Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes (pictured at right), which are dissolved in ethyl alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish. Shellac functions as a tough all-natural primer, sanding sealant, tannin-blocker, odour-blocker, stain, and high-gloss varnish. Shellac was once used in electrical applications as it possesses good insulation qualities and it seals out moisture. Phonograph (gramophone) records were also made of it during the pre-1950s, 78-rpm recording era.

Shellac is one of the few historically appropriate finishes (including casein paint, spar varnishes, boiled linseed oil and lacquer) for early 20th-century hardwood floors, and wooden wall and ceiling paneling.

From the time it replaced oil and wax finishes in the 19th century, shellac was one of the dominant wood finishes in the western world until it was replaced by nitrocellulose lacquer in the 1920s and 1930s.

Production

Shellac is scraped from the bark of the trees where the female lac bug, ''Kerria lacca'', Order'' Hemiptera'', Family ''Coccidae'' secretes it to form a tunnel-like tube as it traverses the branches of tree. Though these tunnels are sometimes referred to as "cocoons", they are not literally cocoons in the entomological sense. This insect is in the same family as the insect from which cochineal is obtained. The insects suck the sap of the tree and excrete "stick-lac" almost constantly. The least coloured shellac is produced when the insects feed on the kursum tree, (''Schleichera trijuga''). It takes about 100,000 lac bugs to make 500 g of shellac flakes. The raw shellac, which contains bark shavings and lac bug parts, is placed in canvas tubes (much like long socks) and heated over a fire. This causes the shellac to liquefy, and it seeps out of the canvas, leaving the bark and bug parts behind. The thick, sticky shellac is then dried into a flat sheet and broken into flakes, or dried into "buttons" (pucks/cakes), then bagged and sold. The end-user then crushes it into a fine powder, mixes it with ethyl alcohol prior to use to dissolve the flakes and make liquid shellac.

Liquid shellac has a limited shelf life (about 1 year), hence it is sold in dry form for dissolution prior to use. Liquid shellac sold in hardware stores is clearly marked with the production (mixing) date, so the consumer can know whether the shellac inside is still good. Alternatively, old shellac may be tested to see if it is still usable: a few drops on glass should quickly dry to a hard surface. Shellac that remains tacky for a long time is no longer usable. Storage life depends on peak temperature, so refrigeration extends shelf life.

The thickness (strength) of shellac is measured by the unit "pound cut", referring to the amount (in pounds) of shellac flakes dissolved in a gallon of denatured alcohol. For example: a 1-lb. cut (said as "one pound cut") of shellac is the strength obtained by dissolving one pound of shellac flakes in a gallon of alcohol. A 5-lb. cut is the strength of five pounds of shellac flakes dissolved in a gallon of alcohol. Most pre-mixed commercial preparations come at a 3-lb. cut. Multiple thin layers of shellac produce a significantly better end result than a few thick layers. Thick layers of shellac do not adhere to the substrate or to each other well, and thus can peel off with relative ease; in addition, thick shellac will obscure fine details in carved designs in wood and other substrates.

Shellac naturally dries to a high-gloss sheen. For applications where a flatter (less shiny) sheen is desired, products containing amorphous silica, such as "Shellac Flat," may be added to the dissolved shellac.

Shellac naturally contains a small amount of wax (3%-5% by volume), which comes from the lac bug. In some preparations, this wax is removed (the resulting product being called "dewaxed shellac"). This is done for applications where the shellac will be coated with something else (such as paint or varnish), so the topcoat will adhere. Waxy (non-dewaxed) shellac appears milky in liquid form, but dries clear.

Colors and availability

Shellac comes in many warm colors, ranging from a very light blond ("platina") to a very dark brown ("garnet"), with all shades of brown, yellow, orange and red in between. The colour is influenced by the sap of the tree the lac bug is living on, as well as the time of harvest. Historically, the most commonly-sold shellac is called "orange shellac", and was used extensively as a combination stain and protectant for wood paneling and cabinetry in the 20th century.

Shellac was once very common any place paints or varnishes were sold (such as hardware stores). However, cheaper and more abrasion - and chemical-resistant finishes, such as polyurethane, have almost completely replaced it in decorative residential wood finishing such as hardwood floors, wooden wainscoting plank paneling, and kitchen cabinets. These alternative products, however, must be applied over a stain if the user wants the wood coloured; clear or blond shellac may applied over a stain without affecting the tint of the finished piece, as a protective topcoat. "Wax over shellac" (an application of buffed-on paste wax over several coats of shellac) is often regarded as a beautiful, if fragile finish for hardwood floors. Many luthiers still use shellac to ''French polish'' fine acoustic stringed instruments, but these purists are rare now, as shellac is replaced by synthetic plastic lacquers and varnishes in many workshops.

