Definitions

shellac

shellac

[shuh-lak]
shellac, solution of lac in alcohol or acetone. In commerce the name is applied to the resinous substance (lac) itself rather than to the solution. It ranges in color from orange to light yellow depending upon the extent to which it has been purified; the darker shellacs are the less pure. When bleached it is known as white shellac. Applied to surfaces such as wood and plaster, the solution forms a hard coating upon evaporation of the solvent. Shellac is widely used as a spirit varnish, as a protective covering for drawings and plaster casts, for stiffening in the manufacture of felt hats, in making sealing wax, and in electrical insulation.

Shellac is the commercial resin marketed in the form of amber flakes, made from lac, the secretion of the family of lac-producing insects, though most commonly from the cultivated Kerria lacca , found in the forests of Assam and Thailand.

Origin

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 is 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 golden yellow/ golden brown flakes, 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 turned into a moulding compound when mixed with woodflour and moulded under heat and pressure methods, so it is classified as thermoplastic. But old mouldings tend to become thermoset; that is, they suffer chemical reactions over time and are no longer fusible.

Properties

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. It is used in the traditional "French polish" method of finishing furniture, and fine viols and guitars. Shellac refined for industrial purposes either retains its natural wax content or is refined wax-free by filtration. 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. 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.

Historical introduction to the West

Shellac was in rare use as a dyestuff for as long as there was a trade with the East Indies. Merrifield cites 1220 for the introduction of shellac as an artist's pigment in Spain. This isn't unreasonable, given that lapis lazuli as ultramarine pigment from Afghanistan was already being imported long before this.

In areas where small caskets or reliquaries were decorated, then a significant number of them were protected with shellac, and from an early period. Painting was done with egg tempera over gesso. Shellac was also used as an adhesive and sealer over inlay work, such as ivory or abalone inlay.

The use of overall paint or varnish decoration on large pieces of furniture was first popularised in Venice (then later throughout Italy). There are a number of 13th century references to painted or varnished cassone, often dowry cassone which were deliberately impressive as part of dynastic marriages. The definition of varnish is not always clear, but it seems to have been a spirit varnish based on gum benjamin or mastic, both traded around the Mediterranean. At some time, shellac began to be used as well. An article from the Journal of the American Institute of Conservation describes the use of infrared spectroscopy to identify a shellac coating on a 16th century cassone. This is also the period in history where "varnisher" was identified as a distinct trade, separate from both carpenter and artist.

Another consumer of shellac is sealing wax. Woods' ‘The Nature and Treatment of Wax and Shellac Seals’ discusses the various formulations, and the period when shellac started to be added to the previous beeswax recipes.

The "period of widespread introduction" would seem to be around 1550 to 1650, when it moves from being a rarity on highly decorated pieces to being a substance that's described in the standard texts of the day.

Products

Shellac was used from mid-19th century to produce small moulded goods like picture frames, boxes, toilet articles, jewellery, 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.

Although advancement in plastics have rendered shellac obsolete as a moulding 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. Shellac is used as a hard-drying adhesive for tubular cycle tires, particularly for track racing Orange shellac is also the preferred adhesive for reattaching ink sacs when restoring vintage fountain pens.

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

See also

References

External links

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