ARTICLES & REVIEWS
by Zinsser Co. Staff
Shellac has an Ancient History
Not much is known regarding the very early history of shellac. In the Vedic period about 3,000 years ago it was called Laksha. One of the Vedic books contains an account of a whole palace constructed entirely out of lac resin.
Ancient Chinese and Indian civilizations used the dye extracted from lac for dyeing silk and leather and as a cosmetic rouge and a coloring for head ornaments. The superior adhesive quality of the resin made it useful for setting jewels and sword hilts as well as repairing broken pottery. The residue left after the extraction of the dye was made into a grinding wheel for jade: a technique still in use today.
It was in the field of medicine, however, that the most extensive applications for lac were discovered. It was prescribed either as an emollient, or as a stimulant to tissue growth or in the treatment of gum hemorrhages and menstrual disorders. In veterinary medicine lac was mixed with lard and the paste used to fill the cavities in the hooves of horses and cattle.
Europe Discovers the Wonders of Shellac
Following the historical journey of Marco Polo to the Orient in the late 13th Century, shellac and its by-products began to make their way into European commerce and industry. Accounts dating as far back as 1534 describe the cultivation, harvesting, processing and use of lac in extraordinary detail.
By the mid 17th Century shellac resin, shellac dye, and shellac wax were used with increasing frequency by painters not only to create their masterpieces, but also to provide them with a protective finish.
Shellac became the preferred finish for craftsmen and artisans—it was the coating of choice for fine furniture, woodcarvings, and turnings. To this day some of the finest museum pieces still have their original shellac finish.
The Golden Age of Shellac
Ironically, it wasn't until the mid-19th Century that shellac was commonly used as a clear finish. Until that time it was processed mainly for the dye that was extracted from the lac after it was harvested. This rich, reddish-purple colorant was highly prized and much sought after by the textile trade in both Europe and America because it was an excellent substitute for cochineal, a dye imported from Spanish colonies in Mexico.
In 1856 an English chemist named Henry Perkin succeeded in synthesizing a mauve-colored dye from an aniline derivative of coal tar. His discovery forever changed the future course of the shellac industry. As the demand for natural lac dye declined, the demand for shellac varnish began to increase. Production plants began springing up throughout Europe, most notably in Germany, which soon developed a reputation for manufacturing the finest shellac in the world. Efforts were also being concentrated on producing colorless shellac. As far back as the 1830s shellac chemists discovered that by chlorinating an alkaline solution of shellac they could remove almost all of the color and then precipitate the resin. The result was a pale, straw-colored varnish that excelled any oil-base varnish for clarity.
1849: Shellac Comes to America
By the middle of the 19th Century, Germany was the center of shellac bleaching for all of Europe. One such bleachery, located in the town of Mainz, employed a man named William Zinsser as one of its bleaching foremen. Confident of his technological skills and convinced that a good market for bleached shellac either existed or could be created in the United States, Zinsser and his family emigrated to America.
The next eighty years witnessed a veritable explosion in the commercial applications for shellac. It was used extensively as a binder in the manufacture of gramophone records from the turn of the century well into the 1950s, when manufacturers began using vinyl to press record albums. Shellac was used to make shoe polish, felt sizing for men's hats, hair spray, floor wax, pharmaceutical, candy and fruit coatings, printing inks, adhesives, grinding wheels, paper and foil coatings, and electrical insulators.
From the turn of the century through the 1950s home builders and painting contractors used shellac as a sealer for plaster walls and a fast-drying varnish for interior woodwork, trim and floors. Today this beautiful original finish can still be seen in many older homes.
The Rise of Lacquers and Polyurethane
The development of synthetic resin compounds early in the 20th Century together with advances in varnish formulation heralded the end of shellac's industrial and architectural dominance. Ironically, many of these newly developed resins—such as Bakelite and similar phenolic-based compounds—were created by researchers attempting to synthesize shellac.
