Everything about Shellac totally explained
Shellac is the commercial resin marketed in the form of amber flakes, made from
lac, the secretion of the familiy 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's 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's 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 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 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's 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 isn't 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 the dynastic shenanigans of marriage. The definition of varnish isn't 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 (rather obvious) 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's 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's 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's 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 tyres, 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.
Further Information
Get more info on 'Shellac'.
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