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Care and Conservation of Wooden Bowls and Vessels

This information is made available as a public service, for use by collectors of wooden bowls and vessels. It was adapted from an Art Conservation Handout published by the Bishop Museum, Hawai’i State Museum of Natural and Cultural History. The original article is a bit over the top for most of us.

Wooden bowls and vessels are susceptible to damage from insects, humidity, light and dust. The following is a summary of how these forces work to accelerate deterioration of wooden objects and the steps which can be taken to limit the damage.

These recommendations are intended for use with fine art objects rather than utility bowls. However, the section on cleaning and polishing wooden objects is applicable to bowls and vessels that see frequent use in your home.

Insects

Wood-boring insects such as termites are a major source of damage to wood.

Your wooden objects should be examined periodically for the presence of holes and frass (fine pellet or sawdust droppings). If a bowl or vessel has any cracks, as many do, these are sites of exposed wood which are particularly vulnerable to insect infestation. If an infestation is discovered, the object should be placed into a heavy duty plastic bag immediately and an expert consulted regarding the proper procedure for freeze sterilization.

Humidity

Relative humidity is a measure of how much moisture there is in the air. Wood can absorb and desorb moisture as the humidity rises and falls. Very low humidity will cause wood to dry out and crack. Rapid fluctuations in humidity are not good either because the expansion and contraction of the wood will result in a weakening of the wood, particularly if the wood has cracks or old repairs. When repairs such as butterfly repairs, patches or plugs were made to the object, a different wood than that which the object was made frequently was used. Because different woods expand and contract differently in response to changes and fluctuations in humidity, constant swings in humidity can over time result in a loosening of the old repairs in the object.

Wooden objects in areas subject to high humidity are at risk of mold growth.

Mold is a problem because it is visually disfiguring and can have a bad affect upon the surface finish of a bowl or vessel. Mold growth grows best given the following conditions: high humidity, lack of air circulation, and the presence of a food source such as dust or oils to grow on. Mold can be avoided by keeping your wooden art away from areas of high humidity such as bathrooms and away from areas where leaks or mold have been a problem in the past.

Air circulation in the room where your wood art is kept can be increased by running fans and routinely airing out closets or cabinets where art is stored. Dehumidifiers can be used in rooms which do not have sufficient air circulation. Keeping your art free of dust (see section on DUST) will also aid in preventing the growth of mold.

If mold is found, it must be removed from the surface of the object. Take the bowl or vessel to a clean work surface. Using a dry, soft, natural bristle paint brush, lift the fluffy mold growths from the surface of the object. A vacuum nozzle should be carefully held above the area being brushed to collect the mold is it is lifted. The nozzle should not be allowed to come into contact with the surface of the object to avoid scratching the surface.

Light

Light is a form of energy and can do damage. All visible materials absorb light. The amount of damage done to these materials depends upon the energies contained by the light, the intensity of the light, and the length of time that the light is kept on the object. Light damage is cumulative. Once the molecular structure of a material has been altered by light, it cannot heal itself. Sunlight and fluorescent light are the most damaging types of light because they contain high energy short wavelength ultraviolet (UV) radiation.

Exposure to light can result in the breaking of chemical bonds between atoms in the molecules that make up materials. Broken chemical bonds in materials result in visible changes in the condition of objects. In wooden objects these changes will be seen as bleaching and fading of the wood. On objects that have been shellacked or have shellac fills, the effects of light damage can result in degradation of the shellac finish which will be seen as cracking and darkening of the shellac finish.

To lessen the damage caused by ultraviolet light, wooden objects should never receive direct sunlight or be illuminated by bright spot lights. Direct sunlight can also cause the object to get hot. Elevated temperatures will have a drying effect upon the wood and possibly damage the finish as well. The ultraviolet component in sunlight can be lessened by the use of clear or partially shaded acrylic films which can be applied to window glass. The ultraviolet light given off by fluorescent light can be diminished by the use of ultraviolet absorbing filters which can be placed over the fluorescent light tubes. If the object is in a display case, the case should be made of an ultraviolet light absorbing acrylic.

Dust

Dust can damage objects by attracting insects and mold. Dust is fine organic or inorganic suspended particulate matter ranging in size and coarseness. Natural particulates in the air include, mold spores, pollen, proteinaceous particles from dead insects and inorganic, mineral based dirt. The organic material in dust tends to be hygroscopic. This means that moisture in the air is retained and even attracted by the dust. The moisture collected in dust provides the necessary humidity for the growth of mold. Mold growth on the surface of an object can result in discoloration, staining and marring of the finish. The organic matter found in dust is also a good absorber of atmospheric pollutants such as sulphur dioxide and chlorides which provide the necessary conditions for mold growth. Chloride salts are hygroscopic and can lead to staining and weakening of wood.

Another danger of dust is that concrete particles and other inorganic mineral components are crystalline. The sharp, hard spikes and edges of these articles can be abrasive and act as fine sand paper, marring polished and finished surfaces as dust is rubbed or dragged across the surface.

Finally, the insect parts, pollen, plant particles and grease found in dust is an attractive food source for a range of insects. The warm, moist, food-rich environment formed by the dust provides an excellent nesting site for insects.

Insects which begin by feeding and nesting in dust may eventually infest and damage the wood object.

Good housekeeping is the best means of combating dust. Weekly cleaning of exposed surfaces will prevent the build-up of dust and help keep your art from suffering chemical, fungal and mechanical damage. There are various methods of safely removing dust. The finish on your wooden art can be damaged by the solvents used in commercial spray cleaners and these should not be used. Commercials cleaners can also contain a variety of other unknown ingredients which have the potential for damaging your art. Any product containing silicones or oils should not be used, as these materials tend to build up, darken, and become sticky and difficult to remove over time.

