THE materials used by the painter and decorator in the exercise of his craft are so numerous that to do full justice to them would require far more space than is at our disposal.
Paint is a generic term for a large number of different types of material, ranging from colour wash to the finest enamel. To the decorator, however, the word invariably refers to an oil or gloss paint, of which there are four components – the pigment, the vehicle or medium, the thinner, and the drying agent.
I. The pigment provides body, opacity or obscuring power, and colour.
Pigments are derived from various metals, earths, and other sources, and the degree of fineness to which they can be ground is a highly important consideration. They are usually supplied to the painter in paste form, ground in linseed oil, turpentine, or gold size, although they are also avail able in dry form. In most paints, the bulk of the pigment is a basic white, with or without the addition of coloured pigments or stainers.
II. The vehicle or medium is a drying oil, such as linseed oil, a varnish, or a mixture of both. Its function is to enable the particles of pigment to be applied to a surface in a uniform coating and to remain attached to that surface. Linseed oil dries mainly by oxidation – that is, by absorbing oxygen from the air – changing, in doing so, from a fluid into a tough, leathery substance known as linoxyn. The process, it should be noted, is a gradual one which continues long after the paint is dry to the touch, and brings about an alteration in the structure of the oil as the result of which the linoxyn is not soluble in turpentine.
The proportion of vehicle to pigment governs, to a great extent, the degree of gloss which the paint film possesses, as well as the durability of the coating.
The thinner is a term sometimes applied to the liquid portion of the paint as a whole, but more generally to the volatile liquid which is added to the paint to make it more fluid and thus more easily put on. For oil paints, the thinner traditionally employed is turpentine, though mineral spirit (known as ‘ white spirit’ or ‘ turpentine substitute ‘) is widely used, especially by paint manufacturers, since it is considerably cheaper: provided it is of good quality, it serves equally well for the purpose as pure turpentine.
The drying agent, or drier, is a substance available in paste, liquid, 26 or powder form, which is added to the paint to accelerate oxidation of the film. Driers are essential ingredients since without them the paint would take too long to dry for practical purposes, but the more sparingly they can be used the better, since they tend to shorten the life of the film.
I. The Pigment-White Pigments
Of the various pigments used in the making of paints, the white are the most important, since they are employed in the great majority of paints not only because of the very large amounts of white paints and enamels used in decoration, but because white pigments are employed to reduce or break down the strength of coloured pigments, and also, in many cases, to give them opacity. They include the following:
White lead is one of the oldest of manufactured pigments, and though it now has many competitors, it still enjoys great popularity in the paint trade.
White lead is prepared by many processes. In the stack or old Dutch process the metallic lead is moulded into ‘ wicket’ or ‘ buckle ‘ form grids with longitudinal bars within a frame. These are placed in layers on platforms covered with spent tan in a room on the floor of which are open earthenware pots containing dilute acetic acid. A stack is built up of several platforms, each covered with spent tan. Then the room is sealed and the fumes rising cause the tan to ferment, which in turn acts on the lead and corrodes it into hydrocarbonate. The process is over in from 80 to 100 days. To activate the giving off of vapour, bars of lead are sometimes placed in the acetic. On removal the lead is ground between corrugated rollers, screened, ground in water, and then dried in stoves. In the chamber process the metallic lead is hung in thin strips over poles in a room which is then closed and suitable corroding gases introduced through stoneware pipes. Oxidation is complete in about eight weeks. In the Carter process, an American method, lead is reduced to a powder by means of high-pressure steam blasts, is then placed in wooden drums which are slowly revolved while dilute acetic acid and carbonic acid gas are introduced into them. After twelve days corrosion is complete, and a brilliant white, very fine powder produced.
Properties: White lead has many advantages: it possesses great opacity or hiding power – greater than that of almost any other pigment – exceptional adhesiveness and resistance to rain and moisture. Although, owing to its high specific gravity, its spreading capacity is not particularly great, it works beautifully under the brush – a fact which has no doubt contributed to its popularity among painters.
White lead mixes well with linseed oil – with which it has what is sometimes described as a natural affinity – other oils, and turpentine. It is used both as a white paint and as a base for mixed paints, for interior or exterior application. For certain purposes, such as, for example, on out-side woodwork, it has few equals and no superior.
The colour of white lead depends on the degree of purity of the raw material, any slight suggestion of yellow being frequently counteracted by the addition of a trace of ultramarine. If a white-lead paint is applied to a surface to which little or no light has access, the paint will become yellow, but will bleach out white again if exposed to the sunlight. It should be added that all white paints have a tendency to yellow in the dark, but in lead it is somewhat more in evidence than in those made from other pig-ments.
White lead is perfectly stable in pure air, but if exposed in a sulphurous atmosphere will darken. For the same reason it should not be mixed with vermilion (mercuric sulphide), or the latter will discolour, as will chrome green.
Both as a pigment and as a base, white lead requires to be finely ground to ensure its brightness of colour and durability. Disintegration, in a white lead paint, usually takes the form of ‘ chalking M or ‘ checking ‘: by the first is implied the loss of oil from the exposed portion of the paint film, leaving the lead lacking in binder, so that it brushes or rubs off. Since every paint must perish, sooner or later, chalking, from the painter’s point of view, is possibly the best way in which it can do so, for since the process is gradual, the under layers continue to protect the surface until the time comes to repaint, when the task of preparing the latter will be much less arduous than if the old paint had failed by cracking or peeling.
When ‘ checking ‘ takes place, the surface shows a number of minute, criss-cross fissures. These at first affect only the outer layers of the paint film, and normally take some considerable time to work their way through the entire coating.
Reduced White Lead: Owing to its cost, white lead is frequently reduced or adulterated, though by specifying ‘ Genuine English Stack-made White Lead ‘ and dealing with a reputable firm, the decorator may be sure that he is obtaining a pure material. The pigment most usually employed for reducing it is barytes; if added in reasonable proportions, the latter does not necessarily impair the efficiency of the white lead, but if the decorator, having ordered genuine white lead, has any reason to suppose that it has been reduced with barytes, a simple test can be made.
Take a portion of the material about as big as a hazel nut and place it in a clean glass jar, add some slightly diluted nitric acid and stir. If the white lead is pure it will be completely dissolved by the nitric acid, and if any residue is left, it may be regarded as barytes or some other adulterant.
