Difference between revisions of "Shellac"

Latest revision as of 09:11, 20 September 2010

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 denatured 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, odor-blocker, stain, and high-gloss varnish. Shellac was once used in electrical applications as it possesses good insulation qualities and it seals out moisture. Gramophone (or phonograph) discs were also made of it during the pre-1950s, 78-rpm recording era.

Shellac is often the only historically appropriate finish for early 20th-century hardwood floors, and wooden wall and ceiling paneling.

From the time it replaced oil and wax finishes in the 1800s, shellac was the dominant wood finish 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, Laccifer (Tachardia) lacca Kerr, Order Hemiptera, Family Coccidae^[1] deposits it to provide a sticky hold on the trunk. 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 are parasitic upon the kursum tree, (Schleichera trijuga). 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 up into flakes when dried, or dried into "buttons" (pucks/cakes), and then bagged and sold. The end-user then mixes it with denatured alcohol on-site a few days prior to use in order to dissolve the flakes and make liquid shellac.

Liquid shellac has a limited shelf life (about 1 year), hence it is also sold in dry form for dissolution prior to use. Liquid shellac sold in hardware stores is clearly marked with the production (mixing) date, so that 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.

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 be peeled off with relative ease; in addition, thick shellac will fill in (and thus ruin) 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,^[2] 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 that the topcoat will be able to stick. Waxy (non-dewaxed) shellac appears milky in liquid form, but dries clear.

Colours and availability

Shellac comes in many warm colours, ranging from a very light blond ("platina") to a very dark brown ("garnet"), with all shades of brown and yellow and 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 on wood paneling and cabinetry in the 20th century.

Shellac was once very common, being available any place paints or varnishes were sold (such as hardware stores). However, less expensive, more transparent, and more abrasion- and chemical-resistant finishes, such as polyurethane, have almost completely replaced it in the world of decorative residential wood finishing such as for hardwood floors, wooden wainscoting and plank paneling, and kitchen cabinets. These alternative products, however, must be applied over a stain if the user wants the wood coloured; shellac wasn't applied over a stain, as it was orange/amber in colour by itself, and so functioned as a combination stain and protective topcoat. "Wax over shellac" (an application of buffed-on paste wax over several coats of shellac) is often regarded as a beautiful finish for hardwood floors.^[3]

Properties

Shellac is a natural 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 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 and methanol, shellac yields a coating of superior 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]

History

The earliest record of shellac goes back 3000 years, but shellac is known to have been used earlier.^[4] According to some sacred Hindu texts, 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^[5] 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.^[6] This is also the period in history where "varnisher" was identified as a distinct trade, separate from both carpenter and artist.

Another consumer of shellac is sealing wax. Woods's The Nature and Treatment of Wax and Shellac Seals^[7] 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 this application, it lends an extremely warm, inviting, homely glow. 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 and guitars.

Shellac was used from mid-19th century to produce small moulded goods like picture frames, boxes, toilet articles, jewelry, inkwells and even dentures. 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 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.^[8] Shellac is used as a hard-drying adhesive for tubular cycle tires, particularly for track racing^[9]

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 (alternatively, confectioner's glaze). Because of its alkaline properties, shellac-coated pills may be used for a timed enteric or colonic release.^[10] It is also used to replace the natural wax of the apple, which is removed during the cleaning process.^[11] 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.^[12]

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 as an alcoholic solution in much the same manner as it is applied to timber. For multilayer windings, the whole coil was submerged in the shellac solution and then removed, drained and placed in a warm place to allow the alcohol to evaporate. The shellac then holds the turns in place, provides extra insulation and prevents movement and vibration, reducing buzz and hum. In motors and generators it also provides a medium for transfer of forces 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.

As a natural resin, shellac has similarities to other natural resins such as Myrrh and Frankincense.

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 systems in fireworks formulae.

Trivia

• It takes about 100,000 lac bugs to make 500 g of shellac flakes.^[4]
• Shellac is UV-resistant, and does not darken as it ages (though the wood under it may do so on its own, as in the case of pine).^[4]
• Shellac scratches less easily than most usual lacquers, and damaged areas can easily be touched-up with another coat of shellac (unlike with polyurethane) because the new coat merges with and bonds to the existing coat(s), but shellac is much softer than Urushi lacquer for instance, which is far superior in regards to both chemical and mechanical resistance.

Gallery

• Schellak.jpg

  Blonde shellac flakes.

• Shellac.JPG

  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 and kitchen cabinets.

• Shellac flakes closeup.JPG

  Closeup of Waxy #1 Orange (L) and Dewaxed Bona (R) shellac flakes. The former -- orange shellac -- is the traditional shellac used for decades to finish wooden wall paneling and kitchen cabinets.

• Shellac board.JPG

  "Quick and dirty" example of a pine board coated with 1-5 coats of Dewaxed Dark shellac (a darker version of traditional orange shellac).

See also

• Bakelite
• Zein

References

1. ↑ ^{1.0 1.1} Merk Index, 9th Ed. page 8224.
2. ↑ American Woodworker: Tips for Using Shellac
3. ↑ Naturalhandyman.com
4. ↑ ^{4.0 4.1 4.2 4.3} Naturalhandyman.com
5. ↑ Merrifield, Mary (1849). Original Treatises on the Art of Painting. Mineola, N.Y.: Dover Publ. ISBN 0486404404. 
6. ↑ "Furniture finish layer identification by infrared linear mapping microspectroscopy". JAIC (Journal of the American Institute of Conservation). 31 (2, Article 6): 225 to 236. 1992. 
7. ↑ Woods, C. (1994). "The Nature and Treatment of Wax and Shellac Seals". Journal of the Society of Archivists (15). 
8. ↑ Out Your Backdoor: Shellac & Twine makes Handlebar fine
9. ↑ mounting-tubulars Mounting Tubular Tires by Jobst Brandt
10. ↑ Shellac film coatings providing release at selected pH and method - US Patent 6620431
11. ↑ US Apple: Consumers - FAQs: Apples and Wax
12. ↑ Shellac, WoodworkDetails.com

• 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

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