S. R. Garcia, I. M. Godfrey and S. Lussier
Wood is a widely used and well known material. Although its general properties are familiar to most people there is an underlying complexity to wood that is not widely appreciated. It is a natural substance made up of countless individual cells. These cells and their contents are composed primarily of:
- polysaccharides (hemicelluloses and cellulose);
- polyphenolics (lignin); and
- extractives which are usually present in small quantities. These may affect the colour and odour of the wood.
Examination of the cross-sectional surface of a tree trunk reveals three distinct zones: the bark, the light-coloured outer region (the sapwood) and the dark-coloured inner region (the heartwood). It is the heartwood that is used in furniture, building, sculpture and the thousands of other common uses to which wood is put (Figure 1).
The tangential and radial directions may be identified on the cross-sectional surface (Figure 2). Wood properties vary in these and the longitudinal direction. For instance, when wood dries its shrinkage is greatest in the tangential (5 - 10 %) and least in the longitudinal (less than 0.1 %) directions.
From the time a tree is felled it begins to lose moisture, first at the surface and eventually from deeper within the wood. The rate at which green timber dries depends on environmental conditions and the type of wood. It is a challenge to timber specialists to dry wood without warping, cracking and splitting. Seasoned dry wood continues to interact with its surroundings, absorbing or releasing water in response to changes in the relative humidity of its environment.
Wood samples are usually classified as either hardwoods or softwoods. These classifications are not related to the hardness or the softness of the materials, being based primarily on the presence of pores (hardwoods) or their absence (softwoods). Balsa for example, despite its inherent softness, is a hardwood.
There are also fundamental differences in the usual physical characteristics of the trees from which these timber types are derived. These are listed below:
|deciduous or evergreen||evergreen|
|flowers or seeds||cones or seeds|
|complex cellular structure||less complex|
Damage to wood and wood products may be caused by:
- poor handling, inadequate packing for transport or exposure to changing relative humidity (physical degradation);
- heat (thermal degradation);
- light and UV radiation (photochemical degradation);
- biological agents such as fungi, bacteria or insects (biological degradation); and
- acidic or alkaline chemicals (chemical degradation).
Poor handling and packing, exposure to fluctuating relative humidity levels and insects usually cause physical damage to wood whereas heat, light, fungi, bacteria and corrosive chemicals cause chemical changes to the wood substance. These latter changes can lead to the complete destruction of the wood.
Wood may be physically damaged by either poor handling or as a result of stresses induced by changing moisture gradients. Its relative softness means that wood can be easily damaged by contact with harder and sharper objects.
Excessively hot and dry conditions cause wood to shrink and crack. Conversely, cold and damp conditions cause wood to swell and warp. Substantial damage may be caused by large, rapid fluctuations in relative humidity levels. This is particularly so if wooden parts are closely joined and if their respective grains run contrary to one another. The restriction of free movement often leads to warping and cracking.
The rate of change of relative humidity levels and the way in which the wood has been sawn (along or across the grain, radially) determine whether bowed, cupped, twisted, cracked or split wood will result from exposure to inappropriate or changing relative humidity conditions.
The expansion and contraction of wood may damage either the wood itself or materials attached to it (Figure 3). Paint layers, for example, will tend to craze and flake if the underlying wood is subjected to alternating periods of expansion and contraction.
Physical damage may occur if no precautions are taken when wooden objects are transported from one climatic region to another. For example, wooden sculptures transported from tropical regions to drier areas commonly develop serious cracks as moisture is lost from the wood. Similar damage may occur if items composed of wood or other organic substances are exposed to hot sunlight during the day and cold, moist conditions at night.
The most severe form of thermal degradation is obviously the complete destruction of wood by burning. The individual components of wood (hemicellulose, cellulose and lignin) are affected by heat before the state of combustion is reached.
Heat weakens the bonds between fibres, leading to increased brittleness and fragility. For example the embrittlement of newspapers with age can be attributed largely to thermal degradation of hemicelluloses present in the ground wood pulps. The extent of degradation is determined by the type of wood concerned, the temperature and the duration of exposure. As chemical breakdown usually occurs more rapidly in an aqueous environment, the presence of moisture increases the effects of thermal degradation.
Physical damage will occur if a wooden object is placed close to a heat source. The resultant loss of moisture often results in shrinking and cracking of the wood. Thermal degradation is considered to be the most serious threat to wood kept indoors.
Chemical, mechanical and light energy factors combine to contribute to the deterioration of wood that is exposed outdoors (weathering). The general appearance and surface finishes of historic structures are often affected.