Lead Poisoning: The greatest disadvantage of white lead is that it is highly poisonous. In France, and in certain other countries, its use in paint is prohibited, and in Great Britain special precautions are prescribed by law both in paint manufacture and in painting. There is very little risk in manufacture, since employees are under direct supervision, hygienic measures can be enforced, and mechanical means adopted to reduce the handling of dangerous materials to a minimum. It is not always so simple for the master-painter to exercise the same degree of control, and consequently the majority of those cases which occur are among painters.
The Lead Paint (Protection Against Poisoning) Act of 1926 has fortunately considerably reduced the risk of disease. It is generally agreed that the latter is caused largely by inhaling lead dust, and one of the main provisions of the Act prohibits the rubbing down of a lead-painted surface by a dry sandpapering process and makes it compulsory for the employer to provide soap, water, nail-brushes, and towels for his operatives.
No one will deny that white lead is extremely poisonous, yet the risk of disease from it is frequently much exaggerated. The greatest safeguard is personal cleanliness, and provided this is observed, there is no reason why poisoning should ever occur. Many painters, who have been using white lead for year after year, have not only been immune themselves, but have never come into contact with a case, and of those which are reported not a few prove, on investigation, to be due to other causes than white lead.
Basic Sulphate of Lead
This material, which is also known as sublimed white lead, is produced by melting lead ore, or galena, in a furnace and forcing draughts of air through it. Its colour is not quite so good as that of white lead, nor in a paint does it work so well under the brush. Though not absolutely non-poisonous, it is far less toxic than white-lead carbonate, and its colour is not affected by impure air. It requires slightly more oil to form a paste.
Until 1932 the description ‘ white lead ‘ was held to apply exclusively to white-lead carbonate, and when the term is used by itself, it is still, in the majority of cases, taken to imply this material. In the year in question, however, a decision taken in the course of a case in the law courts established the fact that basic sulphate could also be described as ‘ white lead,’ though a qualifying phrase should be added to distinguish it from white-lead carbonate.
Leaded zincs are prepared in the same way as is basic sulphate of lead, the galena being replaced by mixed ores of lead and zinc. The proportions may be varied. The product is generally sold in paste form, and it is claimed that it combines the qualities of white lead and zinc oxide.
Zinc Oxide (Oxide of Zinc)
A brilliantly white pigment widely used in the manufacture of enamel paints of good quality, and frequently employed in other forms of paint where white lead is not desirable. It is non-poisonous, its chief disadvantage being its lack of opacity, which is considerably below that of white lead. Owing to the fact that it is not nearly so dense as the latter and requires more oil in grinding, it is fre-quently asserted that it does not cover so well, but while it is thinner when painted on a surface, it will be found that, weight for weight, its covering capacity is probably equal to that of white lead.
Zinc oxide keeps its colour well, and can be mixed with practically every other pigment. It is extremely durable and is not discoloured by sulphur gases. After application the paint film becomes very hard, and failure, when it occurs, usually takes the form of cracking.
Zinc oxide has no siccative or drying action on linseed oil, and consequently some form of added drier is usually essential. Patent or paste driers are not, as a rule, advisable, a liquid drier being generally more suitable.
Lithopone is a valuable white pigment, consisting of about 66 per cent, of sulphate of barium or blanc fixe, the remainder being zinc sulphide, sometimes with a small proportion of zinc oxide. It was originally introduced by Mr. J. B. Orr, of Widnes, Lancashire.
It has a specific gravity of 4-2. Mr. Orr originally called his pigment ‘ Charlton white,’ and afterwards it went under the name of’ Orr’s white.’ After his practical experience, the pigment was introduced in large quantities in America and the Continent, and was called ‘ Lithopone,’ or ‘ Lithophone,’ the latter being the usual Continental spelling.
The large proportion of sulphate of barium contained in the pigment might lead one to suppose that it was inferior in covering qualities or opacity. As a matter of fact, however, the artificial barium sulphate possesses excellent body, and lithopone is distinguished not only for such good body, but also for the fact that it spreads over so large a surface. In the former respect it equals or is superior to white lead, and in the latter it certainly is superior. It is the base of practically all the ‘ undercoats ‘ sold by paint manufacturers for use under enamels and ordinary painted work. The particles are very fine and mix well with oil. Very little rubbing down is required, and enormous quantities of the pigment are now used, particularly for flat wall finishes. It is also the base of many water paints.
Lithopone is non-poisonous and is not liable to discoloration by sulphur fumes, such as are found in industrial areas. But while it makes an excellent paint for inside work, it cannot be recommended for use in a finishing paint intended for outside application. It lacks durability and does not stand up well to exposure, breaking down, under normal conditions, after about a year. It possesses, moreover, one peculiar and serious disadvantage – a tendency to darken to a deep grey when exposed to the sunlight. The work may look perfect when completed, but a few hours of sunlight will gradually darken its hue until it becomes a dull lead colour. When night falls, it will resume its original whiteness, but daylight will bring a repetition of the phenomenon. Many theories have been advanced as to the cause of this curious change, but the exact reason has yet to be determined.
It is only fair to add, however, that this defect is by no means common, and though it is still liable to make its appearance, it occurs less frequently than was the case in the early days of the use of the pigment in paint.
A comparatively new addition to the range of white pigments and one which has great possibilities in decorative work. drying of pigments, to squeeze them in paste form through perforated plates whence they emerged in drop-shaped pieces.
Vine Black is prepared from vine twigs, wine lees, or grape husks by a process similar to that used in the making of bone blacks. It gives a rich, dense black with a faint bluish tone. It mixes readily with water but is not suitable for use in oil paints.
Graphite, known also as Blacklead or Plumbago, is a compound consisting mainly of carbon with small amounts of alumina, silica, lime, and iron. It is not so black as gas- or lampblack but possesses exceptional opacity and spreading power. It is completely inert and is unaffected by acids or alkalis or by heat. It is used for many other purposes than for paint, notably in the manufacture of pencils: a considerable proportion of the graphite employed in paints is used for coatings for ferrous metals. It was formerly fairly extensively employed in primers for iron- and steelwork but it has no anti-corrosive properties, its value in this connection lying chiefly in the fact that, owing to the structure of its particles, it has good moisture resistance. It would thus seem to be more suitable in a paint intended for application over a good rust-resisting primer.
