Shellac

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Some of the many different colors of shellac Shellac varities.png
Some of the many different colors of shellac
Shellac in alcohol Shellac liquid.jpeg
Shellac in alcohol

Shellac (/ʃəˈlæk/) [1] 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 and dissolved in alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish. Shellac functions as a tough 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 and 78 rpm gramophone records were made of it until they were replaced by vinyl long-playing records from the 1950s onwards.

Contents

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 largely replaced by nitrocellulose lacquer in the 1920s and 1930s.

Etymology

Shellac comes from shell and lac , a calque of French laque en écailles, "lac in thin pieces", later gomme-laque, "gum lac". [2] Most European languages (except Romance ones and Greek) have borrowed the word for the substance from English or from the German equivalent Schellack’’.[ citation needed ]

Production

Lac tubes created by Kerria lacca Kerria-lacca.jpg
Lac tubes created by Kerria lacca
Drawing of the insect Kerria lacca and its shellac tubes, by Harold Maxwell-Lefroy, 1909 02-Indian-Insect-Life - Harold Maxwell-Lefroy - Kerria-Lacca.jpg
Drawing of the insect Kerria lacca and its shellac tubes, by Harold Maxwell-Lefroy, 1909

Shellac is scraped from the bark of the trees where the female lac bug, Kerria lacca (order Hemiptera, family Kerriidae, also known as Laccifer lacca), secretes it to form a tunnel-like tube as it traverses the branches of the tree. Though these tunnels are sometimes referred to as "cocoons", they are not cocoons in the entomological sense. This insect is in the same superfamily as the insect from which cochineal is obtained. The insects suck the sap of the tree and excrete "sticklac" almost constantly. The least-coloured shellac is produced when the insects feed on the kusum tree ( Schleichera ).[ citation needed ]

The number of lac bugs required to produce 1 kilogram (2.2 lb) of shellac has variously been estimated as 50,000, [3] 200,000, [4] or 300,000. [5] [6] The root word lakh is a unit in Indian numbering system for 100,000 and presumably refers to the huge numbers of insects that swarm on host trees, up to 150 per square inch (23/cm2). [7]

The raw shellac, which contains bark shavings and lac bugs removed during scraping, 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 bugs 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 and mixes it with ethyl alcohol before use, to dissolve the flakes and make liquid shellac.[ citation needed ]

Liquid shellac has a limited shelf life (about 1 year), so is sold in dry form for dissolution before use. Liquid shellac sold in hardware stores is often marked with the production (mixing) date, so the consumer can know whether the shellac inside is still good. Some manufacturers (e.g., Zinsser) have ceased labeling shellac with the production date, but the production date may be discernible from the production lot code. Alternatively, old shellac may be tested to see if it is still usable: a few drops on glass should dry to a hard surface in roughly 15 minutes. Shellac that remains tacky for a long time is no longer usable. Storage life depends on peak temperature, so refrigeration extends shelf life.[ citation needed ]

The thickness (concentration) 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 of shellac is the strength obtained by dissolving one pound of shellac flakes in a gallon of alcohol (equivalent to 120 grams per litre). [8] 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.[ citation needed ]

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. [9]

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.[ citation needed ]

Colours and availability

Shellac comes in many warm colours, ranging from a very light blonde ("platina") to a very dark brown ("garnet"), with many varieties of brown, yellow, orange and red in between. The colour is influenced by the sap of the tree the lac bug is living on and by 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 panelling and cabinetry in the 20th century.[ citation needed ]

Shellac was once very common anywhere 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 panelling, and kitchen cabinets. These alternative products, however, must be applied over a stain if the user wants the wood to be coloured; clear or blonde shellac may be applied over a stain without affecting the colour 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. Luthiers still use shellac to French polish fine acoustic stringed instruments, but it has been replaced by synthetic plastic lacquers and varnishes in many workshops, especially high-volume production environments. [10]

Shellac dissolved in alcohol, typically more dilute than French-Polish, is now commonly sold as "sanding sealer" by several companies. It is used to seal wooden surfaces, often as preparation for a final more durable finish; it reduces the amount of final coating required by reducing its absorption into the wood.[ citation needed ]

Properties

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

Shellac is a natural bioadhesive polymer and is chemically similar to synthetic polymers. [11] It can thus can be considered a natural form of plastic.[ citation needed ]

