Understanding the Use of Varnishes

Varnishes are liquid translucent substances which, when applied to a surface in a thin film, dry on exposure to the air, giving a decorative and protective coating. Most varnishes give a glossy finish though some are made to dry with a mat or semi-mat film.

The two main types of varnish which concern the decorator are oil and spirit varnishes. Oil varnishes consist of a combination of gums or resins, drying oils, drier, and a solvent or thinner, and harden mainly by the evaporation of the solvent and the oxidation of the oil. Spirit varnishes consist of solutions of gums or resins in a solvent and dry entirely by the evaporation of the latter. Oil varnish is by far the more important of the two types and the term varnish, by itself, is invariably taken to imply an oil varnish.

The manufacture of varnish, though superficially a simple operation, is far from being so in practice and requires considerable skill and experience to produce good and uniform results.

The requisite amount of resin or ‘ gum ‘ is first melted or ‘ run ‘ in a special metal pot. The fumes given off during this process are collected by means of a hood and conveyed through pipes to condensers.

Meanwhile, the oil, which is usually linseed oil, is being heated in a separate vessel to about 400 degrees or 500 degrees F. according to the nature of the gum. The correct proportion of hot oil is then added slowly to the melted resin and the mixture is then cooked for a further period, any necessary driers are added, and then the pot and its contents are removed from the fire, allowed to cool until a temperature of about 300 degrees F. is reached, when thinners in the form of turpentine or white spirit are added. The varnish is finally run through a filter press, to remove any impurities, and run into tanks. As freshly-made varnish proves unsatisfactory in use, it is allowed to mature for some months before it is despatched from the factory.

The gums and resins used in the process are exudations from trees, which harden in the air, usually in an amorphous state. The difference is that gums are soluble in water, forming a mucilage, and are insoluble in alcohol. As a rule they are transparent, and are all the products of living trees or bushes. They are chiefly used in dissolved form as ad-hesives, but also to add body and a glossy finish to water-colours. Resins are sometimes transparent, often cloudy, and occasionally opaque in the mass; they are of vegetable origin, but many of them are fossiliferous, the exudation of extinct trees. Others are derived from existing species. The chief difference between them and gums is that they are insoluble in water and often only partially soluble in alcohol. Most of them after being laid on in a diluted form dry with a hard and more or less glossy surface. They are, therefore, important constituents of polishes, varnishes (clear or opaque), and enamel paints, as well as stoving enamels and lacquers. In the trade the terms are employed rather loosely. Many so-called gums are really resins. But there is some excuse for this, as a few of the carbohydrates partake of the nature of both gums and resins. These are valuable, because they dry with a more glossy and a harder surface than gums, but with a slightly less hard, less brittle, and more elastic surface than resins.

Resins

Accroides, or Black Boy Gum

A resin derived from several of the Xanthorrhea species of trees; it comes to us mainly from Australia. It is obtained both from exudations and by extraction from crushed bark and wood. Red and yellow varieties come on the market. Both are transparent and dissolve in alcohol, giving strong yellowish solutions, mainly used for colouring spirit varnishes.

Amber

A fossil resin found in the greensand beds stretching from Norfolk, along the Dutch coast, the shores of the Baltic, across Siberia, and also found in America. Somewhat kindred fossil resins are found over limited areas in Asia. Practically all the amber found in commerce comes from Holland and the Baltic. It has a glossy appearance, and may be clear or cloudy, ranging in colour from pale straw to bright yellow, light brown to orange or greenish brown, and in some cases is almost black and opaque. In the trade besides the terms ‘ transparent ‘ and ‘ cloudy ‘ we find these resins classed as ‘ shining ‘ (pale yellow or greenish), ‘ bone colour ‘ (dull white), and ‘ bastard ‘ (dark and opaque). It is insoluble in water, ammonia, petroleum spirit, benzol, acetic acid, and carbon bisulphide, but partly soluble in alcohol, turpentine, ether, and certain essential oils.

Amber is, however, seldom used for varnishes to-day, on account of its high price and scarcity, and so-called ‘ Amber ‘ varnishes seldom contain amber, the term being mainly taken to denote the colour.

Animi

This resin, which comes from Zanzibar, and is sometimes known as ‘ Zanzibar Copal,’ is (with the exception of amber) the hardest of the fossil resins. It is pale yellow in colour and nearly transparent. It is very expensive and, in consequence, used only for very high-grade varnishes.