A term given to a low grade of pigment prepared from crushed waste coal, slate, and other minerals, with or without the addition of lampblack or other carbon blacks to supply the required density. Mineral black is suitable only for the manufacture of cheap black paints.
A useful range of red pigments is made from natural or artificially prepared oxides of iron. When properly prepared, they have great staining power, durability, resistance to light, acids, and alkalis, opacity and spreading power.
Red Oxide of Iron
Natural deposits of oxides, containing varying proportions of oxide of iron, are found in many parts of the world, including this country. Most of them have a high degree of purity and thus require a minimum of treatment before grinding. The pigment has long found extensive use in the making of protective coatings for iron- and steelwork, although it is now generally recognised that, by itself, it possesses little, if any, rust-inhibitive power: mixed with a pigment such as red lead or zinc chromate it gives a valuable anti-corrosive paint.
Other natural oxides of iron, as chocolate, purple-brown, etc., serve equally well for a similar purpose.
The true Indian red is made from a natural earth found in India, but nowadays this red is more commonly manufactured by the calcination of yellow ochres and ferrous sulphate. It varies in colour but is usually of a somewhat darkish tone, often with a slight purplish hue.
Venetian Red: This is also mainly artificially prepared on a principle similar to that followed for Indian red. It is rather brighter and lighter than the latter, and is widely used for tinting and also for distemper colours.
This has properties similar to those of Venetian red, but it is not so bright as the latter.
This valuable earth colour is obtained by exposing raw sienna to a moderate heat. The resulting product is of a reddish-orange colour, though its shade depends on the source of the raw sienna, as well as the temperature at which it is calcined and the length of time for which the process is carried on. It is extremely transparent in oil and thus finds extensive use as a glazing colour.
This pigment, one of the oldest in the history of painting, is prepared from oxide of lead in two distinct stages. In the first, the metal is converted in the furnace into yellow monoxide of lead, or massicot; in the second, the massicot, on being further heated, takes up more oxygen and forms the familiar bright, strong red powder.
Probably the most important use of red lead in paint is in priming coats for iron and steelwork, for which its anti-corrosive properties make it especially valuable. A small proportion of red lead is usually added to white lead paint employed as a first coating on woodwork, to make the traditional pink primer,’ still widely used in the trade; the presence of the red lead helps to produce a harder film and to assist the oxidation of the oil which has penetrated within the structure of the wood.
Red lead exerts a powerful drying influence on linseed oil and paints made with it set hard within a short time. In spite of its bright colour, it is seldom used for purely decorative purposes, as it does not work well under the brush and is inclined to separate from the oil. An extremely heavy pigment, it tends to settle in the container; for this reason, paint containing any considerable proportion of it must be constantly stirred when in use and should be made up only a short time before it is to be applied.
This difficulty has, however, been overcome in recent years by the introduction of what is known as ‘ non-setting ‘ red lead, which contains more oxygen and in which the pigment remains better in suspension in the medium.
This pigment, the name of which describes its colour, is of much the same composition as red lead and has similar properties. It is employed as a rust-inhibiting primer on ferrous metalwork and also in the manufacture of vermilionette and other lakes.
The use of this pigment dates from very ancient times and probably originated in China. It is a sulphide of mercury and provides a red of extreme brilliance. It has great opacity and tinting strength and is normally fast to light, though occasionally it will blacken on exposure. The conditions which bring about this discoloration are not yet completely understood but the change occurs only very rarely, and not at all when the paint has a protective film of varnish.
Its high cost and, to a minor extent, its rather coarse texture and heaviness greatly restrict its use and the introduction of permanent lake reds has made it more or less obsolete, so far as the decorator is concerned.
This range of pigments is derived from the chromates of lead, barium, zinc, and strontium, and varies in colour from pale yellow to deep orange.
The lead chromes have great tinting strength and considerable opacity, and are moderately fast to light, though they tend to fade on prolonged exposure to sunlight. They can be safely mixed with the white basic and most other pigments, though not with those which, like ultramarine, contain sulphur. They are also affected by alkalis.
Barium chrome, more usually known as ‘ lemon yellow,’ is a very pale yellow, with properties similar to those of the lead chromes, though it has less opacity. It is used chiefly by artists.
Zinc chrome is also pale in colour, with a slight greenish tinge; it is proof against lime but is readily affected by acids and alkalis. Until recently, it found only a limited use in paint manufacture, but research carried out prior to and during the war proved it to possess valuable anti-corrosive properties and it was extensively used for priming metals, particularly in the U.S.A.
Strontium chrome is rather deeper in colour than barium chrome and has similar properties.
This range of pigments is prepared from natural earths, found in various parts of the world, notably in France, Italy, America, and Great Britain. French ochres are the brightest in colour; they have considerable opacity and tinting power. Italian ochres are slightly browner in colour and more transparent, as are those from America, which usually have a slightly reddish cast.
The finest English ochre is the well-known Oxford ochre, but the supply appears to be almost exhausted. Those found in Gloucestershire are more opaque than the Italian variety and have a brownish tinge, while those worked in Derbyshire are somewhat darker and redder in colour.
Ochres dry slowly, and are affected by acids, but are alkali-resisting and fast to light. They mix well in both oil and water.
Like the ochres, which they much resemble, the siennas are natural earth pigments, found principally in Italy. Raw sienna is of a dull, yellowish-brown colour. When roasted at a fairly moderate temperature, it becomes darker and redder, producing the pigment known as Burnt Sienna.
Sienna will grind in oil and in water and will mix with white lead and other bases. Both raw and burnt sienna are affected by acids, though the burnt variety somewhat less than the raw. Both resist alkalis and are fast to light. They are used for staining purposes and play an important part in graining. They are far less opaque than the ochres, but have better drying properties.
This is a yellow pigment used, to a considerable extent, by artists and by decorators in fine work. The tone is quite beautiful, being very bright. The colour is also almost transparent and it is used for glazing. It is expensive, and therefore can only be used sparingly in decorative work.