When wood is kept reasonably dry and exposed to sunlight or UV radiation, the surface tends to turn brown. A grey finish is observed when the effects of light and moisture are combined. The reactions are chiefly due to changes in the lignin caused by exposure to UV radiation. Colour changes are usually restricted to the surface of the wood.
In addition to the problems associated with photodegradation of wood itself, excessive light exposure will cause bleaching of certain dyes or pigments and fading or discolouration of surface finishes.
Given the right conditions, a variety of different organisms will attack wood. These include:
- fungi (such as the brown, white and soft rots); and
- insects (such as termites, furniture beetles and powder post beetles).
The type and quantity of extractives present in the heartwood are important in determining the biological resistance of wood.
The sapwood of most species and the heartwood of many others are prone to attack by a number of different wood-rotting and wood-staining fungi. Wood-staining fungi (mould) flourish in damp, dirty and unventilated conditions. Although moulds merely stain the surface of wooden items, wood-rotting fungi, such as dry rot, severely damage structural timbers. Wood-rotting fungi require moisture, some air, moderate temperatures and the absence of toxic influences to develop.
Termites (white ants), black ants and other insects are also responsible for serious damage to wooden materials. The most commonly occurring wood-boring insects found in museums are the furniture beetle and to a lesser extent, the powder-post beetle.
The furniture beetle usually attacks softwoods such as pine and fir and is commonly found in damp floorboards. It also will attack mature hardwoods such as oak and walnut and is responsible for considerable damage to old furniture.
The powder-post beetle only attacks hardwoods and occurs most frequently in damp timber or in structural timbers of recent buildings.
Wood is generally resistant to a large number of chemicals, but it is still vulnerable to chemical attack, particularly under conditions of high acidity (pH less than 2) or high alkalinity (pH greater than 11). Under these conditions the long cellulose chains and the shorter hemicellulose chains are broken producing a softer, more friable object.
Chemical degradation is enhanced by high temperatures and the presence of iron salts, sulphur dioxide and oxidising agents such as chlorine and nitric acid.
In order to ensure wooden objects are well cared for, take the following factors into account:
- light, temperature and relative humidity;
- handling techniques;
- modes of storage, display and support; and
- protection from insects, fungi and dirt.
To maintain wooden objects in the best condition, the following environmental conditions are recommended:
- relative humidity levels in the range 40 – 60 %, with a maximum variation of 5 % in any 24 hour period;
- temperature range of 15 – 25 °C with a maximum variation of 4 °C in any 24 hour period; and
- light levels of 50 lux for dyed or painted wood, up to 200 lux for undyed or uncoated wood and a maximum of 300 lux for wooden objects that have largely been used outdoors or have otherwise lost their natural colouring. Keep UV radiation levels at or below 75 µwatts/lumen.
Never place wooden objects in direct contact with outside walls or in areas in which large variations in temperature and relative humidity are expected. Avoid placing wooden objects near fireplaces, heaters, air conditioning vents and doorways. Maintaining relative humidity levels below 65 % should ensure that fungal attack does not occur.
Where both metal and wooden components are present in an object, a compromise may be needed. In these cases, it is preferable to bias conditions in favour of wood as this material is more sensitive to changes in environmental moisture levels.
Never expose furniture and other wooden artefacts to direct sunlight. In addition to causing photochemical damage to the wood itself, veneers and glues are likely to be affected, often resulting in lifting, shrinkage, warping and cracking.
Common sense is the best guide when moving or handling any object. Follow the guidelines below:
- seek help when moving large pieces of furniture;
- plan ahead. Clear the pathway to and the final location for the object and move objects slowly and with care;
- grip objects only in areas that can support their full weight such as the rail of a chair and the apron of a table (not the legs or table top);
- remove any detachable pieces before movement;
- lift furniture clear of the floor when moving it. Otherwise the side thrust on feet or legs can place undue pressure on joints; and
- to reduce the risk of dropping wooden objects, do not use gloves.