Following World War I chemists discovered that nitrocellulose dissolved in a powerful solvent mixture produced a crystal clear coating that dried as fast as shellac, thus giving rise to the widespread use of lacquer as a furniture finish. After World War II alkyd varnishes were developed and by the 1950s the public was introduced to the first oil-based polyurethanes. As these finishes displaced shellac as the standard finish coating for interior woodwork and floors, one-by-one America's shellac manufacturers either closed their doors or merged with others.
How Insects Make Shellac
Shellac has the distinction of being the only known commercial resin of animal origin. It is produced by a tiny red insect (Laccifer lacca) which, in its larval stage, is about the size of an apple seed. Swarms of the insects feed on certain host trees, commonly called lac trees, in India and Thailand, the main lac-producing countries.
Their whole life cycle spans six months and is devoted to eating, propagating, and creating lac as a protective cocoon for their larvae.
During certain seasons of the year, these tiny red insects swarm in such great numbers that the trees at times take on a red or pinkish color. When settled on the twigs and branches, they project a stinger-like proboscis to penetrate the bark.
Sucking the sap, they begin absorbing it until they die. In shellac lore this is the feast of death. While they eat they propagate, with each female producing about one thousand eggs before dying.
In the body of the lac insect the digested tree sap undergoes a chemical transformation and is eventually secreted through pores. On contact with the air, it forms a hard shell-like covering over the entire swarm. In time this covering becomes a composite crust for the twig and insects. Only about five percent of the insects amassed on the trees are males. The female is the main shellac producer.
While she is secreting lac, she is preparing herself to die after providing a fluid in which her eggs will mature and from which the future supply of bugs will come, to repeat the process of swarming, propagating and creating the next season's shellac harvest.
The males, having fertilized the hordes of females, also begin their life-ending feast. Although they contribute relatively little more to the shellac crop, they have already assured an ample supply because the females vastly increase their output of lac after being fertilized. The great mass of male and female bugs on each tree gradually becomes inactive as the shell-like covering forms over them. In the sixth or seventh months, the young begin to break through the crust and swarm to new feeding grounds.
Shellac cultivation yields a large crop by helping the larvae find better locations for their feast. This involves simply cutting lac-bearing twigs from an infested tree a few days before the emergence of the larvae. A bundle of such twigs, known as broodlac, is tied to an un-infested tree on which there are many tender new shoots. This results in a higher survival rate of insects and a greater yield of lac since only a little broodlac gives forth sufficient larvae to infest a tree thoroughly. No further attention is needed until shellac is harvested.
How Lac Is Harvested
Shortly after the young have swarmed at the end of the adults' life cycle, natives begin to harvest the lac encrustation from the trees. Only one crop is taken per tree. Young are hatched, however, twice a year. Natives gather millions of encrusted twigs, called sticklac, for transport to simple factories or refining centers where the lac crust is scraped off and processed. The resin is also collected in the forest or orchard by smacking the branches with a wooden mallet. This material is called grainlac. In either case, this is the first step in the harvest of shellac resin.
At refining centers, sticklac is scraped to remove the resin from the twigs and then it is ground (as is grainlac), usually in a primitive, hand-cranked mill. At this stage, the ground lac contains a mixture of resin, insect remains, twigs and other impurities. This is now passed through a coarse screen to remove the larger size twigs. After the lac is ground and the chaff sifted out, it is soaked in water for several hours in large cup-shaped jars. These are about two feet high and have rough serrated inner surfaces.
A ghasandar jumps into the jar and rubs the lac with his feet against the rough surfaces. This action causes the lac seeds to break open, releasing dye and insect remains. The ground lac is rinsed to remove the dye and then spread out on a concrete floor to dry in the sun. The dried resin is called seedlac because of its grain-like appearance and ranges in color from pale lemon to dark red.
Commercial shellac falls into three categories that reflect the processes used in their manufacture: hand-made, machine-made, and bleached.
This process involves a primitive method still used by small local factories to produce flake shellac. Generally, three workers carry out the process from start to finish. They begin by packing seedlac into a long round bag about the shape of a section of two-inch fire hose. These bags vary from 25 to 40 feet in length. Small sections of the long bag are heated uniformly by slowly rotating them over a charcoal fire in an oven called a bhatta.