Properties

Shellac is a natural bioadhesive polymer and is chemically similar to synthetic polymers, and thus can be considered a natural form of plastic. It can be turned into a moulding compound when mixed with wood flour and moulded under heat and pressure methods, so it can also be classified as thermoplastic.

Shellac scratches more easily than most lacquers and varnishes, and application is more labor-intensive, which is why they have replaced shellac in most areas. But damaged shellac can easily be touched-up with another coat of shellac (unlike polyurethane) because the new coat merges with and bonds to the existing coat(s). Shellac is much softer than Urushi lacquer for instance, which is far superior in regards to both chemical and mechanical resistance.

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 blends containing ethanol or methanol, shellac yields a coating of good durability and hardness.

Upon mild hydrolysis shellac gives a complex mix of aliphatic and alicyclic hydroxy acids and their polymers that varies in exact composition depending upon the source of the shellac and the season of collection. The major component of the aliphatic component is aleuritic acid, whereas the main alicyclic component is shellolic acid. According to the Mahabharata, an entire palace was built out of dried shellac.

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.

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 that were made 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 use for shellac is sealing wax. Woods's ''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 described in the standard texts of the day.

Uses

In the early- and mid-20th century, orange shellac was used as a one-product finish (combination stain and varnish-like topcoat) on decorative wood paneling used on walls and ceilings in homes, particularly in America. In the American South, use of knotty pine plank paneling covered with orange shellac was once as common in new construction as drywall is today. It was also often used on kitchen cabinets and hardwood floors, prior to the advent of polyurethane.

It is the central element of the traditional "French polish" method of finishing furniture and fine viols, guitars and pianos.

Shellac was used from mid-19th century to produce small moulded goods like picture frames, boxes, toilet articles, jewelry, inkwells and even dentures. Although advances 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.

Shellac 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. It has always been the preferred hot-melt adhesive for fixing leather saxophone pads into their metal key-cups.

Until the advent of vinyl around the 1940s, most gramophone 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 India ink.

Shellac was historically used as a protective coating on paintings.

Shellac is edible and it is used as a glazing agent on pills (see excipients) and candies in the form of ''pharmaceutical glaze'' (or, ''confectioner's glaze''). 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 is not vegan and most likely not vegetarian either as it may, and probably does, contain crushed insects.

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.

Shellac is an odour and stain blocker and so is often used as the base of "solves all problems" primers. Although its durability against abrasives and many common solvents is not very good, shellac provides an excellent barrier against water vapour penetration. Shellac-based primers are an effective sealant to control odours associated with fire damage.

Shellac was once used for fixing inductor, motor, generator and transformer windings, where it was applied directly to single layer windings in an alcohol solution. For multilayer windings, the whole coil was submerged in shellac solution, then drained and placed in a warm place to allow the alcohol to evaporate. The shellac then locks the wire turns in place, provides extra insulation and prevents movement and vibration, reducing buzz and hum. In motors and generators it also helps transfer force generated by magnetic attraction and repulsion from the windings to the rotor or armature. In more recent times, synthetic resins, such as Glyptol, (Glyptal), have been substituted for the shellac. Some applications use shellac mixed with other natural or synthetic resins, such as pine resin or Phenol-Formaldehyde Resin, of which Bakelite is the best known, for electrical use. Mixed with other resins, Barium sulfate, Calcium Carbonate, Zinc Sulfide, Aluminum Oxide and/or Cuprous Carbonate, (Malachite), shellac forms a component of ''Heat Cured Capping Cement'' used to fasten the caps or bases to the bulbs of electric lamps.

Shellac is used to make felt hats more stiff and water-resistant.

As a natural resin, shellac has some similarities to other natural resins such as Myrrh and Frankincense, although frankincense is only partially soluble in alcohol .

Shellac finds a use in pyrotechnic compositions as a low temperature fuel where it allows the creation of pure 'greens' and 'blues', colours difficult to achieve with other fuel mixes in fireworks formulae.

Gallery

See also

Bakelite

Zein

References

Footnotes

Notations

Shellac Application By Smith & Rodger

External links

Shellac.net and Zinsser, shellac vendors in the United States

DIYinfo.org's Shellac Wiki, practical information on everything to do with shellac

Reactive Pyrolysis-Gas Chromatography of Shellac

Jeff Jewitt, shellac – a traditional finish still yields superb results

Category:Wood finishing materials Category:Food additives Category:Insect products Category:Polymers Category:Resins Category:Waxes Category:Excipients

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