Storage and Display
To store, display and support wooden objects safely, follow these guidelines:
- use stable, inert materials for the construction and support of artefacts (see the chapter Preventive Conservation: Agents of Decay);
- the mode of storage is largely determined by the size of the objects themselves - drawers, shelves, cupboards or even the floor itself may be appropriate;
- if drawers are used for storage, enamelled metal drawers are preferred. Wooden drawers may also be used but avoid chipboard and other composition boards;
- store flat objects on level surfaces and use specially constructed supports or padding for objects with irregular surfaces;
- padding, such as polyethylene or polypropylene foam, may be used but only if the object’s stability is not compromised;
- large flat-bottomed objects may be stored on the floor but should be raised on padded blocks to allow for air circulation;
- use dust covers for shelving and keep furniture clean and dust-free;
- do not treat historic furniture which forms part of a collection as furniture. For example, do not sit on the chairs in such a collection;
- be careful what is placed on a piece of furniture as sharp objects may scratch the surface and hot items, condensation or liquid spills can badly affect surface finishes (‘blooming’, staining or even dissolution of the finish);
- maintain a stable, clean environment;
- place bubble wrap and dust covers over large wooden objects;
- do not store or consume food and drink near wooden artefacts; and
- inspect objects regularly, looking for signs of insect attack such as flight holes or frass that may have fallen from such holes. Regular inspection is even more critical for objects which cannot be stored under cover.
Large wooden objects kept outdoors should be placed under cover on a concrete pad to protect them from weathering elements and to prevent easy access by black and white ants. Additional protection from weathering can be attained by maintaining painted or varnished surfaces. Dust these objects regularly.
Use stands to raise wheeled objects from the ground. This takes the weight of the vehicle from the wheels and reduces access by ants.
If transporting wooden objects between regions of differing relative humidity, take precautions to allow the object to acclimatise to its new environment. Packing and equilibration regimes are described elsewhere (see the chapter Handling, Packing and Storage).
Before treating a wooden object examine it carefully, noting significant features which may affect the type of treatment chosen. Factors such as the type of original surface, the presence of maker’s marks, evidence of usage and previous repairs or alterations need to be considered when deciding on a treatment strategy (Figure 4).
As wooden objects and certain surface finishes are particularly sensitive to moisture, dry cleaning methods are recommended. The abrasive nature of dust must also be considered when contemplating cleaning, especially with objects that have highly polished surfaces. Only clean wooden surfaces that are either unfinished, painted or those with a robust surface finish.
For all wooden surfaces, but especially unsealed and polished surfaces (including French polished), the best approach is to use a soft lint-free cloth or a soft brush and vacuum cleaner to remove dust and loose dirt.
Objects that have a surface that is not permeable to water (for example, waxed, polyurethane finish) may be cleaned using the approach described above or by using a sponge that has been slightly moistened in water. Do not allow water to remain on the surface of the wood.
Do not use wet cleaning techniques on polished wooden surfaces.
Consolidation or Repair
Wooden objects requiring conservation or restoration can range from large structures and vehicles to extremely fine, delicate carved objects.
Understanding an object’s past and possible future roles may affect the type of treatment undertaken. For instance, repairs to a horse-drawn vehicle that is to be used will differ from those for a similar object that is to be placed on static display.
There is a variety of techniques and consolidants that may be used to ‘repair’ damaged or friable wood. These repairs include consolidating friable components, filling gaps and gluing dislodged pieces. Materials used to repair wooden objects include resins, waxes and adhesives. Some of these, not all of which are recommended for use by the way, are listed below:
- resin and wax;
- polyester resins (for example, Paraloid B72);
- polyvinyl butyral;
- polyvinyl acetate;
- epoxy resins (for example, Araldite);
- cellulose nitrate; and
- silicone rubbers;
Unfortunately, inappropriate materials are often used to replace missing wood or as fillers for gaps and cracks. Some of these include newspaper, plaster, concrete, bitumen and motor car body filler.
The type of adhesive and the filler type will be determined by the proposed use of the object. If the object is to perform a structural role then the combination of materials will be different from those that would be used for non-structural fills. For structural fills where movement of the wood is likely to be slight, an epoxy resin or micro-balloon fill has been recommended (Grattan and Barclay 1988). The use of a fill of epoxy resin and sawdust is described elsewhere (see the chapter Case Studies). A summary of traditional fillers is provided in Appendix 5.
As the techniques and chemicals used in consolidation are specialised, seek the advice of a conservator.
Usually older style furniture is constructed and held together by dowels or pegs and animal-based glue. Although these glues are quite strong, they deteriorate with age, become very brittle and are easily broken, especially if excessive stress is placed upon the joints. Screws, brackets and hold down clamps (table tops) were used in later furniture.
