Shellac

From Encyclopedia Jr, free information reference for Kids

See Shellac (band) for the post-punk band.

Shellac is a brittle or flaky secretion of the lac insect Coccus lacca, found in the forests of Assam and Thailand. Freed from wood it is called "seedlac." Once it was commonly believed that shellac was a resin obtained from the wings of an insect (order Hemiptera) found in India. In actuality, shellac was obtained from the secretion of the female insect, harvested from the bark of the trees where she deposits it to provide a sticky hold on the trunk. There is a risk that the harvesting process can scoop the insect up along with the secretion, leading to its death. 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 yellow/ brown pellets, this possibly providing the basis for the "Wing Source Story." Shellac is a natural polymer and is chemically similar to synthetic polymers, thus it is considered a natural plastic. It can be molded by heat and pressure methods, so it is classified as thermoplastic.

A 19th century picture frame with a shellac finish.

It 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. It is used in the traditional "French polish" method of finishing furniture, and fine viols and guitars. Shellac is also used as a finish for certain former Soviet Bloc small arms' wood stocks, such as the stock of the AK-47. Shellac refined for industrial purposes either retains its natural wax content or is refined wax-free by filteration. Orange shellac is bleached with sodium hypochlorite solution to form white shellac and also is produced in wax-containing and wax-free form. Because it is compatible with most other finishes, shellac is also 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.

It was used beginning in the mid-19th century to produce small goods like picture frames, boxes, toilet articles, jewellery, inkwells and even dental plates.

Shellac is edible, and it was used as a glazing agent on pills and candies. When used for this purpose, it has the food additive E number E904. There were concerns that this coating is not vegetarian as it may contain crushed bugs. In the tablet manufacture trade, it is sometimes referred to as "beetlejuice" for this reason. Shellac is known to cause allergies on contact, resulting in skin irritations.^[1]

Shellac is now considered obsolete as a moulding compound, but its use in industries continues as there are no synthetic or natural substitutes in a number of processes. For examples, it is used as an outer fruit coating to prevent post-harvest decay (e.g. apples); in dental technology it is used in the production of custom impression trays; it is used in the optical trade; and it was used in the production of gramophone (phonograph) records until about 1950 (see gramophone record for details). It is used by many cyclists as a protective and decorative coating for their handlebar tape.^{[citation needed]} Orange shellac is also the preferred adhesive for reattaching ink sacs when restoring vintage fountain pens.^[2]

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

[edit] See also

• Pharmaceutical glaze
• Zein

[edit] References

1. ^ Le Coz C-J., Leclere J-M., Arnoult E., Raison-Peyron N., Pons-Guiraud A., Vigan M., (2000). Allergic contact dermatitis from shellac in mascara. Contact Dermatitis 46(3), pp. 149-152(6)
2. ^ http://www.richardspens.com/?page=ref_info/glossary/S.htm