Cadmium Yellow is a sulphide of the metal cadmium. If pure it is very permanent. Cadmium yellow can be ground in any vehicle, and mixes easily with any other pigment except chrome yellow, emerald green, or other colouring matter containing lead or copper, which turns the pigment black. It is sometimes adulterated with sulphur, which causes it to blacken on exposure to the air. Very pale tints are suspicious. Cadmium yellow is expensive, and is only suitable for the finest of decorative work.
These are made from natural earths which occur in many parts of the world, the finest quality coming from the island of Cyprus. Originally this was imported into this country via Constantinople and was consequently known as ‘ Turkey Umber ‘ – a name by which it is still known in the trade. Deposits of the earth are found also in Italy and France and also in this country, in Derbyshire.
Raw umber varies from yellowish brown to dark brown in colour, according to its source. It can be heat-treated, to produce Burnt Umber, which has a warmer, darker colour.
Both raw and burnt umber can be ground in water, oil, or turpentine and safely mixed with other colours and bases, being valuable owing to their durability and fastness to light and alkalis. The raw variety is unaffected by acids, in which, however, burnt umber is partially soluble. Both are somewhat transparent and are consequently useful for glazing and in graining work. Umbers, owing to their manganese oxide content, exert a strong drying influence on linseed oil.
This pigment, named after the great seventeenth-century artist who used it extensively, is made from a natural earth, found in various places, but especially in Germany; it is sometimes known as Cologne Earth or Cassel Earth, from its places of origin.
Vandyke brown is prepared in a similar fashion to other earth pigments, though it is also made artificially by the slow burning of cork refuse, beech-wood bark, and other decomposed vegetable matter. It is also produced by mixing lampblack or other good black with iron oxide and toning with yellow ochre, but the result lacks the richness of tone found in the genuine pigment, although it has the advantage of being durable and fast to light, whereas true Vandyke brown is unreliable in these respects. It can be ground in oil, turpentine, or water, but has poor drying properties.
Sepia is a valuable rich dark reddish-brown pigment, obtained from glands in the common cuttle-fish or squib (Sepia officinalis), containing the secretion. The contents of these glands when dry are treated with dilute alkali, which causes precipitation of the colouring matter. This is further refined by reprecipitation by means of an acid. It can be ground in water or oil. Sepia has high staining qualities, and is fairly stable to light. It is rather expensive, and only used for high-class decorative painting. It is one of the favourite colours for decoration in monochrome.
This popular and widely used pigment is made from Prussian blue and chrome yellow, with barytes, the proportion of which varies according to the quality of the green. It is made in three tints – pale, middle, and deep – ranging from a light, rather yellowish green to a deep blue-green.
Brunswick greens, if of good quality, have excellent staining power and cover well. They are, however, only moderately permanent on exposure to weather and tend to turn blue or yellow in time, depending on whether the yellow chrome or Prussian blue content fades the first. Since both chrome and Prussian blue are readily affected by alkali, Brunswick green is unsuitable for distemper colours, nor should it be used in oil paints on surfaces containing alkali liable to be activated – as, for instance, new Portland cement.
This pigment is an oxide of the metal chromium and is prepared in various ways. It has a slightly yellowish tinge but is remarkably permanent and unaffected by acids, alkalis, or heat, and has great opacity. It mixes well with white and all other colours, and is used both for oil paints and for good-grade washable distempers.
This pigment, also known as Guignet’s Green, is also an oxide of chromium, differing only slightly in composition and properties from chromium oxide. It is equally stable and permanent, but more brilliant in colour and rather more transparent in oil.
A name given to a mixture of viridian, zinc chrome, and barytes. As the name implies, the product is fast to light and does not discolour on exposure.
Terre Verte, or Terra Verde
This is a natural silicate of iron and magnesia, first discovered in Verona, but also obtained from Cyprus, the Mendip Hills, etc. It varies considerably in strength and tone, but always gives a good green with a slight bluish-grey tinge. The natural mineral only requires to be ground and can then be mixed in water or oil and with most other pigments. It is permanent to light, and, though containing iron, is not darkened by sulphur or sulphurous fumes. It is not brilliant and has but a poor body. Terre Verte is also known as Veronese Green and Ochre Green. Copper greens are sometimes substituted for the natural earth, but whereas the latter are not darkened by the action of sulphuretted hydrogen or dissolved by dilute acids, the copper greens are blackened by sulphuretted hydrogen and dissolved by acids.
This colour pigment is one of the most brilliant greens which can be obtained, but it possesses the disadvantage of being very poisonous, consisting, as it does, of a very large proportion of arsenious copper. The colour is almost identical with the green of the solar spectrum. The average composition may be taken at about 33 per cent, of copper oxide, and 58 per cent, of arsenious oxide, the balance being made up of various products. The pigment must not be mixed with any other which contains sulphur, such as, for instance, ultramarine, cadmium yellow, etc. Its actual use is largely restricted to that of an insecticide, known as ‘ Paris green.’
Malachite, or Mountain Green, is a pigment prepared from carbonate of copper found in malachite stone. It is ground in oil and water, is permanent and does not influence other colours when mixed with them. It is, however, expensive, and only available for the finest work.
Cobalt Green, also known as Rinman’s Green and Zinc Green, is a mixture of oxides of cobalt and zinc. It has a bright tint. The brightness is increased by the addition of a little phosphoric acid. Cobalt green is permanent and can be safely mixed with other colours and can be used on lime.
Natural ultramarine is prepared from the ground colouring matter of lapis lazuli. But this is very costly and is only used for artists’ colours and in ceramics. The characteristic colour is a deep, brilliant blue, often having a slightly greenish hue, and in some samples changing to violet. By heating while exposed to air a red pigment is obtained. Ultramarines have good body and are permanent. The ultramarine of commerce, used by decorators, is an artificial preparation of china clay, soda-ash, sodium sulphide, sodium sulphate, and rosin. The mixture is roasted, ground, and washed.
Ultramarine is fast to light and unaffected by alkalis, but not by acids. It is widely employed both for oil paints and also in the preparation of distempers and water paints. When used in oil paints, it tends to settle and cake in the container and is also liable, on occasion, to separate out from the medium after application, producing a somewhat streaky finish. Having relatively little opacity, it is extensively used for glazes. Another use to which it is frequently put is in small additions to white pigments to counteract any tendency to yellowness.