If restoring period or historic furniture, take care to use correct restorative methods. Incorrect procedures can undermine both the value and the historical importance of the furniture. Research into the methods of construction of furniture is very important, often yielding valuable information which can guide any restoration process. For example, if animal glues and dowelling were used originally, use the same materials in any restoration/conservation treatment. If a new piece has to be constructed and fitted to an old object, use original methods. Retain any remaining damaged, original pieces for future repairs.
Note, despite the recommendation above, there is some support for using modern materials so that repairs and restoration work can be distinguished from the untreated components.
Staining and Marking of Furniture
Staining or marking is virtually inevitable on furniture that has been used throughout its life. These stains and marks often enhance the character and patina of the piece. Some attempts to eradicate staining and marks have damaged some fine pieces of furniture.
As the removal of stains and marks may involve the loss of both wood and patina, consider the available options before bringing the sander and sandpaper into action. Seek advice from a conservator. Drastic treatment may result in the loss of value of the piece, especially if problems are encountered with reproducing the surface finish and colour (often difficult tasks).
A variety of materials, including brass, ivory and different woods, is used for inlays. Their differing compositions and variations in the way they react to changing environmental conditions complicate repairs. Extremes of temperature and humidity will affect each of the materials differently.
The treatment of inlays is highly specialised. Seek professional advice before attempting any treatment.
Surface finishes applied to wooden objects include:
- shellac (French polish);
- oils; and
Exercise care when selecting a surface coating to apply to a wooden object. The purpose of the coating may be to either protect a friable surface or to confer a pleasant sheen to a surface that has become dull. In the past it was customary to apply drying oils, such as linseed oil, or waxes such as beeswax. The former suffer from the disadvantage that they yellow with age, a tendency that is accentuated under tropical conditions and they become very difficult to remove after prolonged ageing. The latter often leave the surface of the wood slightly sticky. Sticky areas attract dust, particularly in the crevices in the wood. This creates favourable conditions for the laying of eggs by wood-boring insects.
Depending on the nature of the object, waxes may be suitable as protective surface coatings as long as care is taken to choose a suitable wax mixture which leaves a relatively hard, non-sticky surface finish. Such a wax mixture may be prepared using some commercially available synthetic waxes. A useful formulation and method of preparation is given below:
|microcrystalline wax||100 g|
|polyethylene wax||25 g|
|white spirit||230 g|
- melt the waxes together and stir well to ensure that they are thoroughly mixed; and
- pour the molten mixture quickly into the white spirit and stir constantly while cooling so that a smooth paste is obtained.
Several varieties of these waxes are available and the sheen of the resultant wax film can be varied by altering the grades or the proportions of the waxes used. The high dispersive power of the microcrystalline wax helps to remove ingrained dirt from the wood as well as providing a protective coating of pleasant appearance.
Formulations made by mixing beeswax and carnauba with turpentine have been found to work well.
Several suitable commercial wood waxes are available and these give satisfactory results. Avoid waxes containing silicones (this is usually stated on the container) as they are very difficult to remove. This may cause problems if subsequent treatments are necessary.
Only use coloured waxes after careful thought as these obviously alter the appearance of the wood. Clear finish waxes are preferred.
French polished furniture has a beautiful and lustrous finish but it can be damaged easily. As French polish is shellac dissolved in methylated spirit, any wet object or alcohol will either mark or dissolve the surface. Restoring damaged surfaces will usually require the services of a professional restorer.
Polyurethane finishes are clear, hard and resistant to wear but are virtually impossible to remove without damage to the underlying wood. Their use on period or antique furniture is not recommended.
Oil-based finishes also give a lustrous shine but unless wiped down carefully can leave a ‘tacky’ finish which will attract dust. Oils which may be used on furniture include poppy seed, walnut and Danish oils.
Veneer is a thin layer, normally of some exotic timber, laid and glued over a frame of a different type of wood. If animal glue has been used and is deteriorating, the veneer may lift. If regluing is attempted, all traces of the old glue must be removed to ensure a flat finish is obtained. Refer large areas of lifting veneer to experts in the area.
Upholstery is a general term which includes carpets, bed hangings, curtains, chairs, sofas and the components of seat furniture. Information concerning the deterioration, care and conservation of these objects may be obtained by reading the appropriate chapters that deal with the particular material types associated with these objects.
