Shellac

Blonde shellac

Shellac is the refined form of a natural, resinous, organic secretion of the tiny lac insect and that is used commercially as a wood sealer and finisher, floor polish, protective candy coating, and so forth. Lac is the name given to the actual secretion of the insect, and shellac technically is the refined lac in flake form (Bryk 2002). However, the term also is used for refined lac that is suspended in an alcohol-based solvent, such as the thin shellac varnish used to finish wood (Bryk 2002).

Source: Lac insects

Lac is the resinous secretion of a number of species of lac-producing insects. Lac insects belong to such genera as Metatachardia, Laccifer, Tachordiella and others of the superfamily Coccoidea of the Homopera order; however, members of two families— Lacciferidae and Tachardinidae—appear to be more prominent in terms of lac secretion (ASB 2008).

Of the many species of lac insect, the one that is commercially cultured is Kerria lacca, which is also known by such synonyms as Laccifer lacca, Tachardia lacca, and Carteria lacca. Kerria lacca belongs to the lac insect family Kerriidae, one of some 28 families of scale insects and mealy bugs comprising a large group of about 8000 described species of plant sucking insects, a few of which produce similar natural products (e.g., cochineal and crimson). Thousands of these tiny insects colonize branches of suitable host trees and secrete the resinous pigment.

As a member of the Homoptera order ("true bugs"), Kerria lacca is hemimetabolous. In other words, members of this species go through an incomplete metamorphosis or simple metamorphosis whereby the larvae resemble the adults somewhat, and their life cycle involves the three distinct stages of egg, nymph, and the adult stage, or imago—that is, there is no pupal stage characteristic of insects undergoing complete metamorphosis.

In Kerra lacca, the insect starts as a nymph that is only about 0.6 millimeters long (ASB 2008). It settles on a host plant gregariously and there may be on average 150 such larva per square inch of twig (ASB 2008). The insects project protrusions into the tree, penetrating the bark, and suck up the sap, which is chemically altered in the insects' bodies (Bryk 2002). When exuded onto the tree branch, this excretion forms a hard covering. Larva began secreting this lac after a day or two of settlement. As the insects are in close proximity, the lac secretions from adjacent cells coalesce with each other and forms a shell-like covering over the entire swarm (ASB 2008; Bryk 2002).

After the first molt, the male and female larvae lose their legs, antennae, and eyes, and after the third molt, the mouthparts in the male larvae become atrophied, they stop feeding, and they fertilze with the female (ASB 2008). The females lac output increases greatly after fertilization (Bryk 2002). The female may lay 1000 eggs before dying; after hatching, the new larva break through the crust and swarm out (Bryk 2002).

Kerria lacca is primarily cultured in India and Bangladesh on the plants Zizyphus mauritiana and Z. jujuba (ASB 2008). It is also cultivated in India on Dhak (Butea monosperma) and Kusum (Schleichera oleosa); in Thailand on the rain tree (Samanea saman) and the pigeon pea (Cajanus cajan); in China on the pigeon pea and Hibiscus species; and in Mexico on the Barbados nut (Jatropha curcas).

Production

The coated branches of the host trees are cut and harvested as sticklac. Workers cut these encrusted branches and they are sent to places to be refined, either by hand or mechanically (Bryk 2002). Essentially, the harvested sticklac is removed from the branches and crushed and sieved to remove impurities. The sieved material is then repeatedly washed to remove insect parts and other soluble material. The resulting product is known as seedlac. The prefix seed refers to its pellet shape, which resembles seeds. Seedlac is the raw material to produce shellac, which is made from the seedlac by hand or mechanical equipment, the later usually using a heat-or-solvent-based process (Bryk 2002).

The natural coloration of lac residue is greatly influenced by the sap consumed by the lac insect and the season of the harvest. Generally in the trade of seedlac there are two distinct colors: the orange Bysacki and the blonde Kushmi. When purified, the chemical takes the form of golden yellow/ golden brown flakes. Bleaching may be used to remove the residual red color and give a clear shellac. Otherwise, the shellac remains orangish after the refining process.

Shellac is offered for sale in the flake form or may be suspended in denatured alcohol, with the later more popular with the consumer (Bryk 2002).

Shellac refined for industrial purposes either retains its natural wax content or is refined wax-free by filtration.