This pigment, so called because it was discovered by a Berlin colour maker in 1704, is prepared by mixing a solution of ferrous salt with one of potassium ferrocyanide. Its most notable characteristic is its great staining power. It is fast to light and can be mixed with most pigments. It withstands acids but is very susceptible to alkaline attack, which causes it to turn a brownish shade. For this reason it is unsuitable for use in the preparation of distempers or for use in oil paints intended to be applied over a ground containing active alkali. It discolours, also, when exposed to heat. It was also known in former times as Berlin Blue, but this name has now almost fallen out of use.
A name given to the finest qualities of Prussian blue. Chinese blue has a slight bronze sheen.
A mixture of Prussian blue and barytes. Pale tints of this mixture are sometimes sold under the name of Celestial Blue.
This blue is similar in general properties to Prussian blue, but it is paler in colour. It has, to a large extent, been superseded by Brunswick blue.
This pigment is an oxide of the mineral cobalt and is notable for its brilliance and purity of tone. It is extremely stable and is not affected by acids, alkalis, or heat, besides being fast to light. It can be mixed with other paints and ground in oil or water. In oil, however, it is very transparent and is thus employed frequently for colour glazing. It is, however, too costly as a rule for general work.
This differs slightly in composition and method of preparation from cobalt blue, which, however, it much resembles, though it is paler than the latter and has a slight greenish cast.
This is a relatively new pigment, introduced in 1935, since when it has been fairly extensively used in the preparation of high-grade finishes. It is a beautiful colour, with a staining power greater even than that of Prussian blue. It has the advantage of being fast to light, unaffected by acids or alkalis, and of being able to withstand high temperatures without changing colour.
This important class of pigments has seen great developments in recent years. Originally, the name was applied to pigments or dyes derived from various plants, woods, and insects, as, for example, madder roots, logwood, and cochineal, but nowadays it has a far wider application and includes pigments produced by striking dyes or transparent pigments upon inert bases, such as alumina, barytes, blanc fixe, orange lead, or zinc oxide. Most of the natural lakes are now obsolescent and the great majority of lake pigments are prepared from artificial dyes. The natural lakes still produced on a limited scale include:
This brilliant colour is derived by steeping the dried cochineal insect, collected from the leaves of certain cacti in South America and Mexico. It is insoluble in water, turpentine, or oils but soluble in alkalis, in which its colour deepens. It can be ground in oil or water, but has no permanence and fades badly after a comparatively short period of exposure.
Crimson Lakes, Scarlet Lakes, are prepared from cochineal precipitated on alumina. Although the colours are rich and brilliant, they are extremely fugitive and should not be used for any paint likely to be subjected to the rays of the sun.
These are derived from colouring matter found in the roots of the madder plant (chiefly the Rubia tinctoria, but also one or two other varieties). They are remarkable for their brightness and also for their fastness to light. Though still used to a certain extent by artists, natural madder lakes have been superseded by artificial alizarine so far as paints for decorators are concerned.
This deep-blue pigment is derived from a variety of plants grown in India and the East. It has strong tinting properties but lacks opacity and is not fast to light. Very little of the natural indigo is now prepared but it still finds some use as a pigment for artists.
A range of yellow lakes is prepared from various trees, plants, and flowers. The most important, so far as the decorator is concerned, is that known as Dutch Pink, sometimes called Italian Pink. It is made by colouring a white base, such as Paris white, with a decoction of quercitron bark, procured from a tree which grows in South and Central America, and in the U.S.A. One of the uses of this pigment is as a glaze for subduing the brightness of a colour, as when, for example, it is desired to match paintwork which has faded.
The great majority of lake colours now in use are prepared from synthetic dyes, precipitated on to suitable bases. There are many hundreds of these dyes, covering practically every colour, and their manufacture is a highly complex process. As with most other products, the quality varies considerably, but the best lakes made artificially are remarkably permanent and give excellent results.
A good deal of misconception still exists in the decorating trade concerning the use, by paint manufacturers, of what are commonly known as ‘ extenders ‘ – a term which originated in America. These may be described as inert substances, lacking in opacity, which are added to specific pigments for a variety of reasons. Barytes, asbestine, and whiting are among the substances in question.
It is true that extenders are almost invariably cheaper than the pigments to which they are added, and that in consequence their use means a saving of money to the manufacturer. Employed in moderation, however, they fulfil certain definite and valuable functions which vary according to the nature of the paint in which they are incorporated. In some cases, they supply physical properties which are lacking in the main pigment or pigments. They may, for example, help to keep the pigment particles better in suspension in the medium, thus tending to prevent undue sediment. Again, they may possess the property of reducing gloss and can, for this reason, play a useful part in the preparation of flat paints and enamels.
Firms of repute use in the paints they make only sufficient extenders to fulfil certain requirements and, employed in this way, these substances must not be looked upon as adulterants, since they do not affect the durability of the paint, but endow it with properties it would not otherwise possess. Only when they are used in excess, with the sole idea of lowering the cost of the paint, irrespective of its behaviour in service, can they be considered harmful.
II. The Vehicle or Medium
As already stated, the vehicle or medium of a paint may be a drying oil, a varnish, or a mixture of both. Varnishes will be considered elsewhere and we will here deal only with the principal oils used in paints.
Although in recent years new forms of drying oils have been developed or discovered for use in paints, linseed oil still remains the most widely used paint vehicle, not only because of its own valuable properties but because it is obtainable in abundant quantities more or less wherever flax is grown. The decorator who makes up his own finishes rarely, if ever, uses any other kind of oil.
Up to the end of the last century, North Russia was the most important source of supply and Baltic oil was regarded in the painting trade as the best of its kind. Nowadays, the Argentine is probably the chief exporting country, though Southern India, the U.S.A., and Canada also produce and export considerable quantities.
The principal method of obtaining the oil is by grinding or pressing the flax seed, though a good deal is now extracted by means of solvents. Before crushing, other seeds, such as rape or hemp, which are often found among the flax, are removed by machinery. The oil, when it first emerges from the press, contains a certain proportion of impurities, in the form of water, natural salts, and albuminous matter, and these, too, must be removed. The presence of these impurities gives the oil a cloudy appearance; in storage it clarifies and the mucilage (commonly known as ‘ foots ‘) sinks to the bottom of the tank. Formerly, the method employed was to expose the oil for many months in glass-covered shallow tanks; on ageing, in this way, it became progressively lighter in colour. This process produces an oil which is probably superior in quality to that obtained by any other means, but it is too slow and costly for ordinary purposes, and the bulk of the oil produced is now refined by chemical means.