Discussion in this section concentrates on the wooden frame, rather than with the webbing, springs, stuffing, lining, cover and other non-wood upholstery components. Despite this focus, these latter components are important (Burgin 1990, Twichel 1990, Perkins 1990). Some general aspects related to their care and conservation are listed below:
- if extremely disintegrated material is replaced, clean and store it;
- use materials such as plywood and perspex to support deteriorated webbing;
- use soft brushes and low suction vacuum cleaning to clean stuffing;
- place and mould stuffing to its original shape during replacement;
- stabilise acidic hessian by using water poultices; and
- use ethafoam if reconstruction of the under-upholstery is necessary for display purposes and the original materials are unable to be used.
As long as the original fabric has been retained, the colours, patterns and types of textiles used on covers often provide evidence of the style of a period. The conservation of covers is somewhat contentious, especially the issue of removal of the cover for treatment. Replacement is often difficult and cannot always be accomplished. Apart from cleaning robust covers with soft brushes and low suction vacuum cleaners their conservation is best left in the hands of conservators.
The frame can provide useful information about the history of an object. Breaks in the joints or members and nailing patterns, for instance, may give an insight into past wear, abuse and repairs.
Repairing a frame raises many ethical questions regarding the use of adhesives, the replacement of parts and the general extent of intervention (James 1990, Podmaniczky 1990, Graves 1990). The following principles are useful guides when considering conservation of a frame:
- if part of the frame has to be replaced, document this and retain the removed section;
- minimise intervention so that as much of the original object is retained as possible;
- if an adhesive is required for a mechanical joint, use a reversible adhesive, such as PVA;
- analyse and document any original adhesive;
- strengthen weak components by using metal plates to bridge joints or low molecular weight consolidants; and
- when re-upholstering, minimise damage to the frame by using the original holes or by nailing into plugs of white pine which have been used to fill holes.
Heavy Vehicles or Machinery
Although every artefact must be assessed individually, repairs to heavy wooden equipment and horse-drawn vehicles normally require strong consolidating agents. As maximum strength is required, animal glue is not recommended (see the chapter Case Studies).
The application of oils to the wood is not advised as dust quickly adheres to it. This in turn attracts moisture and subsequent mould development. Preventive measures, such as those outlined earlier in this chapter are recommended (see the chapter Preventive Conservation: Agents of Decay).
- Carefully check wooden objects for insect infestation or mould before adding them to a collection.
- Keep temperature and relative humidity within the ranges 15 – 25 °C and 40 – 60 % respectively with maximum variations of 4 °C and 5 % in any 24 hour period. Hot, dry conditions will cause wood to shrink and crack while excessively moist conditions will cause it to swell and warp.
- Never expose wooden objects to direct sunlight. In addition to causing photochemical damage, veneers and glues may be affected, resulting in lifting, warping or cracking.
- Do not put objects on top of polished surfaces as they could scratch or otherwise damage the surface finish.
- When storing large objects, place bubble wrap over them and dust covers on top.
Burgin, A., 1990, Eighteenth- and nineteenth-century chair and sofa treatments, Upholstery Conservation, Preprints of a Symposium held at Colonial Williamsburg, East Kinston, N.H., American Conservation Consortium Ltd, pp. 323-337.
Valerie Dorge, V. and Carey Howlett, F. (Eds), 1998 , Painted Wood: History and Conservation, Proceedings of a symposium organized by the Wooden Artifacts Group of the American Institute for Conservation of Historic and Artistic Works and the Foundation of the AIC, Colonial Williamsburg Foundation, Williamsburg, Virginia 11–14 November 1994, The Getty Conservation Institute, Los Angeles.
Grattan, D.W. and Barclay, R.L., 1988, A study of gap-fillers for wooden objects, Studies in Conservation, vol. 33, pp. 71-86.
Graves, L., 1990, Conservation of an 1811 sofa with nearly intact original upholstery, Upholstery Conservation, Preprints of a Symposium held at Colonial Williamsburg, East Kinston, N.H., American Conservation Consortium Ltd, pp. 356-376.
James, D., 1990, Upholstery - A Complete Course, Guild of Master Craftsman Publications, East Sussex.
Perkins, Z.A., 1990, Conservation of a Leon Marcotte side chair, Upholstery Conservation, Preprints of a Symposium held at Colonial Williamsburg, East Kinston, N.H., American Conservation Consortium Ltd, pp. 409-421.
Podmaniczky, M.S., 1990, Wooden frame conservation techniques, ibid, pp. 29-41.
Twichel, J., 1990, Experiments with ‘tackless’ upholstery: Successes (and trials) at SPNEA, ibid, pp. 232-245.
Unger, A., Schniewind, A. and Unger, W., 2001, Conservation of Wood Artifacts: A Handbook, Springer-Verlag, Berlin.