Shellac Information
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 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.
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.
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Production
Shellac is scraped from the bark of the trees where the female lac bug, Kerria lacca (Order Hemiptera, Family Coccidae[1]) 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.[2] 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 grams of shellac flakes.[3] 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 (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. 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,[4] 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 in 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. [5] [6]
Properties
A decorative medal made in France in 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, 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.[1]
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).[3]
History
The earliest record of shellac goes back 3000 years, but shellac is known to have been used earlier.[3] According to the Mahabharata, an entire palace was built out of dried shellac.[3]
Shellac was in rare use as a dyestuff for as long as there was a trade with the East Indies. Merrifield[7] cites 1220 for the introduction of shellac as an artist's pigment in Spain. Lapis lazuli a
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