Traces of gummy matter in the crude oil remain in solution in the oil, but turn insoluble and cause a jelly-like deposit to separate out when the oil is heated – as when boiled oil or varnish is produced. It is more economical to remove these traces by preliminary refining than as ‘ foots ‘ from the finished product. Removal is effected in various ways; the oil may, for instance, be treated with Fuller’s earth which absorbs the glutinous matter: alternatively, caustic alkali can be added to neutralise the free fatty acids and form a soap which separates out; or, again, the oil can be treated with sulphuric acid. After treatment, the oil is allowed to settle until it is clear and is then refiltered. The refined oil is paler in colour and perfectly limpid.
Linseed oil dries by absorbing oxygen from the air, and this property of absorbing oxygen is increased by heating the oil for some hours and adding a proportion of driers in the form of lead oxide and manganese oxide. The latter decompose in the heated oil and form soluble metal compounds of the oil itself, the heating of the oil in the presence of these drying agents imparting a dark colour to the oil. Whereas raw linseed oil tends to become lighter when exposed to strong sunlight, boiled oil is inclined to become slightly darker. There are various types of boiled oil, some of which are extra pale and some extra-quick drying.
In the preparation of various types of paints, varnishes, and other finishes, linseed oil treated in a number of ways may be employed. It may, for instance, be thickened or ‘ bodied ‘ by having a current of air blown through it. ‘ Blown ‘ oil, as it is called in this form, is slightly thicker in consistency than ordinary boiled oil and yellower in colour. Its smell is reminiscent of that of oiled silk.
The decorator seldom encounters blown oil, which has a tendency to form rather thick, soft skins. For the manufacturer, it has useful properties, being rather more elastic and moisture-resisting than boiled oil, as well as paler in colour.
In the preparation of certain forms of slow-drying enamels and enamel paints, what is known as ‘ Stand ‘ oil is frequently used. This form of linseed oil originated in Holland about 1865; it is produced by heating the oil without the addition of any driers, until the required degree of stoutness is obtained. Cooking proceeds for several hours at a temperature of just below 6oo° F., and steps are taken to ensure that the oil does not become too dark in the process.
This, again, is essentially a manufacturer’s oil and is seldom used by the decorator to make up his own paints in this country, though some continental painters occasionally employ it for this purpose. Stand oil is a viscous fluid, slightly thicker than treacle. It yields a film which is rather soft and is, therefore, often combined with varnish in order to harden it. An enamel properly made with first-class stand oil has great durability and elasticity.
Raw and Boiled Oil Compared
So far as the ordinary decorator is concerned, the choice lies between raw and boiled oil and a great deal has been written on their comparative merits. Probably the majority of painters prefer raw oil for general purposes. It works more freely under the brush, dries reasonably quickly and is capable of satisfying the suction of surfaces of average absorption. It is at its best when used with white lead for which it has a natural affinity; this can be seen in the ability of the pigment to absorb the oil even in the presence of water. With zinc oxide, raw oil is not so suitable; unless stand oil or a similar ‘ bodied ‘ oil is used, or a suitable varnish added to raw oil, zinc oxide tends to produce rather a brittle film.
Boiled oil gives a film which has a better initial gloss and which dries more quickly than when raw oil is used, and these are unquestionably advantages. Against this must be put the fact that a boiled-oil paint does not brush out so readily, and probably is not quite so durable. The film remains softer and consequently boiled oil is not usually satisfactory for use in undercoats. Where the surface is unduly porous, boiled oil is to be preferred, since there is less tendency than in raw oil for an undue proportion of the vehicle to be absorbed, leaving the pigment underbound on the surface. For pigments which are poor driers, such as Vandyke brown or the carbon blacks, boiled oil has some advantages.
Tung Oil, or China Wood Oil, is derived from the nut of Aleurites cor data, a tree which flourishes in China. The oil is of pale-yellow to brownish tinge and is slightly viscous. It is used almost entirely by manufacturers for the production of high-grade enamels, varnishes, and other finishes. When dry, a film of tung oil is duller and more opaque than that of linseed oil and has a curious crinkled appearance. A peculiar characteristic is that it slowly gelatinises on heating. It is regarded as one of the most valuable drying oils for paints, since it has great powers of moisture resistance and dries rapidly. It is largely used for making high-grade quick-drying finishes.
The main source of supply of tung oil is China and during the Sino-Japanese and the recent World War exports from that country practically ceased. The irregularity of the supply in previous years, the high price, and the rather crude methods of collection by the Chinese, led to experiments in cultivating the trees being carried out in various countries, notably in the U.S.A. and in certain parts of the British Empire. The results, especially in America, are promising, but it must be a long time before the trees yield on a large-enough scale to make much appreciable difference.
This oil, derived from the nuts of a tree which grows in Brazil, has long been known to have possibilities for use in paints, but it was not until the Sino-Japanese War, when supplies of tung oil began to be shut off, that any serious attempt to make use of it was begun. Since it possesses properties not unlike those of tung oil, it seems probable that it will play an important part in the finishes of the near future. Hitherto, although the trees usually yield heavy crops of nuts, the immense distances to be covered in collecting them, and transport difficulties in general, have restricted the amount of oil which reaches the paint manufacturer, and most of it is used in America.
The inclusion in this list of castor, which has long been regarded as a typical non-drying oil, needs a brief explanation. The drying of oils depends on their taking up oxygen from the air and their ability to do so is determined by what is usually known as their ‘ degree of un-saturation.’ All natural oils are mixtures of various fatty acids combined with glycerine and there are found in all of them mixtures of drying and non-drying glycerides, the relative proportion of which determines whether or not the oil will dry.
In castor oil, the low degree of unsaturation is due to the fact that the glyceride has been chemically combined with water, which prevents the oil from taking up oxygen. Attempts had been made from time to time prior to the last war to remove this water and, when the shortage of tung oil became acute, efforts were intensified, with the result that it was found possible to produce a ‘ dehydrated ‘ castor oil which would dry quickly, have good resistance to alkalis and good durability. This was used on a considerable scale in the war, notably in the production of synthetic-resin finishes.