Benzoin or Benjamin Gum

A balsamic resin derived from Styrax benzoin, this comes from countries in the Indian Ocean. It is derived by tapping the trees, and the exudation for the first three years forms first-class resin, yellowish white, soft and fragrant. After that, the quality falls off and the resin is darker in colour and hard. Siam benzoin is sold in flattish opaque ‘ tears,’ and in large amber masses enclosing white patches. It is brittle and melts at ioo° G. Sumatra benzoin is marketed in large dark blocks enclosing white masses, and is hard. These resins are used in varnishes.

Copal

This name is given to several kinds of semi-fossil resin, chiefly derived from Trachylobium verrucosa trees, and found in the ground near their roots. These trees grow mostly in West and Central Africa. It is insoluble in water and alcohol, ether and chloroform. When ground fine and digested in ether it swells, and on being melted gives off an oily liquid. The residue sets into a brittle mass which is soluble in ether, turpentine, chloroform, etc. Copal is useful for varnishes required to dry with a hard lustrous surface.

Dammar (or Damar): A resin derived from Dammara orientalis, and imported from Singapore in nodules of to 2 in. It is clear white to straw colour, but is sometimes covered with a powdery crust, is soluble in turpentine, ether, petrol spirit, and oil, while both alcohol and amyl alcohol convert it into a white jelly. Penang Dammar is similar to the above, and both are valued for dissolving in oils to produce coach- and cabinet-makers’ varnishes; for pictures and paper it is dissolved in benzol, which gives a clear, hard, but rather friable varnish.

Dragon’s Blood

A resin excreted by the Calamus draco tree, growing in Eastern Asia. It is imported in cylindrical sticks, 12 to 14 in. long, and I to 1 in. thick, but is usually marketed in a ground form. It is of a blackish-brown colour, of opaque appearance in the mass, but crimson when sliced thin. It is soluble in alcohol, benzol, petroleum spirit, shale spirit, etc. Besides the resins coming from the East, dragon’s blood is also exported from Socotra, Zanzibar, Teneriffe, and Mexico.

Elemi

A resin which is known under two names: (1) Manila gum elemi is derived from Canarium commune, growing in the Philippine Islands. The best quality is white, the second grey. It is naturally soft and granular, becomes plastic at between 75 degrees to 8o° C, and melts at 120 degrees G. It is partially soluble in alcohol and more thoroughly in ether. (2) Mexican elemi is derived from Amyris elimifera, is darker and heavier than Manila elemi, and comes from Mexico, Brazil, and Mauritius. It is particularly useful for mixing with shellac or sandarac when it is desired that a varnish should retain elasticity.

Gamboge: A resin yielded by the Garcinia morella tree, this has a bright-yellow colour. It is soluble in alkaline solutions. Colouring matter is extracted by maceration in ether, which, on evaporation, leaves a transparent, vitreous, and brittle mass, reddish orange in appearance, but giving a deep yellow after being powdered. Formerly used for colouring varnishes, it is not permanent to light and has mainly been superseded by artificial dyes for this purpose.

Kauri, or Cowree

A fossil resin found in New Zealand in lumps sometimes attaining over 100 lb. in weight. Alcohol, turpentine, and benzol convert it into a gelatinous mass; ether dissolves it completely. Kauri is used in the preparation of high-grade varnishes and is expensive.

Lac (Shellac)

This resin is not a natural exudation from a tree but is prepared from the secretion of insects which feed on the sap of a variety of trees which grow almost exclusively in India. It varies in colour from yellow to orange. The deposits of lac are formed mainly on twigs and small branches, and in this condition are known as ‘ stick-lac.’ After being separated from the wood, the product is refined by native labour, purified, graded according to colour, melted by heat, and, after it has set in sheet form, is sold as shellac.’ The best quality is orange in colour and is known as ‘ orange shellac.’ Other forms in which it is marketed are ‘ button lac,’ which is the same as shellac but made in thick, round lumps, and ‘ garnet lac,’ which is similar to button lac but is in thick, flattish pieces. By treating shellac with hot alkaline solutions, the colouring matter can be removed and it is then sold as ‘ bleached ‘ or ‘ white ‘ shellac.