When removing dust from your art the abrasive quality of dust must be kept in mind. A gentle stream of clean air blown over the surface can remove a large quantity of abrasive dust. Brushing the surface with a soft brush such as a white boar's hair Chinese pipe brush or Japanese hake brush rather than a dust cloth will further reduce abrasion. The sift bristles will pick up dust rather than drag it across the surface as would a dust cloth. Brushes are available from local art supply stores. When removing dust from a hard, textured surface, a stiffer brush may be needed to work the dust out of the cracks and crevasses frequently found on wooden bowls or vessels. The stiffer brush should be used only after as much dust as possible has been removed using air or soft brush.

Cleaning

Over a period of time, as a result of frequent handling and accumulations of dust and dirt, your bowl or vessel may take on a dull, lifeless appearance which detracts from its natural beauty. To clean a wood object, it should first be dusted carefully using a soft brush to remove the dust from the surface. If possible, when brushing dust from the surface, the dust should be directed into the nozzle of a small, low power vacuum cleaner to completely remove the dust, rather than merely redistribute it.

After the surfaces of the object has been dusted, surface accretions and other dirt can best and most safely be removed by using a clean, damp cloth. The use of "dusting" aerosol sprays which are sprayed onto a dust cloth should be avoided as these can introduce a wide variety of contaminants and solvents into the wood which may attack the finish of the wood.

If the object has acquired paint scuff marks from being in contact with painted surfaces such as shelves or walls, the paint, if it is a water-based paint, can usually be removed by gently rubbing with a damp cotton swab such as a Q-Tip. Paint removers should never be used.

Polishing

Spray or liquid polishes should be avoided. Emulsion polishes which are water-based products containing waxes, oils, detergents and solvents can have extremely powerful cleaners which leave a bright, but short-lived sheen on the surface. These may also contain harmful abrasives.

Oil polishes are troublesome as well. There are two basic types of oils: drying oils and non-drying oils. Drying oils, such as linseed and tung oil "dry" through a chemical reaction with the air. The drying process makes the oil increasingly difficult to remove over time. As the drying oil ages it also tends to become yellow or brown. Non-drying oils, such as lemon oil and mineral oil remain liquid on, or in the surface of the wood. Dust and other airborne contaminants will easily stick to the wet surface. Therefore, drying and non-drying oils should not be used to maintain wooden objects.

The most beneficial polish to use on a bowl or vessel is a semi-solid, micro-crystalline wax such as Renaissance ̈ wax. This wax does not stain or discolor and will provide a very hard, moisture resistant protective layer to the surface of your wooden object. Waxes are among the most stable of materials and the severe deterioration problems of drying and non-drying oils are not encountered with micro-crystalline waxes. When using Renaissance ̈ wax, a little goes a long way. The wax should be applied using a small, clean, lint-free cloth. A thin coating of wax should be applied to a small area of the bowl or vessel, then buffed out with a clean, soft, lint-free cloth. If too large an area is waxed at one time and the wax allowed to become too dry, it will be difficult to buff out the wax. Therefore, it is easier and more effective to work on a small area at a time, then move along to the next small area making sure to overlap them slightly so as to provide even coverage. It is important to use only a thin coating of wax.

If over the years, your bowl or vessel has acquired a variety of nicks, scratches and gouges, you may find that if too much wax is used these surface defects will become filled with wax which when dry, will be very difficult to buff out. Fortunately, because Renaissance ̈ wax is so durable and stable the object should require re-waxing only very infrequently, depending upon how often it is handled. The need for a new application of wax can be easily determined. If the surface can no longer be buffed to an appropriate sheen, it is safe to assume that the wax has worn off.

Utility objects that see frequent use around the home may be polished using a beeswax/oil formulation. The oil shouldn’t contain silicones or solvents. The oil should be a vegetable oil (e.g., jojoba oil, coconut oil) that is relatively stable and will not go rancid over time. Carnauba wax formulations used for cars should be avoided, as they contain oils, silicones, and other petroleum distillates that can harm you and the object being polished. If you have access to a Beall buffer, a relatively pure carnauba wax that is sold in stick form can be used to polish the bowl or vessel.

Storage

The goals of safe storage are to avoid insects, mold, dust, light damage and damage due to mishandling. Art objects should be stored in a clean, dry, dark storage space. If the object is placed in a plastic bag, this will limit access of dust and insects. The storage space should not be one that is excessively hot. Nor should the object be stored in such a way that it is constantly being moved in order to gain access to other items in the storage space. The more often an object is handled or moved, the greater the likelihood that it will come to harm. It is important to remember than no matter what precautions are taken, problems can occur with time. It is essential, therefore, that all stored materials be examined at least once every three months for signs of insect activity, mold growth or any other observable change.

Display

Wooden objects should not be exposed to display conditions of excessive light, heat or humidity, or conditions that will foster mold growth or insect infestation. Therefore, select an area where the object will not receive direct sunlight and will be out of harm's way. It should be in an area where excessive moisture or leaks are not a problem.

If the object is displayed as a free standing object, placed on a shelf or piece of furniture for instance, there should be an interleaving material separating the bowl or vessel from the object it is placed on. This interleaving material will prevent the bottom of the object from getting scratched and abraded or from picking up scuff marks of paint if the object is placed on a painted surface. Similarly, if the object is placed on a shelf, it should be far enough away from the wall to avoid damage that may ensue from the object hitting the wall.

A purposely built exhibit case will protect the bowl from dust and insects. The case should use ultraviolet absorbing acrylic. The case should also provide for the maintenance of a stable relative humidity. This can be accomplished through the incorporation of silica gel desiccant into the case design. If you wish to do so, please consult an expert as misuse can result in damage to wooden objects.