This oil, which up to the present has been used only on a relatively small scale as a paint vehicle, is produced from the seeds of a plant grown in China and the Far East. Its properties are akin to those of linseed oil. Its drying properties are not particularly good but can be improved by the addition of suitable drying agents.
Poppy Oil; Walnut Oil
These oils, the first derived from the seeds of the opium poppy and the second from the ordinary walnut, are almost exclusively used in the preparation of artists’ colours.
Fish Oil, prepared from the menhaden fish, is being largely employed in the United States in the preparation of paints, replacing linseed oil, which it closely resembles. It has a slightly brownish colour, but when refined is of a pale straw tint; it is not used to any great extent for paints in this country, mainly due to its odour, which is somewhat objectionable.
III. The Thinner
So far as the decorator who makes up his own paints is concerned, the two main forms of thinner are turpentine and turpentine substitute, usually referred to as white spirit.
Turpentine is the oldest and still the most important solvent or thinner for oil paints, although substitutes are extensively used for it, particularly by paint manufacturers and by many decorators. The best of these substitutes are probably quite as satisfactory as genuine turpentine in most respects and, in one or two, may even be superior to it, but the average painter, especially if he be of the old school, still as a general rule prefers to use genuine turpentine whenever the cost of the work allows him to do so.
The source of turpentine is the resinous exudation from certain trees of the pine family in the U.S.A., France, Spain, Greece, India, the U.S.S.R., Scandinavia, and a few other countries. America is by far the greatest producer, turning out each year more than twice the amount made by the other countries put together. Actually, so far as essential qualities are concerned, the properties of all true turpentines differ very little, no matter from what country they originate, and between the best grades of each there is not much to choose. There are distinct differences in smell, Russian turpentine, in particular, having rather a smoky, disagreeable odour, and some differences in colour. It is due mainly to prejudice, however, that in this country there is a strong preference for American turpentine.
Turpentine is produced both from the living trees and from pine stumps, fallen branches, and trimmings of trees which have been felled, the latter type being known as ‘ wood ‘ turpentine. There is some slight difference in the chemical composition of the two types, but they are the same for all practical purposes. In the U.S.A., turpentine is classified according to the four methods by which it is produced as ‘ gum spirits of turpentine ‘ (otherwise known as ‘ spirits of turpentine ‘ or ‘ oil of turpentine ‘), ‘ steam-distilled wood turpentine,’ ‘ destructively distilled wood turpentine,’ and ‘ sulphate wood turpentine.’
The crude gum is collected in most turpentine-producing countries in much the same way, by making a cut in the trunk of the pine so as to sever the resin ducts and affixing to the tree a cup or container into which the resin exudes. A certain amount of the gum is distilled in crude stills but – at all events, in America – the bulk of it is dealt with in industrial plants by modern methods. The gum is loaded into copper pot stills with a little water. When the still is heated, turpentine and water vapours are expelled and condensed in the copper coil, whence the two liquids flow into a separator; here they divide into two layers, with the turpentine on top, and are automatically drawn off. In some plants, the turpentine layer is further dehydrated by being passed through a device containing rock salt which absorbs any water not previously accounted for.
Wood turpentine is produced by a steam distillation process, in which the wood is ground to powder and placed in the heated still, into which steam is forced. The volatile components are freed by the heat and carried to the condensing coil by the steam. The destructive distillation process involves placing pine branches and stumps in a retort, which is sealed and heated, the volatile components being carried off and condensed. Sulphate wood turpentine is recovered as a by-product when paper pulp is produced from pine wood.
Properties: Good-quality turpentine should be colourless and should evaporate almost completely on exposure to the air, though a slight gummy residue, hardly visible to the naked eye, may be left at times if the turpentine has been kept in a container which has not been completely sealed.
It is sometimes alleged that turpentine may have toxic effects on the human body and a number of diseases, including Bright’s disease, painters’ colic, and hardening of the arterial tissues, have been attributed to it. Some years ago, a Government Commission investigated the matter but was unable to find any foundation for these suggestions. On the other hand, it cannot be denied that some people are affected by turpentine vapour which is liable to cause them headaches and consequent nausea.
Properties as a Thinner: The main reasons why turpentine is so excellent a thinner for oil paints can be summarised as follows:
It is a good solvent of linseed oil and other drying oils, but does not dissolve an old hard paint film, so that fresh paint can safely be applied on top of the old without seriously disturbing the latter. It probably has some very slight superficial softening effect on an old coating (unless the film is very hard): this may help to make the new coating adhere more firmly.
It evaporates at a rate satisfactory for brush application in normal conditions and sufficiently slowly to keep the paint at a suitable working consistency. It gives a good flow to the paint mixture and helps it to work easily under the brush and level out to a film of uniform thickness.
It has a comparatively high flash point and thus can be handled with safety. Being itself of a resinous nature it has an affinity to wood and is therefore a desirable ingredient of paint for use on wood surfaces. Its ability to carry oxygen into and through the paint film helps the latter to dry and harden.
Turpentine should not be kept in metal containers unless the metal is treated with shellac or other impervious material to prevent actual contact with the fluid. It is best stored in glass or earthenware jars which must be kept carefully sealed; if it is exposed to the air for any length of time it is liable to take on a syrupy consistency.
The purified resin of the common larch. A yellowish, viscous body, soluble in volatile and fixed oils, ether, and alcohol.
From the painter’s point of view, the chief objection to pure turpentine is its price and consequently turpentine substitutes are widely used. They are mainly obtained by distilling and refining petroleum; crude petroleum includes a number of spirits and oils which have the same proportionate chemical composition and similar properties, but which evaporate at different rates and boil at different temperatures. The main divisions into which the original product is separated by repeated distillation are called ‘ fractions,’ the lowest boiling fraction being light petrol and the heaviest, a range of lubricating oils. In between, in order of heaviness, are heavy petrol, benzine, white spirit, and petroleum spirit.