Shellac is soluble in alcohol and is the chief resin used in the preparation of spirit varnishes and French polishes. It is insoluble in turpentine or oil, and for this reason is widely employed for ‘ knotting,’ used by the painter for preventing resinous exudations from knots, or colours subject to bleeding, from affecting paint or varnish coatings applied on top of them.

Mastic

A resin obtained from several lentisc trees, growing in Northern Africa. It is soluble in turpentine, alcohol, amyl alcohol, and acetone, but not soluble in ether. It is a compound in fine varnishes for pictures, etc., and when mixed with turpentine and linseed oil forms the megilp used as a medium for water-colours. The cake variety, in large pieces, is the best. Small tear mastic is also used for varnishes and is known as gum juniper.

Rosin

A brittle resin obtained as a residue after distillation of turpentine. It is transparent to translucent, pale yellow to brown. The best quality is known as ‘ window glass,’ the second as ‘ amber.’ ‘ Virgin rosin ‘ is the crude resin before distillation, known also as gum thus, which is used in the preparation of some of the coarser varnishes. Rosin softens at 75 degrees G. and melts at ioo° C. It is soluble in amyl alcohol, turpentine, ether, acetone, benzol, petroleum spirit, coal-tar naphtha, but not in water. When distilled it yields rosin spirit and rosin oil. It is used for cheap varieties of varnishes and varnish-paints, to which it imparts initially a high gloss, but it forms films which are not durable and which soon crack and disintegrate.

Sandarac

A North African resin, yielded by Callitris quadrivalvis trees; it comes on to the market in cylindrical pieces; its melting-point is 150 degrees C. Sandarac is soluble in alcohol, methylated spirit, amyl alcohol; only slightly so in benzol and petroleum spirit. It is employed in the preparation of spirit and oil varnish, giving a hard bright surface.

Gums

Gum Arabic, or Gum Acacia, comes mainly through Aden and Bombay. It is a true gum, derived from several of the acacias. It is known on the market under various names, sometimes indicative of quality or merely of origin. Thus there are Picked Turkey, White Sennaar, Gum Senegal, Soudan or Suakim gum (second quality), Mogador gum, Brown Barbary, Cape gum, Indian gum, Wattle gum (from Australia). These vary partly as regards their adhesive qualities, more especially as regards colour. (Clear or cloudy, water white to light brown.) All dissolve in water, the best qualities taking up one and a half times their weight of water, forming a thick mucilage. They are not soluble in alcohol; if this be added to a water solution precipitation takes place; if subacetate of lead is added a white opaque jelly results. Alumina sulphate thickens the mucilage and adds to its adhesive properties. Acacia spinosa produces the gum Siris of India. Gum Ghatti is also from an acacia, but unlike the others is only formed into a jelly on being soaked in water.

Gum Tragacanth, or Gum Dragon, is an exudation from the astragalus shrubs of Asia Minor. It is transparent, soluble in water, and used both to produce water gums and for mixing in translucent colourless varnishes. It comes on the market in: (1) leaf form, (2) vermicelli.

Dextrine, or British Gum, is manufactured by heating starch for several hours to between 212° to 275 degrees G., and adding an acid. ‘ White ‘ dextrine is of a pale yellow; ‘ Canary ‘ a bright to dark yellow; inferior qualities brown. It has a sweetish taste, is soluble in water, and has excellent adhesive qualities.

Bitumens

Bitumen is a term loosely applied to a number of plastic substances, varying in colour from dark brown to black, and with a characteristic odour, which occur naturally in many parts of the world, notably in Trinidad, Switzerland, and certain districts in the U.S.A., and are also produced artificially. They include:

Asphalt

Asphalt, or asphaltum, is a natural mixture of carbons, found in many different forms, some rock-like and some semi-plastic, of a dark-brown colour, melting at about ioo° G., giving off an acrid odour; it is combustible, insoluble in water and alcohol, slightly soluble in turpentine and easily so in coal-tar naphtha. In its purified condition asphalt is largely used in the preparation of some paints and such varnishes as black japan, Brunswick black, Berlin black, etc. For paint manufacture native asphalt is ground fine, with or without the addition of bitumen or other pitchy residues, and mixed with a suitable thinner and driers or, to the above, resin and colouring matter may be added. The solvents used in asphalt paints are naphtha, benzol, petroleum, turpentine, carbon disul-phide, and tetrachloride. Asphalt in solution is used for impregnating felt and paper to be employed as damp-proofing materials, either as damp courses, flat against walls or under roofs, on floors, etc. Such materials are commonly fixed in place by nails, but hot melted bitumen mastic may be used as a cement.

