Shellac is a versatile natural resin derived from the secretions of an insect. It serves as a protective coating or finish across various applications. This unique substance originates from a biological process, undergoing several refinement steps before becoming the familiar product used in many industries. Its composition primarily involves a natural resin, often dissolved in a solvent for practical use.
The Source of Lac Resin
The primary component of shellac is a resinous secretion produced by the female lac insect, Kerria lacca. These insects are native to subtropical and tropical regions, thriving on specific host trees in countries like India, Thailand, and Myanmar. The insects infest the branches of these trees, such as the palas (Butea monosperma) and kusum (Schleichera oleosa), forming a thick, encrusting layer.
The lac insect secretes this resinous material as a protective covering for itself and its larvae. This natural defense mechanism accumulates a hard, amber-like substance on tree branches. Farmers then harvest these resin-coated branches, known as sticklac, from the host trees. Sticklac contains not only the raw lac resin but also tree bark, insect remains, and other impurities, which must be removed during processing.
Processing Raw Lac into Shellac
Transforming raw sticklac into usable shellac flakes involves several stages of purification. Initially, the harvested sticklac is crushed into smaller pieces to separate the resin from most of the wood and other debris. This crushed material then undergoes a washing process, often with water, to remove water-soluble impurities and insect bodies. The resulting product, known as seedlac, still contains some impurities.
Following washing, the cleaned lac (seedlac) is melted using heat, sometimes in combination with a solvent, to further separate the resin from any remaining insoluble impurities. The molten lac is then filtered through cloth or fine mesh screens to achieve a purer form. This refined molten lac is then stretched into thin sheets, which are allowed to cool and harden. These brittle sheets are subsequently broken into flakes, creating the dry shellac product.
The Role of Solvents and Additives
Shellac flakes, in their dry form, are not directly applicable as a coating; they require dissolution in a suitable solvent to create a liquid solution. Ethyl alcohol, also known as ethanol, is the most common solvent used to dissolve shellac flakes. Other alcohols, such as methanol or isopropanol, can also be used. The alcohol acts as a carrier, allowing the shellac resin to be spread evenly over a surface, and it evaporates after application, leaving a hard, protective film of shellac.
Commercial shellac products may contain other substances in addition to the dissolved resin. Some shellac, for instance, might retain a natural wax component that is part of the original lac secretion, while “dewaxed” shellac has this wax removed for specific applications. Dyes may also be added for coloring purposes. These additives are present in minor quantities, with the lac resin and solvent forming the bulk of the solution.