, during the pulping process lignin will also be removed from within the cell wall as well as from between the cells; this is both acceptable and desirable, since lignin imparts a greyish coloration to the pulp, which is unaccept‑ able for the production of white paper. However, it is not possible to remove all the lignin without also dissolving most of the hemicelluloses that not only add to the mass of pulp produced, but also impart a measure of adhesion between the fibres. Thus, a compromise has to be reached in determining how far to progress with the chemical reaction, and the decision depends on the require‑ ments of the end product.

Frequently, though not always, the initial pulping process is terminated when a quarter to a half of the lignin still remains and this is then removed in a subsequent chemical operation known as bleaching, which, though expensive, has relatively little effect on the hemi‑celluloses. The yield of chemical pulp will vary con‑ siderably depending on the conditions employed, but it will usually be in the range of 40–50% of the dry mass of the original timber. The yield of pulp can be increased to 55–80% by semi‑chemical pulping.

Only part of the lignin is removed in an initial chemical treatment that is designed to soften the wood chips. Subsequent mechanical treatment separates the fibres without undue damage. These high-yield pulps usually find their way into card and board‑liner, which are extensively used for packaging where ultimate whiteness is not a prerequisite.

Other Chemical Processes

Brief mention must be made of the destructive dis‑ tillation of timber, a process that is carried out either for the production of charcoal alone or for the additional recovery of the volatile by‑products such as methanol, acetic acid, acetone and wood‑tar. The timber is heated initially to 250oC, after which the process is exothermic. Distillation must be carried out either in the complete absence of air, or with small, controlled amounts of air.

Timber can be softened in the presence of ammonia vapour as a result of plasticisation of the lignin. The timber can then be bent or moulded using this process, but, because of the harmful effects of the vapour, the process has never been adopted commercially.

THERMAL PROCESSING

Good dimensional stabilisation and an improvement in durability can be obtained by heating timber for short periods of time to very high temperatures (250–350oC). A reduction of 40% in movement has been recorded after heating timber to 350oC for short periods of time (Rowell and Youngs, 1981). It is possible to achieve a reduction in swelling of from 50 to 80% at lower temperatures (180–200oC), again in short periods of time, by heating the sample in an inert gas at 8–10 bar (Giebeler, 1983). The use of lower temperatures (120–160oC) in the presence of air necessitates exposure for several months in order to achieve a similar reduction in swelling.

It appears that it is the degradation of the hemicelluloses that reduces the propensity of the timber to swell (Stamm, 1977). Unfortunately, all these treatments result in thermal degradation of timber with considerable loss in strength and especially toughness, unless oxygen is either removed or reduced appreciably, in which case the magnitude of the loss is also reduced considerably.

There has been renewed interest in themal modification of wood over the last decade with the commercialisation of the process by four European companies to produce slightly different products. In the best known and best documented of these products, timber is first pre-heated to 150oC for 48 hours before the temperature is raised to 240oC for up to 4 hours in an atmosphere low in oxygen.

The timber is then allowed to cool and stabilise for 24 hours. The product has been used for cladding and solar shading for over 10 years and it is claimed that it is 50% more stable than untreated softwood, and also that durability is enhanced. Thus, one grade has a durability class to EN350-2 of 3, while a more superior grade has a rating of 2; this performance is in excess of Scots pine heartwood, which has a rating of only 4.

Loss in strength varies from 10 to 30% depending on property, time at elevated temperature, exposure temperature and the degree of reduction of oxygen in the atmosphere. Examples of the use of this product in the UK are given by Suttie (2007). 56.4 Finishes Finishes have a combined decorative and protective function. Indoors they are employed primarily for aesthetic reasons, though their role in resisting soiling and abrasion is also important. Outdoors, however, their protective function is vital.

The combined effects of structure and moisture movement in timber have a most profound effect on the performance of coatings. For example in the softwoods the presence of distinct bands of early- and latewood with their differential degree of permeability results not only in a difference in sheen or reflectance of the coating between these zones, but also in marked differences in adhesion.

In Douglas fir, where the latewood is most conspicuous, flaking of paint from the late‑ wood is a common occurrence. In addition, the radial movement of the latewood has been shown to be as high as six times that of the earlywood, consequently the ingress of water to the surface layers results in differential movement and considerable stressing of the coatings.

In those hardwoods characterised by the presence of large vessels, the coating tends to sag across the vessel and it is there‑ fore essential to apply a paste filler to the surface prior to painting; even with this, the life of a paint film on a timber such as oak is very short. For this reason, the use of exterior wood stains (see later) is common, as this type of finish tends not to exhibit the same degree of flaking.

