Shellac

Anthony Beech

 

  Stunning Japanned cabinet with doors open to reveal draw fronts decorated in oriental style with birds, plants, pagodas, etc in gold on background of deep reds and blacks resembling tortoiseshell  
     
  English Japanned cabinet, 1690-1700: Japanning allowed
European cabinet-makers to imitate oriental lacquer finishes.
Shellac-based recipes for Japanning were published in 1688
in John Stalker and George Parker’s A Treatise of Japanning
and Varnishing
. (Photo: Victoria and Albert Museum, London) 
 

Many resins, oils and waxes have been used over the centuries to provide a glossy finish to fine timbers. One of the most important is shellac, a resin produced from a secretion of the lac beetle (Laccifer lacca), which feeds on tree sap. The insect’s name is derived from the word lakh, the Sanskrit word for the number 100,000, and relates to the number of insects found on infested trees. The tree branches become covered in the secreted material which, in its raw form, is called sticklac. It is harvested extensively in India and, to a lesser extent, in China, Burma and Thailand.

During the process of harvesting, branches are cut from the tree then crushed and washed to remove wood fibres and other contaminants, transforming the sticklac into a usable material called seedlac. Further refinement is achieved by heating the seedlac to its melting point and filtering it through a cloth onto a cool surface to form discs of buttonlac, which will be familiar to traditional polishers as a key ingredient of shellac.

The seedlac or buttonlac can then be dissolved in alcohol. This dissolved form of shellac is sold as French polish under trade names such as ‘special pale polish’ or ‘button polish’ depending on the exact preparation. The natural product contains a small quantity of wax produced by the lac beetle which is sometimes removed during production to create ‘de-waxed’ shellac. The weight of shellac dissolved is described as ‘the cut’. This term refers to the ratio of shellac to alcohol in the preparation, so one pound of shellac dissolved in a gallon of alcohol is called a ‘one-pound cut’. Most commercial preparations are a three-pound cut, which can then be further diluted as required or used neat for ‘bodying up’.

Other off-the-shelf shellac preparations may have added ingredients to adapt the properties of the shellac, such as melamine to produce ‘heat-resistant’ polish. These modified shellacs have their place alongside modern cellulose and acrylic lacquers for finishing new cabinet work and furniture but are not generally suitable for period furniture or period architectural features because they are likely to be less reversible and less stable.

The exact nature of the shellac product varies depending on the species of tree which the lac beetle feeds on. This results in different colours and properties which were historically exploited to achieve different finishes ranging from deep brown to orange and amber tones. Use of the more opaque shellacs can be problematic as building a ‘grain-filled’ finish can require the application of so much shellac that the wood surface is partially obscured, resulting in an unsatisfactory, muddy finish. The finish can also become patchy during the ‘cutting back’ process (see ‘Application’ below).

Other issues arose historically with the grain-fillers used to prepare the wood for polishing. These were popular in the late 19th and early 20th centuries as a way of speeding up the polishing process. They were often prepared using chalk and other materials which were coloured to match the wood surface. As a result, the finish on much Edwardian furniture displays white flecks in the grain where the grain-filler has been bleached by ultraviolet light.

HISTORIC USE

Shellac has been used for wood finishing since at least the 13th century but it has had many other uses. Its most common early use was for fixing dye and it has been used in the food, pharmaceutical and cosmetic industries. Its thermoplastic properties also leant themselves to the production of cast material before the advent of modern plastic alternatives, including early gramophone records.

  Discs of reddish-brown buttonlac in a porcelain crucible  
  Discs of buttonlac, a key ingredient of shellac (Photo: www.cornelissen.com)  

Early uses in cabinet-making include the creation of ‘Japanned’ finishes. References to the use of shellac appear in Stalker and Parker’s A Treatise of Japanning and Varnishing published in 1688. This included recipes for the preparation of shellac-based varnishes and drawings of suitable oriental designs. The process of Japanning was an attempt by European craftsmen to replicate the appearance of Japanese and Chinese lacquerware, which was highly fashionable and was imported by wealthy collectors alongside oriental porcelain. Some confusion exists regarding the differences between oriental lacquer and shellac-based finishes. Oriental lacquer differs from shellac in its use of raw ‘urushi’ lacquer direct from the tree. It can only be used in its country of origin as it is difficult to store and cannot be transported in its raw form.