With a melting point of 75 °C (167 °F), it can be classed as a thermoplastic; [12] used to bind wood flour, the mixture can be moulded with heat and pressure.[ citation needed ]

Shellac scratches more easily than most lacquers and varnishes, and application is more labour-intensive, which is why it has been replaced by plastic in most areas. But damaged shellac can easily be touched up with another coat of shellac (unlike polyurethane, which chemically cures to a solid) 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 with regard to both chemical and mechanical resistance.[ citation needed ]

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 denatured ethanol) for application, shellac yields a coating of good durability and hardness.[ citation needed ]

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. [13]

Shellac is UV-resistant, and does not darken as it ages (though the wood under it may do so, as in the case of pine). [4]

History

The earliest written evidence of shellac goes back 3,000 years, but shellac is known to have been used earlier. [4] According to the ancient Indian epic poem, the Mahabharata, an entire palace was built out of dried shellac. [4]

Shellac was in rare use as a dyestuff for as long as there was a trade with the East Indies. Merrifield [14] cites 1220 for the introduction of shellac as an artist's pigment in Spain. Lapis lazuli, an ultramarine pigment from Afghanistan, was already being imported long before this.[ citation needed ]

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 using infrared spectroscopy to identify shellac coating on a 16th-century cassone. [15] This is also the period in history where "varnisher" was identified as a distinct trade, separate from both carpenter and artist.[ citation needed ]

Another use for shellac is sealing wax. Woods's The Nature and Treatment of Wax and Shellac Seals [16] discusses the various formulations, and the period when shellac started to be added to the previous beeswax recipes.[ citation needed ]

The "period of widespread introduction" would seem to be around 1550 to 1650, when the substance moved from being a rarity on highly decorated pieces to being described in the standard texts of the day.[ citation needed ]

Uses

Historical

In the early- and mid-twentieth century, orange shellac was used as a one-product finish (combination stain and varnish-like topcoat) on decorative wood panelling used on walls and ceilings in homes, particularly in the US. In the American South, use of knotty pine plank panelling 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.[ citation needed ]

Until the advent of vinyl, most gramophone records were pressed from shellac compounds. [17] [18] From 1921 to 1928, 18,000 tons of shellac were used to create 260 million records for Europe. [7] In the 1930s, it was estimated that half of all shellac was used for gramophone records. [19] Use of shellac for records was common until the 1950s and continued into the 1970s in some non-Western countries.[ citation needed ]

Until recent advances in technology, shellac (French polish) was the only glue used in the making of ballet dancers' pointe shoes, to stiffen the box (toe area) to support the dancer en pointe. Many manufacturers of pointe shoes still use the traditional techniques, and many dancers use shellac to revive a softening pair of shoes. [20]

Shellac was historically used as a protective coating on paintings.[ citation needed ]

Sheets of Braille were coated with shellac to help protect them from wear due to being read by hand.[ citation needed ]

Shellac was used from the mid-nineteenth century to produce small moulded goods such as picture frames, boxes, toilet articles, jewelry, inkwells and even dentures. Advances in plastics have rendered shellac obsolete as a moulding compound.[ citation needed ]

Shellac (both orange and white varieties) was used both in the field and laboratory to glue and stabilise dinosaur bones until about the mid-1960s. While effective at the time, the long-term negative effects of shellac (being organic in nature) on dinosaur bones and other fossils is debated, and shellac is very rarely used by professional conservators and fossil preparators today.[ citation needed ]

Shellac was used for fixing inductor, motor, generator and transformer windings. It was applied directly to single-layer windings in an alcohol solution. For multi-layer 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 locked the wire turns in place, provided extra insulation, prevented movement and vibration and reduced 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, shellac has been replaced in these applications by synthetic resins such as polyester resin. 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, aluminium 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.[ citation needed ]

Current

It is the central element of the traditional "French polish" method of finishing furniture, fine string instruments, and pianos.[ citation needed ]

Shellac, edible, is used as a glazing agent on pills (see excipient) and sweets, in the form of pharmaceutical glaze (or, "confectioner's glaze"). Because of its acidic properties (resisting stomach acids), shellac-coated pills may be used for a timed enteric or colonic release. [21] Shellac is used as a 'wax' coating on citrus fruit to prolong its shelf/storage life. It is also used to replace the natural wax of the apple, which is removed during the cleaning process. [22] When used for this purpose, it has the food additive E number E904.[ citation needed ]