Pitch is a solid or thick viscid substance left after the dry distillation of carbons (coal, wood, petroleum) carried to a point before the residue becomes a hard solid, 1.e. coke. The longer the heat is applied, the darker, harder, and more brittle becomes the residual pitch. It varies in colour from a light brown to a black. Pitch is used in a crude form as a paint when melted; it is also employed as an ingredient in the manufacture of black paints, varnishes, and japans.

Spirit Varnishes

The most important resin which is employed in the making of spirit varnishes is shellac, though sandarac, mastic, copal, amber, and others are frequently used. The process of manufacture, as usually carried out in this country, is comparatively simple. The gums and solvents are placed in a wooden barrel which is made to revolve horizontally until complete solution takes place. Methylated spirit is generally employed as a solvent.

French Polish

This is a clear varnish for use on wood where only a very thin coat is required. There are many recipes for its manufacture, but a good-quality polish can be made by dissolving 1J lb. of orange shellac in 1 gal. of methylated spirit, then adding 2 oz. of gum sandarac and 2 oz. of gum benzoin. Before applying, the wood is ‘ filled ‘ by rubbing on a mixture of plaster of Paris and tallow. Then the polish is put on by rubbing with a soft cloth dipped in methylated spirit. The rubbing should be in a circular direction, light, then hard. A finishing-off coat is given consisting of the above polish diluted by adding methylated spirit.

Knotting

The purpose of knotting is to protect paint against knots and other resinous exudations in wood, and, except in very bad cases, it is usually effective in doing so, provided that it is of good quality. In theory, it consists of a solution of pure shellac in pure methylated spirit, the proportions being from 2 to 2 1/2 lb. shellac to each gallon of spirit. In practice, however, the shellac portion is often partly or entirely replaced by a less-expensive substitute; even rosin is occasionally employed for this purpose, thereby increasing the trouble which the application of the knotting is intended to prevent. When purchasing shellac knotting, therefore, decorators should stipulate that it is guaranteed pure and free from rosin.

Lacquers

The term ‘Lacquer’ is rather loosely used in the trade. It is generally applied to spirit varnishes, made from shellac, dammar, mastic, sandarac, or a similar resin, dissolved in a suitable solvent. Coloured lacquers are prepared by adding a certain amount of colouring matter in the form of a dye, so that the solution is transparent. Following American practice, the name ‘ lacquer ‘ is sometimes used to denote cellulose or synthetic-resin varnishes. It may, on occasion, also refer to oriental lacquer, a product quite different from other resinous materials. Oriental lacquer, which is derived from trees which flourish in China and the Far East, is the basis of the well-known oriental lacquer ware and, except in this form, is seldom imported into this country.

Gold Size

Gold Size is a varnish-like material, used as a mordant, or adhesive, for gold leaf, or as a medium for certain classes of primers and fillers. It is made with a comparatively small proportion of oil, a high percentage of resin, driers, and volatile thinner. It has excellent binding properties and, as it dries rapidly, is extensively employed for quick-drying finishes which are not required to have much durability.

Black Varnishes (Japans)

This is a generic name given to a range of finishes prepared by adding various grades of bitumen to heat-treated oil, with the necessary driers, and, after cooling, volatile thinners. In the best-quality black japan, a proportion of resin is included, to produce a harder, stronger, and more lustrous film. Many different grades of asphalts and pitches are employed for the cheaper black varnishes, an inferior variety being Brunswick black, which is sometimes wilfully or inadvertently sold as black japan. Apart from the question of durability, in which black japan is greatly superior, an important difference is that Brunswick black will ‘ bleed through ‘ and discolour a white or pale-coloured paint applied on top of it, whereas black japan is, or should be, immune from this defect.

Black japan is not always easy to apply owing to the difficulty of determining when it is completely dry. It may be quite dry to the touch but still soft underneath, with the result that freshly applied varnish will soften it up and give it a greenish tinge instead of the dense-black appearance which it should possess. The best method of ensuring a really good finish in black japan work is to allow as much time as possible between coats.

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