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White 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.-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
hydrocarbon ate.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 outside 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 pigments.
White lead is perfectly s.table 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" 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 tha't 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.-TThe 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 propor.. tions 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 frequently 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. Litbopone.-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 banum 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. Titanium Oxide.-A comparatively new addition to the range of white pigments and one which has great possibilities in decorative work.
It is a brilliant white, non-poisonous, and unaffected by heat or acids.
It has great obscuring power and exceptional body and density.
It is completely inert and possesses no drying power of itself; consequently, when used alone in paint, driers are essential, probably the best results being obtained from the incorporation of a mixture of the linoleates of lead and cobalt, or of lead and manganese.
I t mixes well with other pigments which may be advantageously added to make good some of its shortcomings: used by itself, it produces rather a soft film with a tendency to chalk, so that zinc oxide added to it will, generally speaking, produce a harder, better-wearing film.
The addition of white lead, moreover, will reduce the proportion. of driers which have to be added, and thus increase the durability of the paint on exposed surfaces.
The chief disadvantage attached to the use of titanium is its expense, which has prevented it from being so widely employed as it deserves to be.
Antimony Oxide.-Although many attempts have been made at
various times to use antimony white as a paint pigment, it was not until 1919, when it was introduced under the proprietary name of" Timonox," that it proved successful.Antimony oxide has many valuable properties, and makes a useful substitute for white lead, where objection is taken to paints made with the latter.
It possesses great opacity and compares favourably with white lead in this respect, and is extremely durable.
It is non-poisonous.
When used by itself, however, it is somewhat soft, a fault which can be corrected by combining it with a pigment such as zinc oxide.
It is completely inert and does nothing to promote the drying of the oil content of a paint, so that the addition of driers is essential.
Barytes.-Barytes is a white mineral largely used by paint manufacturers, the chemical name being" sulphate of barium."
Natural barytes is made from heavy spar which is found in various parts of the country, notably Derbyshire and Cornwall.
It is first broken into lumps, then reduced to powder, after which it is bleached to free it from the yellow colour which often arises from the oxide of iron contained.
This bleaching is done by adding hydrochloric or sulphuric acid, followed by thorough cleaning in pure water, and finally drying in a stove.
This pigment is but little understood by the average decorator, who regards it as being at all times an adulteration.
This is, however, by no means the case.
Its use is sometimes quite necessary, as in the production of the lakes, Brunswick green, etc., when it becomes the base upon which the actual colouring matter is struck.
The fact that it is perfectly inert, is not affected by acids or alkalis, and that it is comparatively cheap, renders it a valuable adjunct to the paint-maker's resources, provided that it is used in moderate quantities and with discretion.
Its one drawback, and a very serious one, is that it almost wholly lacks body or opacity.
When ground in oil it is so transparent that half a dozen coats will not hide the grain and knots in wood.
It is used, to an enormous extent, as an extender or cheapener of paints.
Blanc Fixe is a heavy, brilliant white pigment, possessing excellent covering powers.
A by-product in the manufacture of dyers' chemicals, it is an artificial barium sulphate, actually a precipitate of soluble barium salt and sulphuric acid sulphate.
It is used in the preparation of Orr's zinc white, lithopone, the lakes, and in the printing of wallpapers.
Zinc Wbite.-A generic term once applied exclusively to Zinc Oxide, but now employed to include all paints having a zinc pigment as a basis.
Wbiting.-A preparation of chalk, the produ'ct being a dull, white, rather soft powder.
It is insoluble in pure water but readily soluble in dilute acids.
It is slightly alkaline in reaction, and there is consequently some risk in mixing it with pigments such as Prussian blue, chrome yellow, or Brunswick green, all of which are susceptible to alkaline attack.
Although opaque when used with water, it becomes almost transparent in oil, and is consequently of little value in the making of oil paints.
It is largely employed in the making of distempers and water paints and is the basis of putty which is produced by grinding whiting in linseed oil.
The finest grade of whiting is known as Paris white.