Shellac coating applied with either a standard or modified Huon-Stuehrer nozzle, can be economically micro-sprayed onto various smooth sweets, such as chocolate coated peanuts. Irregularities on the surface of the product being sprayed typically result in the formation of unsightly aggregates ("lac-aggs") which precludes the use of this technique on foods such as walnuts or raisins (however, chocolate-coated raisins being smooth surfaced, are able to be sprayed successfully using a modified Huon-Stuehrer nozzle).[ citation needed ]

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.[ citation needed ]

Shellac has traditionally been used as a dye for cotton and, especially, silk cloth in Thailand, particularly in the north-eastern region. [23] It yields a range of warm colours from pale yellow through to dark orange-reds and dark ochre. [24] Naturally dyed silk cloth, including that using shellac, is widely available in the rural northeast, especially in Ban Khwao District, Chaiyaphum province. The Thai name for the insect and the substance is "khrang" (Thai: ครั่ง).[ citation needed ]

Wood finish

Wood finishing is one of the most traditional and still popular uses of shellac mixed with solvents or alcohol. This dissolved shellac liquid, applied to a piece of wood, is an evaporative finish: the alcohol of the shellac mixture evaporates, leaving behind a protective film. [25]

Shellac as wood finish is natural and non-toxic in its pure form. A finish made of shellac is UV-resistant. For water-resistance and durability, it does not keep up with synthetic finishing products. [26]

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. [27]

Other

Shellac is used:

  • in the tying of artificial flies for trout and salmon, where the shellac was used to seal all trimmed materials at the head of the fly.[ citation needed ]
  • in combination with wax for preserving and imparting a shine to citrus fruits, such as lemons.[ citation needed ]
  • in dental technology, where it is occasionally used in the production of custom impression trays and (partial) denture production.[ citation needed ]
  • as a binder in India ink.[ citation needed ]
  • for cycling, as a protective and decorative coating for bicycle handlebar tape, [28] and as a hard-drying adhesive for tubular cycle tyres, particularly for track racing. [29]
  • for re-attaching ink sacs when restoring vintage fountain pens, the orange variety preferably.[ citation needed ]
  • for fixing pads to the key-cups of woodwind instruments.[ citation needed ]
  • for Luthier applications, to bind wood fibres down and prevent tear out on the soft spruce soundboards.[ citation needed ]
  • to stiffen and impart water-resistance to felt hats, for wood finishing [30] and as a constituent of gossamer (or goss for short), a cheesecloth fabric coated in shellac and ammonia solution used in the shell of traditional silk top and riding hats.
  • for mounting insects, in the form of a gel adhesive mixture composed of 75% ethyl alcohol. [31]
  • as a binder in the fabrication of abrasive wheels, [32] imparting flexibility and smoothness not found in vitrified (ceramic bond) wheels. 'Elastic' bonded wheels typically contain plaster of paris, yielding a stronger bond when mixed with shellac; the mixture of dry plaster powder, abrasive (e.g. corundum/aluminium oxide Al2O3), and shellac are heated and the mixture pressed in a mould.
  • in fireworks 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.[ citation needed ]
  • in watchmaking, due to its low melting temperature (about 80–100 °C (176–212 °F)), shellac is used in most mechanical movements to adjust and adhere pallet stones to the pallet fork and secure the roller jewel to the roller table of the balance wheel. Also for securing small parts to a 'wax chuck' ( faceplate ) in a watchmakers' lathe.[ citation needed ]
  • in the early twentieth century, it was used to protect some military rifle stocks. [33]
  • in Jelly Belly jelly beans, in combination with beeswax to give them their final buff and polish. [34]
  • in modern traditional archery, shellac is one of the hot-melt glue/resin products used to attach arrowheads to wooden or bamboo arrow shafts.[ citation needed ]
  • Sanding sealer, is a solution of shellac dissolved in alcohol widely sold to seal sanded surfaces, typically wooden surfaces before a final coat of a more durable finish. Similar to French Polish but more dilute.[ citation needed ]
  • as a topcoat in nail polish (although not all nail polish sold as "shellac" contains shellac, and some nail polish not labelled in this way does)[ citation needed ]
  • in sculpture, to seal plaster and in conjunction with wax or oilsoaps, to act as a barrier during mold-making processes[ citation needed ]
  • as a dilute solution in the sealing of harpsichord soundboards, protecting them from dust and buffering humidity changes while maintaining a bare-wood appearance.[ citation needed ]

Related Research Articles

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Lac resinous secretion of lac insects

Lac is the resinous secretion of a number of species of lac insects, of which the most commonly cultivated is Kerria lacca.

