Shellac
Shellac (/ʃəˈlæk/)[1] is a resin secreted by the female lac bug on trees in the forests of India and Thailand. Chemically, it is mainly composed of aleuritic acid, jalaric acid, shellolic acid, and other natural waxes.[2] 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 seals out moisture. Phonograph and 78 rpm gramophone records were made of shellac until they were replaced by vinyl long-playing records from 1948 onwards.[3]
For the rock band, see Shellac (band).From the time shellac replaced oil and wax finishes in the 19th century, it was one of the dominant wood finishes in the western world until it was largely replaced by nitrocellulose lacquer in the 1920s and 1930s. Besides wood finishing, shellac is used as an ingredient in food, medication and candy such as confectioner's glaze,[4] as well as a means of preserving harvested citrus fruit.[5]
Etymology[edit]
Shellac comes from shell and lac, a partial calque of French laque en écailles, 'lac in thin pieces', later gomme-laque, 'gum lac'.[6] Most European languages (except Romance ones and Greek) have borrowed the word for the substance from English or from the German equivalent Schellack.[7]
Colours and availability[edit]
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.
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.[16]
Shellac dissolved in alcohol, typically more dilute than as used in 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.
History[edit]
The earliest written evidence of shellac goes back 3,000 years, but shellac is known to have been used earlier.[19] According to the ancient Indian epic poem, the Mahabharata, an entire palace was built out of dried shellac.[19]
Shellac was uncommonly used as a dyestuff for as long as there was a trade with the East Indies. According to Merrifield,[20] shellac was first used as a binding agent in artist's pigments in Spain in the year 1220.
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.[21] 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.[22] The widespread use of shellac seals in Europe dates back to the 17th century, thanks to the increasing trade with India.[23]
Uses[edit]
Historical[edit]
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.
Until the advent of vinyl, most gramophone records were pressed from shellac compounds.[24][25] From 1921 to 1928, 18,000 tons of shellac were used to create 260 million records for Europe.[12] In the 1930s, it was estimated that half of all shellac was used for gramophone records.[26] Use of shellac for records was common until the 1950s and continued into the 1970s in some non-Western countries, as well as for some children's records.[27][28]
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.[29]
Shellac was historically used as a protective coating on paintings.
Sheets of Braille were coated with shellac to help protect them from wear due to being read by hand.
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.[30]
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.[31]
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 location 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.
Current uses[edit]
It is the central element of the traditional "French polish" method of finishing furniture, fine string instruments, and pianos.[32]
Shellac, being 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.[33] 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.[34] When used for this purpose, it has the food additive E number E904.[35]
Shellac is an odour and stain blocker and so is often used as the base of "all-purpose" 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.[36]
Shellac has traditionally been used as a dye for cotton and, especially, silk cloth in Thailand, particularly in the north-eastern region.[37] It yields a range of warm colours from pale yellow through to dark orange-reds and dark ochre.[38] 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).
Blonde shellac flakes
Dewaxed Bona (L) and Waxy #1 Orange (R) shellac flakes. The latter—orange shellac—is the traditional shellac used for decades to finish wooden wall paneling, kitchen cabinets and tool handles.
Closeup of Waxy #1 Orange (L) and Dewaxed Bona (R) shellac flakes. The former—orange shellac—is the traditional shellac used for centuries to finish wooden wall paneling and kitchen cabinets.
"Quick and dirty" example of a pine board coated with 1–5 coats of Dewaxed Dark shellac (a darker version of traditional orange shellac)