When first the medium fractions of petroleum spirit began to be used as thinners for paints and varnishes, they were far from satisfactory for the purpose. They had an unpleasant odour, due to the presence of traces of sulphur and other impurities, while there was no exact specification governing the rate of evaporation; there were often, too, small percentages of oily distillate. Since then, great improvements in refining and fractionating have been made and good-quality white spirit is now available at a reasonable price: incidentally, although all kinds of turpentine substitute are commonly referred to in the paint and painting trades as ‘ white spirit,’ this term should properly be applied to petroleum distillate which exactly conforms to the British Standard Specification for white spirit; other petroleum distillates are better referred to as turpentine substitutes.
When it was first put on the market many years ago, decorators, varnish manufacturers, and other users of turpentine were frankly opposed to it, and it must be admitted that, in its early days, there was some excuse for their prejudice against it. Since that time, however, great improvements have been made in its production, and although some people are still averse to its use, it is generally agreed that if it is free from heavy or greasy end-fractions and of a highly refined quality, there can be no logical objection to its use, and true economy will be affected thereby.
Some enthusiasts claim that, for certain purposes, white spirit is better than pure turpentine, and in one or two cases there is probably some truth in this assertion. In the preparation of the wax-type flat finishes and varnishes, for instance, turpentine is apt to prove too good a solvent for the particles of wax than is desirable, and any wax not in solution is swollen into rather large flat particles, whereas, if white spirit is used, they are maintained in finer form.
For lead paints and quick varnishes containing a high proportion of lead driers, however, pure turpentine, or, failing this, a mixture of pure turpentine and good-quality white spirit, is preferable to white spirit by itself, while in high-gloss enamels and good-quality varnishes turpentine is generally considered the correct thinner to use. For any proprietary brand of paint, white spirit should not be employed as a thinner by the decorator unless its use is clearly indicated as permissible by the manufacturer.
The heavier fractions, such as kerosene or lamp oil, are not satisfactory for thinning paints and varnishes; they contain a substantial amount of heavy oily petroleum which does not evaporate – or only very slowly – in a normal room temperature. The practice of adding paraffin to retard the drying of flat wall finishes or oil scumbles which set too rapidly under the brush in warm weather is consequently to be condemned.
A number of other solvents are employed in the making of paints, varnishes, and enamels, but as they concern the manufacturer rather than the painter there is no need to discuss them at any length in this work. They include:
A colourless liquid, derived from coal tar. Owing to its great penetrative properties, it is used as a solvent for dyes employed for wood stains. It is also an ingredient of many paint removers.
Another coal-tar distillate, distilled in two fractions – light solvent naphtha and heavy solvent naphtha. The former is chiefly employed as a solvent for rubber but the latter is fairly extensively used in paint manufacture for certain quick-drying paints and in primers for woodwork. Unlike turpentine or white spirit, it has solvent action on old-paint films and thus, except in modified form, is unsuitable for paints other than primers. It has exceptional penetrative properties and fairly strong germicidal powers.
This spirit is distilled from various hardwoods and is used largely in the manufacture of spirit varnishes.
This is an alcohol, with a small proportion of adulterant and distinctive colouring matter, in order to make it unfit for drinking. One of its principal uses, so far as the painting trade is concerned, is as a solvent for shellac, to produce knotting and French polish. In the U.S.A., it is known as ‘ denatured alcohol.’
This colourless, limpid fluid is well known for its ‘ pear-drop ‘ odour and is extensively used in the preparation of nitrocellulose lacquers.
This highly inflammable organic solvent is extensively used in the manufacture of cellulose ester lacquers, aeroplane dopes, and paint removers. It is one of the few solvents which will act upon linoxyn (oxidised linseed oil), and this makes it particularly valuable as a paint remover, though it evaporates rapidly when exposed to the air.
IV. The Drying Agent
Driers (also known as Dryers) are substances added to the mediums used in many paints and varnishes to enable the oil to absorb oxygen at a sufficient rate to dry within a reasonable time. Without them, air-drying oil paints would, in most cases, take many days to dry, and ordinary painting processes would be quite unworkable.
The function of driers is to combine with oxygen and pass it on to the drying oil in the paint or varnish, thus accelerating the drying or oxidation of the film. Once the latter has begun to harden, the absorption of oxygen is slowed down; if this were not the case and the oxidation continued at the initial rate, the film would become hard and brittle and would soon disintegrate; as it is, a certain amount of progressive oxidation goes on even after the film is hard because the driers, though less active, continue to exert their influence so long as further oxidation is possible.
The drying agents commonly used are mainly compounds of certain metals, notably lead, manganese, and cobalt. Their properties vary in strength and character: lead compounds have a powerful action but are liable, in certain circumstances, to darken the oil to which they are added. Manganese compounds do not bring about this discoloration but their action, though energetic, is more erratic and less dependable than that of lead. Cobalt has a stronger effect than either lead or manganese, but is also the most erratic, and for this reason is usually combined with lead. It is common practice, in the making of paint, to combine two or more of these drying agents and thus to produce a more stable agent.
The compounds referred to, and certain other drying agents, are used by manufacturers in the preparation of ready-mixed finishes made in bulk, but are too powerful to be employed by the painter in their original form. The driers which he buys are therefore substantially reduced. They reach him in two main forms – paste (or patent) drier, and liquid drier.
There are wide variations in the quality, efficiency, and formulation of paste driers, but in the main they consist of one or more of the drying agents referred to above, with the addition of inert pigments such as barytes, Paris white, or similar pigments which have little opacity in oil and which can consequently be added to dark-coloured paints without making them noticeably lighter or paler.
The ingredients for paste driers are ground in oil, but they frequently contain small percentages of water which is added to dissolve the metallic drying salts. For this reason, they are not compatible with gum and oil varnishes, and there is some risk in adding them to paints which contain the latter. Paste driers are, however, widely used for decorators5 paints and are popular with some grainers since they have the property of preventing the graining colour from flowing together after being combed, and thus act as a kind of megilp.
These include liquid oil driers, terebine, and gold size. The first-named uses linseed oil as the main reducing agent and is available in pale form to avoid discoloration of white or light-coloured paints. It is not so powerful as terebine; this is prepared in various ways, usually by heating linseed oil, adding lead and manganese, and continuing the heat treatment until these are dissolved and the mixture very dark: on cooling, it is thinned with turpentine and white spirit. Pale terebine is also available.
Gold size, though not primarily a drier, is often used in that capacity. It dries quickly to a hard film, without much gloss.