While a helper twists the far end of the bag, the operator, called a karigar, holds the hot end of the bag and squeezes the molten lac through the pores of the bag. The helper at the far end continues to pinch the bag by twisting it, forcing more lac toward the karigar.
The karigar lets the oozing shellac fall on the hearth stone, which has been moistened with water, and scrapes the surface of the bag periodically with a spatula. To prepare for the next step, he repeatedly picks up lac from the hearth-stone with an iron spatula and puts it on the rotating bag, basting it back and forth to get a viscous, uniform melt.
This soft lac is then turned over to a bhilwaya, who works it into thin sheets. With a strip of palm leaf, he spreads the molten lac over a ceramic jar filled with hot water, and then pulls off a piece about two feet square and ¼ inch thick.
Standing before the fire with the sheet, the bhilwaya manipulates it to soften it uniformly. At that point he uses his hands, feet, and teeth to stretch it into a paper-thin sheet about 5 feet by 4 feet. This is laid aside to cool and harden, after which it is broken into flakes.
If there is a demand for it, the bhilwaya will opt instead to make button lac. Instead of stretching the molten lac into sheets, the bhilwaya takes the lac from the twisting bag with a spatula and spreads it out in the form of a circular disk or button about one to three inches in diameter. Before it hardens completely the button is stamped with the seal of the manufacturer.
Shellac manufactured by modern mechanical methods is called machine-made shellac, mainly to distinguish it from shellac made by the indigenous—and, frankly, more fascinating—hand technique. There are two processes: one based on melting (heat process) and the other on solvent extraction.
In the heat process, seedlac is melted on steam-heated grids. The molten lac is forced by hydraulic pressure through a fine wire screen. The filtered shellac, still molten, is collected and transferred to a steam-heated kettle from which it is dropped onto rollers. It is squeezed out on the rollers, coming off as a thin sheet to be broken into flakes. The thickness of the flake is controlled by adjusting the roller pressure. All flake shellac made by this process contains wax.
The solvent process produces three types of shellac:
Although most of the bright red lac dye can be removed with activated carbon, some shade of red-orange remains. For many applications, however, a nearly colorless film is preferred. Kusmi shellac, while extremely light in color, is very expensive and not available in commercial-scale quantities.
The development of shellac bleaching in the early 19th Century solved this problem with a relatively inexpensive process that could produce enormous quantities of very pale-colored shellac
The bleaching process involves dissolving seedlac, which is alkali-soluble, in an aqueous solution of sodium carbonate. The solution is then centrifuged or passed through a fine screen to remove insoluble material. The next step is bleaching the cooled solution with dilute sodium hypochlorite to the desired light color. The shellac is then precipitated from solution by the addition of dilute sulphuric acid, filtered off, washed with water, ground, and dried in vacuum driers.
The final product has a granular consistency and is dissolved in alcohol to give a milky, creamed-honey colored solution. Both clear and amber bulls eye shellac contain from 3% to 5% natural shellac wax, which gives them their somewhat milky appearance in the container but does not affect the clarity of the dry film.
While more efficient equipment and machinery have replaced much of the work that was done by hand in the 19th and early 20th Centuries, the bleaching process itself has not fundamentally changed in over 100 years.
What Does the Term Pound Cut Mean?
Pound cut is a term unique to the shellac industry. It refers to the number of pounds of shellac dissolved in one gallon of alcohol. A 3-lb. cut contains approximately 29% shellac; a 2-lb. cut contains about 22% shellac.
As a rule the following pound cuts are used:
Does Shellac Have a Shelf Life?
Freshly made shellac dries very quickly to a hard, durable and water-resistant finish. However, because it is a natural material, shellac is also perishable and after six months an ordinary solution of shellac and alcohol begins to undergo a chemical change: it gradually takes longer to dry; the dried film is softer and more prone to scratches and water damage. This change is even more pronounced for bleached shellac. After 18 months it may take hours to dry or not even dry at all and is considered to have expired. Exposure to heat also accelerates the expiration process.
Common Shellac Myths
Shellac's Great Properties
Shellac has such remarkable properties that if it were just recently discovered it would be hailed as a miracle finish of the 21st Century.