Around the time of Stalker and Parker’s treatise, shellac was also being used to formulate varnishes which were applied by brush and created a finish for cabinet-work but which did not produce the completely flat and grain-filled surface typical of the later technique of French polishing. Many other varnish types were also then available. These were based on drying oils such as linseed oil, copal, sandarac and mastic, which were extracted directly from trees and plants. Towards the end of the 18th century the technique of applying shellac using friction to achieve a grain-filled, bright, flat surface developed. This was well-suited to veneered surfaces which were often decorated with marquetry and inlay. Satinwood was one of the most fashionable veneers and responded particularly well to the bright finish of shellac, which enhanced the ‘satin’ patterns in the grain. This bright finish continued to grow in popularity into the Regency period when highly polished rosewood took over as the veneer of choice for fashionable furniture.

The eclectic nature of Victorian furniture later in the 19th century and the ease of trade with other parts of the world meant that mahogany, walnut and oak were all used by cabinet-makers. The majority of this work received a shellac finish of some sort, ranging from opened grained oak to highly polished mahogany and walnut. Some of the best shellac finishes were produced at this time and many of them were sufficiently stable that they can be found in good order today.

Unfortunately, the Victorian period also saw a great deal of 17th and 18th-century furniture inappropriately given a shellac finish, often removing or covering earlier oil and wax finishes. Many late 19th-century interiors also boast excellent examples of French polished surfaces, staircases, panelling and display cabinets. These will often have original finishes which can be revived or uncovered from beneath later layers of opaque varnish or shellac.

APPLICATION

The familiar term ‘French polish’ refers not to the product itself, as is often thought, but to the technique of applying shellac in thin layers. The shellac is applied with a polishing ‘fad’ or rubber made by compressing polishers’ wadding or cottonwool into a covering of fine, lint-free cotton. The wadding is then saturated with liquid shellac and the sides are wound to tighten the cover, forming a firm pad which can be held between the thumb and forefingers. After building up a base-layer the surface is then worked in circular or figure-of-eight movements to push the soft shellac into the grain, then in straight lines to pull the surface flat and remove any circular ‘whip’ marks.

  Glazed display case containing museum artefacts with sample draws beneath display compartment and framed section above it for holding interpretive material  
  French polished display case at the Sedgwick Museum of Earth Sciences, Cambridge  

The surface is then allowed to dry before it is lightly ‘cut back’ with fine sand paper. Traditionally, pumice was sometimes used to assist the flattening process and fill the grain. This is less common in modern polishing workshops as it can compromise the quality and stability of the shellac film. After cutting back to remove any ridges, the application process is repeated until the desired level of finish is achieved. Many traditional polishers used linseed oil to lubricate the fad so that more polish could be applied before the stickiness began to cause the fad to drag and damage the surface. This can be problematic, however, because if all the oil is not removed in the spiriting off or waxing back stages, it can spoil the finish by bleeding through or creating matt patches.

Once the desired full-grained surface has been achieved, the spiriting off or pulling over process begins. More alcohol is introduced into the mix to flatten and polish the surface of the shellac film to a bright finish with no lines or whips. This bright finish can then be ‘burnished’ with a liquid abrasive compound to produce a bright finish or matted back with wax and fine wire wool to achieve a softer, mellow finish which is well suited to late 18thcentury furniture.

RESTORING AND CONSERVING SHELLAC-BASED FINISHES

Traditional shellac-based finishes remain semi-soluble in alcohol many years after application and can therefore be removed or reworked with alcohol. In many cases finishes with superficial damage can be reworked without the need to clean back and re-polish. If the shellac film is not heavily degraded it is preferable to repair the surface rather than re-polish. Minor damage and signs of wear can be carefully re-saturated with a weak shellac solution and a fine brush. This will often be sufficient to return the damaged area to its base colour, making the damage less visible. This approach is most effective on period pieces where evidence of use is recognised as part of the object’s history and should be retained.