Lac production is found in Assam and other parts of North-East India, Bangladesh, Myanmar, Thailand, Laos, Vietnam and parts of China. It is also found in Mexico.

Lac was originally cultivated for a purple-red dye derived from soaking the lac bug secretions in water, with this dye used to color silk, leather, and cosmetics (Bryk 2002). In the 19th century, it began to be cultivated as a wood sealer and finisher, with the solubility in alcohol making it particularly popular, since this yielded a quick-drying, tough finish (Bryk 2002).

Properties

A decorative medal made in France in early 20th century molded from shellac compound, the same used for phonograph records of the period.

Shellac is a natural polymer and is chemically similar to synthetic polymers; thus, it is considered a natural plastic. It can be turned into a molding compound when mixed with woodflour and molded under heat and pressure methods, so it is classified as thermoplastic. But old moldings tend to become thermoset, that is, they suffer chemical reactions over time and are no longer fusible.

Shellac is soluble in alkaline solutions such as ammonia, sodium borate, sodium carbonate, and sodium hydroxide, and also in various organic solvents. When dissolved in alcohol—typically blends containing ethanol and methanol—shellac yields a coating of superior durability and hardness and is available in numerous grades.

Uses

Shellac is used in the traditional "French polish" method of finishing furniture, and fine viols and guitars. Because it is compatible with most other finishes, shellac also is used as a barrier or primer coat on wood to prevent the bleeding of resin or pigments into the final finish, or to prevent wood stain from blotching. Lightly tinted shellac preparations are also sold as paint primer. Shellac is best suited to this application because, although its durability against abrasives and many common solvents is not very good, it provides an excellent barrier against water vapor penetration. Shellac based primers are thus an effective sealant to control odors associated with fire damage.

In the late nineteenth and early twentieth centuries, shellac was particularly popular in the United States when houses were being built rapidly, given that shellac was an ideal wood finisher with its quick drying (Bryk 2002)

Shellac was used from mid-19th century to produce small molded goods like picture frames, boxes, toilet articles, jewelery, inkwells, and even dental plates. Until the advent of Vinyl in 1938, phonograph records were pressed from shellac compounds. This use was common until the 1950s, and continued into the 1970s in some non-Western countries. Sheets of Braille were coated with shellac to help protect them from wear due to being read by hand. Shellac is used as a binder in Indian ink.

Shellac is edible and it is used as a glazing agent on pills and candies. Because of its alkaline properties, shellac-coated pills may be used for a timed enteric or colonic release. It is also used to replace the natural wax of the apple, which is removed during the cleaning process. When used for this purpose, it has the food additive E number E904. This coating may not be considered as vegetarian as it may, and probably does, contain crushed insects. In the tablet manufacture trade, it is sometimes referred to as "beetlejuice" for this reason. In some cases, shellac is known to cause allergies on contact, resulting in skin irritations (Le Coz et al. 2000).

Although advancement in plastics have rendered shellac obsolete as a molding compound, it remains popular for a number of other uses. In dental technology, it is still occasionally used in the production of custom impression trays and (partial) denture production. It is used by many cyclists as a protective and decorative coating for their handlebar tape (OYB 2005). Shellac is used as a hard-drying adhesive for tubular cycle tires, particularly for track racing (Brandt 2001).

For some applications a corn protein called zein is a competitive substitute.

References

ISBN links support NWE through referral fees

• Asiatic Society of Bangladesh (ASB). 2008. Lac insect. Banglapedia. Retrieved May 10, 2008.

• Brandt, J. 2001. Mounting tubular tires. Sheldonbrown.com. Retrieved May 10, 2008.

• Bryk, N. E. V. 2002. How is shellac made? In D. S. Blanchfield, ed., How Products are Made: An Illustrated Guide to Product Manufacturing, Volume 7. Detroit: Gale Group. ISBN 0787636436.

• Le Coz, C.-J., J. M. Leclere, E. Arnoult, N. Raison-Peyron, A. Pons-Guiraud, and M. Vigan. 2000. Allergic contact dermatitis from shellac in mascara. Contact Dermatitis 46(3): 149-152.

• Out Your Backdoor (OYB). 2005. Shellac & twine makes handlebar fine. Out Your Backdoor August 21, 2005. Retrieved May 10, 2008.

External links

• DIYinfo.org's Shellac Wiki - Practical information on everything to do with shellac. Retrieved May 10, 2008.