Lacquer

Related Topics:
Lacquerware

In a general sense, lacquer is a clear or coloured coating, that dries by solvent evaporation only and that produces a hard, durable finish that can be polished to a very high gloss, and gives the illusion of depth. In a narrower sense, lacquer consists of a resin dissolved in a fast-drying solvent which is a mixture of naphtha, xylene, toluene, and ketones, including acetone. The word "lacquer" comes from the lac insect (Laccifer lacca, formerly Coccus lacca), whose secretions have been historically used to make lacquer and shellac.

Urushiol-based lacquers

The earliest known lacquers were made in Japan, about 7000 BC.^* These lacquers, made from the resin of the tree Rhus verniciflua, produce very hard, durable finishes that are both beautiful, and very resistant to damage by water, acid, alkali or abrasion. They do not, however, stand up well to ultraviolet light. The active ingredient of the resin is urushiol, a mixture of various phenols suspended in water, plus a few proteins.

Urushiol-based lacquers differ from most other lacquers in that they are slow-drying, water based, and set by oxidation and polymerisation, rather than by evaporation alone. In order for it to cure, a high temperature and humidity is required. The phenols oxidize and polymerize under the action of an enzyme laccase, yielding a substrate that, upon proper evaporation of its water content, is hard and fairly resistant to mechanical stress. Lacquer skills became very highly developed in India and Asia, and many highly decorated pieces were produced. The fresh resin causes severe allergic reactions on human skin, and great care is required in its use.

From China, knowledge of making lacquer ware spread to Korea. Chinese pieces traveled through various trade routes to the Middle East. Known applications of lacquer in China included coffins, plates, music instruments and furniture. Lacquer mixed with powdered cinnabar is used to produce the traditional red lacquerware from China.

The trees must be at least 10 years old before cutting to bleed the resin. It sets by a process called "aqua-polymerization", absorbing oxygen to set; placing in a humid environment (called "furo" or "muro" in Japanese, means "a bath" or "a room") allows it to absorb more oxygen from the evaporation of the water.

Lacquer yielding trees in Thailand, Vietnam, Burma and Taiwan, called Thitsi, are slightly different; they do not contain urushiol, but similar substances called "laccol" or "thitsiol". The end result is similar but softer than the Chinese or Japanese lacquer. Unlike Japanese and Chinese Rhus verniciflua resin, Burmese lacquer does not cause allergic reactions; it sets slower, and is painted by craftsmen's hands without using brushes.

Raw lacquer can be "coloured" by the addition of small amounts of iron oxides, giving red or black depending on the oxide. There is some evidence that its use is even older than 8,000 years from archeological digs in China. Later, pigments were added to make colours. It is used not only as a finish, but mixed with ground fired and unfired clays applied to a mould with layers of hemp cloth, it can produce objects without need for another core like wood. The process is called "kanshitsu" in Japan. Advanced decorative techniques using additional materials such as gold and silver powders and flakes ("makie") were refined to very high standards in Japan also after having been introduced from China. In the lacquering of the Chinese musical instrument, the guqin, the lacquer is mixed with deer horn powder (or ceramic powder) to give it more strength so it can stand up to the fingering.

Because of the toxicity of natural lacquer, it is generally difficult to import it to certain countries. However, it can usually be obtained in small amounts via Japanese katana repair shops online.

There are more than four forms of urushiol which is written as thus:

R = (CH₂)₁₄CH₃ or
R = (CH₂)₇CH=CH(CH₂)₅CH₃ or
R = (CH₂)₇CH=CHCH₂CH=CH(CH₂)₂CH₃ or
R = (CH₂)₇CH=CHCH₂CH=CHCH=CHCH₃ or
R = (CH₂)₇CH=CHCH₂CH=CHCH₂CH=CH₂ and others.

Nitrocellulose lacquers

Quick-drying solvent-based lacquers that contain nitrocellulose, a resin obtained from the nitration of cotton and other cellulostic materials, were developed in the early 1920s, and extensively used in the automobile industry for 30 years. Prior to their introduction, mass produced automotive finishes were limited in colour, with Japan Black being the fastest drying and thus most popular. General Motors Oakland automobile brand automobile was the first (1923) to introduce one of the new fast drying nitrocelluous lacquers, a bright blue, produced by DuPont under their Duco tradename.

These lacquers are also used on wooden products, furniture primarily, and on musical instruments and other objects. The nitrocellulose and other resins and plasticizers are dissolved in the solvent, and each coat of lacquer dissolves some of the previous coat. These lacquers were a huge improvement over earlier automobile and furniture finishes, both in ease of application, and in colour retention. The preferred method of applying quick-drying lacquers is by spraying, and the development of nitrocellulose lacquers led to the first extensive use of spray guns. Nitrocellulose lacquers produce a very hard yet flexible, durable finish that can be polished to a high sheen. Drawback of these lacquers include the hazardous nature of the solvent, which is flammable, volatile and toxic, and the handling hazards of nitrocellulose in the lacquer manufacturing process. Lacquer grade or soluble nitrocellulose is closely related to the more highly nitrated form which is used to make explosives.

Acrylic lacquers

Lacquers using acrylic resin, a synthetic polymer, were developed in the 1950s. Acrylic resin is colourless, transparent thermoplastic, obtained by the polymerization of derivatives of acrylic acid. Acrylic is also used in enamels, which have the advantage of not needing to be buffed to obtain a shine. Enamels, however, are slow drying. The advantage of acrylic lacquers, which was recognized by General Motors, is an exceptionally fast drying time. The use of lacquers in automobile finishes was discontinued when tougher, more durable, weather and chemical resistant two-component polyurethane coatings were developed. The system usually consists of a primer, colour coat and clear topcoat, commonly known as clear coat finishes. It is extensively used for wooden finishing

Water-based lacquers

Due to health risks and environmental considerations involved in the use of solvent-based lacquers, much work has gone in to the development of water-based lacquers. Such lacquers are considerably less toxic and more environmentally friendly, and in many cases, produce acceptable results. More and more water-based coloured lacquers are replacing solvent-based clear and coloured lacquers in underhood and interior applications in the automobile and other similar industrial applications. Water based lacquers are used extensively in wood furniture finishing as well.

Japanning

As Asian and Indian lacquer work became popular in England, France, the Netherlands, and Spain in the 17th century the Europeans developed imitations that were effectively a different technique of lacquering. The European technique, which is used on furniture and other objects, uses varnishes that have a resin base similar to shellac. The technique, which became known as japanning, involves applying several coats of varnish which are each heat-dried and polished. In the 18th century this type of lacquering gained a large popular following. In the 19th and 20th centuries this lacquering technique evolved into the handicraft of decoupage.

References

p.1050
- A concise compilation of technical terms. Attached is a register of all German terms with their corresponding English terms and vice versa, in order to facilitate its use as a means for technical translation from one language to the other.

漆器