Wax

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Resin Solid or highly viscous substance

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Varnish Transparent hard protective finish or film

Varnish is a clear transparent hard protective finish or film. It is neither a paint nor stain. In its native state it has little or no color, but may be pigmented as desired, and is sold commercially in various shades.

Lacquer

The term lacquer is used for a number of hard and potentially shiny finishes applied to materials such as wood or metal. These fall into a number of very different groups.

Nail polish

Nail polish is a lacquer that can be applied to the human fingernail or toenails to decorate and protect the nail plates. The formula has been revised repeatedly to enhance its decorative effects and to suppress cracking or peeling. Nail polish consists of a mix of an organic polymer and several other components that give it its unique color and texture. Nail polishes come in all shades of color and play a significant part in manicures or pedicures.

Carnauba wax Natural plant wax from leaves of the carnauba palm

Carnauba, also called Brazil wax and palm wax, is a wax of the leaves of the carnauba palm Copernicia prunifera, a plant native to and grown only in the northeastern Brazilian states of Piauí, Ceará, Maranhão, Bahia, and Rio Grande do Norte. It is known as "queen of waxes" and in its pure state, usually comes in the form of hard yellow-brown flakes. It is obtained from the leaves of the carnauba palm by collecting and drying them, beating them to loosen the wax, then refining and bleaching the wax. As a food additive, its E number is E903.

Particle board

Particle board, also known as chipboard, is an engineered wood product manufactured from wood chips or jute-stick chips and a synthetic resin or other suitable binder, which is pressed and extruded. Particle board is often confused with oriented strand board, a different type of fiberboard that uses machined wood flakes and offers more strength.

Wood stain

Wood stain is a type of paint used to colour wood.

Primer (paint)

A primer or undercoat is a preparatory coating put on materials before painting. Priming ensures better adhesion of paint to the surface, increases paint durability, and provides additional protection for the material being painted.

Pharmaceutical glaze is an alcohol-based solution of various types of food-grade shellac. The shellac is derived from the raw material sticklac, which is a resin scraped from the branches of trees left from when the small insect, Kerria lacca, creates a hard, waterproof cocoon. When used in food and confections, it is also known as confectioner's glaze, resinous glaze, pure food glaze, natural glaze, or confectioner's resin.

French polish

French polishing is a wood finishing technique that results in a very high gloss surface, with a deep colour and chatoyancy. French polishing consists of applying many thin coats of shellac dissolved in denatured alcohol using a rubbing pad lubricated with one of a variety of oils. The rubbing pad is made of absorbent cotton or wool cloth wadding inside of a piece of fabric and is commonly referred to as a fad, also called a rubber, tampon, or muñeca.

Wood finishing

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Kerriidae

Kerriidae is a family of scale insects, commonly known as lac insects or lac scales. Some members of the genera Metatachardia, Tachardiella, Austrotacharidia, Afrotachardina, Tachardina, and Kerria are raised for commercial purposes, though the most commonly cultivated species is Kerria lacca. These insects secrete a waxy resin that is harvested and converted commercially into lac and shellac, used in various dyes, cosmetics, food glazes, wood finishing varnishes and polishes.

Tung oil

Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree. Tung oil hardens upon exposure to air, and the resulting coating is transparent and has a deep, almost wet look. Used mostly for finishing and protecting wood, after numerous coats, the finish can even look plastic-like. Related drying oils include linseed, safflower, poppy, and soybean oils. The oil and its use are believed to have originated in ancient China and appear in the writings of Confucius from about 400 BC. Raw tung oil tends to dry to a fine, wrinkled finish; the English name for this is gas checking; this property was used to make wrinkle finishes, usually by adding excess cobalt drier. To stop this, the oil is heated to gas-proof it, and most oils used for coating are gas-proofed, also known as "boiled".

Conservation and restoration of wooden furniture

The conservation and restoration of wooden furniture is an activity dedicated to the preservation and protection of wooden furniture objects of historical and personal value. When applied to cultural heritage this activity is generally undertaken by a conservator-restorer. Furniture conservation and restoration can be divided into two general areas: structure and finish. Structure generally relates to wood and can be divided into solid, joined, and veneered wood. The finish of furniture can be painted or transparent.

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  10. French polishing tutorial for guitars
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