The presence of extractives in certain timbers results in an inhibition of drying of most finishes; with iroko and Rhodesian teak, many types of finish may never dry. Contrary to general belief, deep penetration of the timber is not necessary for good adhesion, but it is absolutely essential that the weathered cells on the surface are removed prior to repainting. Good adhesion appears to be achieved by molecular attraction rather than by mechanical keying into the cell structure.

Although aesthetically most pleasing, fully exposed varnish, irrespective of chemical composition, has a life of only a very few years, principally because of the tendency of most types to become brittle on exposure to ultraviolet radiation, thereby cracking and disintegrating because of the stresses imposed by the movement of the timber under changes in moisture content. Ultraviolet light can readily pass through the majority of varnish films, degrading the timber at the interface and causing adhesion failure of the coating. A second type of natural finish that overcomes some of the drawbacks of clear varnish is the water‑repellent preservative stain or exterior wood stain.

There are many types available, but all consist of resin solutions of low viscosity and low solids content. These solutions are readily absorbed into the surface layers of the timber. Their protective action is due in part to the effectiveness of waterrepellent resins in preventing the ingress of water, and in part to the presence of finely dispersed pigments, which protect against photochemical attack.

The higher the concentration of pigments the greater the protection, but this is achieved at the expense of loss of transparency of the finish. Though easy to apply and maintain these thin films, however, offer little resistance to the transmission of water vapour into and out of the timber.

Compared with a paint or varnish the water‑repellent finish will allow timber to wet up and dry out at a much faster rate, thereby eliminating problems of water accumulation that can occur behind impermeable paint systems. The presence of a preservative constituent reduces the possibility of fungal development during periods of high moisture uptake. The films do, however, require more frequent main‑ tenance, but nevertheless have become well established for the treatment of cladding and hardwood joinery.

By far the most widely used finish, especially for external softwood joinery, is the traditional opaque alkyd gloss or flat paint system embracing appropriate undercoats and primers; a three- or four‑coat system is usually recommended.

Multiple coats of oil‑based paint are effective barriers to the move‑ ment of liquid and vapour water, however, breaks in the continuity of the film after relatively short exposure constitute a ready means of entry of mois‑ ture, after which the surrounding, intact film will act as a barrier to the escape of moisture, thereby increasing the likelihood of fungal attack. The effectiveness of the paint system is determined to a considerable extent by the quality of the primer. Quite frequently window and door joinery with only a priming coat is left exposed on building sites for long periods of time.

Most primers are permeable to water, are low in elasticity and rapidly disintegrate owing to stresses set up in the wet timber; it is therefore essential that only a highquality primer is used. Emulsion-based primer/ undercoats applied in two consecutive coats are more flexible and potentially more durable than the traditional resin-based primers and undercoats.

A new range of exterior quality paints – that are either solvent-borne or water-borne formulations – has been produced in the last two decades. Some of the formulations have a higher level of moisture permeability than conventional paint systems and have been described as microporous. These are claimed to resist the passage of liquid water, but to allow the passage of water vapour, thereby allowing the timber to dry out.

However, there appears to be no conclusive proof for such claims. Solvent-borne exterior paints come in many forms, for example, as a three-layer system based on flexible alkyd resins, which produce a gloss finish, or a one-can system that is applied in two coats, and which produces a low-sheen finish. Water-borne exterior paints are based on acrylic or alkyd-acrylic emulsions applied in either twoor three-coat systems. Water-borne systems have a higher level of permeability than solvent-borne systems.

Even more important is the high level of film extensibility of water-borne systems, which is retained on ageing (Miller and Boxall, 1994) and which contributes to their better performance on site than solvent-borne exterior paints. Test work has indicated that the pretreatment of surfaces to be coated with a water‑repellent preservative solution has a most beneficial effect in extending the life of the complete system, first by increasing the stability of the wood surface and thereby reducing the stresses set up on exposure, and second by increasing adhesion between the timber surface and the coating.

This concept of an integrated system of protection employing preservation and coating, though new for timber, has long been established for certain other materials. Thus it is common practice prior to the coat‑ ing of metal to degrease the surface to improve adhesion.

One specialised group of finishes for timber and timber products is that of the flame‑retardant coatings. These coatings are designed to be applied on-site, unlike impregnation treatments, and yet allow timber and wood-based panels to comply with the relevant surface-spread-of-flame regulations. Almost exclusively, they are intumescent in action, i.e. the film expands on heating thereby coating the substrate with an insulated char. Some of the finishes are pigmented, but the vast majority are clear so that the natural beauty of the timber shows through.

The modern versions achieve the necessary performance rating with much thinner coatings than the first-generation products and, more importantly, have a harder surface, thereby reducing their tendency to pick up finger marks. However they are not suitable for external environments. They do not modify the charring rate of timber, but under fire resistance conditions they will delay ignition for approaching fifteen minutes, i.e. during the critical ‘escape’ stage of a building on fire.

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