White ‘bloom’ or haze on shellac caused by water or heat damage can sometimes be ‘burnished’ out with a mild abrasive solution if it is limited to the surface of the shellac, or re-saturated with a weak shellac solution. The bloom is caused by tiny cracks in the surface and polishing with abrasive can bring back a good reflective finish. However, caution must be exercised. Although less invasive than full re-polishing, this technique does remove some of the shellac surface and it is possible to burnish through. This will be indicated by a matt patch where the shellac has been removed to an earlier finish layer or to the bare wood surface. This will then require the polish to be re-built and may mean that the surface has to be completely re-polished to achieve an even finish. The other pitfall of this approach is that it can leave a brighter patch in the worked area compared to the surrounding aged surface. This can be avoided by working over the entire surface and then matting down to the desired level.

Apart from handling damage or water damage the most common reason for the failure of shellac finishes is exposure to unfiltered ultraviolet light, which gradually breaks down the finish until it reverts to a white powder on the surface. Infrared light can also cause damage by heating the surface until it becomes unstable. Unmodified shellac is sensitive to heat and can be damaged by hot cups and plates, which are responsible for a great deal of damage to shellac-finished side tables and dining tables. Alcohol in the form of wine or spirits will also soften shellac causing damage.

When it is necessary to re-polish because the shellac finish has broken down beyond repair, it is often possible to remove much of the damaged finish but retain the shellac which has filled the grain. This means that the subsequent re-polishing takes less time. This approach often allows the retention of much of the patina – the evidence of use and subtle surface colourchanges that make period polished wood so appealing.

  French polished rosewood table with inlaid border around table-top and beaded edge  
  Rio rosewood table from the collection at Burghley House with original French polished finish (Reproduced by the kind permission of Burghley House Preservation Trust)  

The traditional method of removing shellac finishes was to saturate a pad of fine ‘0000’ wire wool with a mix of methylated spirit and raw linseed oil, working this over the surface to soften and remove the shellac. The raw linseed is introduced to slow down the evaporation of the methylated spirit and lubricate the cut of the wire wool. The wire wool becomes clogged with shellac during use and must be replaced periodically. Finally, neat methylated spirit is used on a cotton wool pad to remove any remaining residue. This method has drawbacks, not least the potential risk of contaminating the wood surface with fibres from the wire wool. It is also rather uncontrolled and solvent gels or swabs provide a cleaner, more controlled method of removal.

It is important when carrying out tests on the polished surface to establish the nature of previous finishes and how they may change during cleaning and preparation for re-finishing. Many oxidised finishes which have degraded badly can be revived or repaired. Under no circumstances should ‘sanding down’ be considered as this can remove not only the shellac finish but also all surface patina and the hand-worked surface, which is of historical value. Marquetrydecorated and veneered objects have suffered heavily from this invasive approach and the thinning of the veneer often reaches the point where ground work is visible. The damage caused by sanding down period surfaces is irreversible and should never be considered for period furniture or historic interior woodwork. Once the damaged layer has been cleaned back to a sound surface it is possible to rebuild a shellac finish using traditional techniques. The long-term reversibility and stability of unmodified shellac makes it suitable for use by conservators on historic wood surfaces which were originally shellac-finished and from which it can be removed again. Shellac is not suitable for the in-painting of shellac-bound finishes such as a Japanned or painted finishes because the method of removal for the new shellac would interfere with the original binding.

When re-finishing period furniture and interior woodwork it is important to try to achieve the correct finish for the period. The piece’s place of origin should also be taken into account because a shellac finish may be appropriate for continental furniture from an earlier date than would be appropriate for British-made pieces. Thorough research should be the starting point for any project involving period wood finishing.

 

 

The Building Conservation Directory, 2012

Author

ANTHONY BEECH ACR runs Anthony Beech ACR Furniture Conservation and Restoration, an Icon-accredited workshop based at Burghley House, Lincolnshire. The company provides services ranging from the conservation and restoration of individual pieces to the ongoing conservation of complete collections and interiors.

Email info@furnitureconservation.co.uk

Further information

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