Chapter VII: Whites (3)

It is believed that the production of white lead in Italy during the first ten years of this century was about 300 tons per annum. The total annual production at present is, in round numbers, about 3500 tons, of which 2800 are produced in the Ligurian manufactories, and about 300 in those of Naples. Milan also produces about 300 tons.

The product finds a market almost exclusively in Italy. The annual amount exported is about 300 tons, and goes chiefly to Constantinople. Of the quantity of 2000 tons per annum produced at Cogoleto, about 300 tons is sold mixed with oil. No exact statistics are attainable as to the total amount of Italian white lead which is annually sold other than in the dry state, but it is believed to be little, if any, larger than 300 tons.

When the price of white lead was high, sulphate of baryta was mixed with carbonate of lead to produce white lead of inferior quality, but in consequence of the present price of white lead the use of sulphate of baryta has almost entirely ceased. No other methods of adulteration are known to be in use. In Italy, white lead is universally known as “_biacca_” when it is sold in cakes. When sold in powder it is known as “_carbonato di piombo_” (carbonate of lead). The lead from which all the white lead made in Italy is manufactured comes from the lead mines of the island of Sardinia, with the exception of a very small quantity of argentiferous lead coming from Spain. The acetic acid used in the process of manufacture comes from France. The market price of white lead in Italy is now about 45 lire the quintal (equivalent to 18_s_. per cwt.).

_Lewis’s Process._--Many attempts have been made to substitute for carbonate of lead--the ordinary poisonous white lead, that slowly but surely induces paralysis in those who come in contact with it for any considerable period--some less deleterious pigment. Zinc white has often been put forward as a substitute, and is indeed largely employed; but it is open to the objection of not possessing sufficient body or opacity. Sulphate of lead is not poisonous; but, when prepared in the ordinary way by precipitation, is of a crystalline nature, and, therefore, wanting in both these qualities.

A sulphate of lead has, however, been produced by John T. Lewis, of Philadelphia, by sublimation, which, when treated by the Freeman process, is stated to possess a body and colour superior to the best white lead made by the ordinary process, with the additional merit of being cheaper, while it is also non-poisonous.

In the smelting of lead ore into pig lead, 15 per cent. goes off in fumes, and 10 per cent. are all that it has hitherto been found possible to recover; by this process, however, the whole 15 per cent. are recovered. The sulphate may also be produced from a low quality of galena, or lead ore, which is not suited for smelting into metallic lead, and from the slag formed in the process of smelting.

The plant consists of simple subliming furnaces, iron cooling pipes, suction fans, and a series of flannel or calico bags, arranged vertically in a building, well ventilated, so as to allow the filtered gases to escape, leaving behind the sublimed white lead, which is then merely shaken down from the bags into barrels placed beneath them. The manufacture is carried on at Joplin, Missouri, with four subliming furnaces, about 500 feet of cooling pipes and towers, with suction fans, which drive the fumes into 300 bags, 20 inches in diameter, and 38 feet long, arranged vertically. About 50 tons weekly of white lead are now being produced from waste fumes, slag, and poor ore, by this establishment alone, at a cost not exceeding that given in the following figures:--

£ _s._ _d._

20 cwt. of galena (with 82 per cent, of ore) 8 0 0
Cost of subliming and catching, including repairs,
labour and all expenses 1 10 0
Casks 0 6 0
------------
9 16 0

Freeman’s process (see p. 254) consists of grinding together, in a dry state, under great pressure, and consequently with great friction, sulphate of lead and sulphate of zinc. While neither of these two substances alone possesses good body or opacity, when treated by this process they are so changed in character that the new substance is stated to be superior in these respects to the best form of ordinary white lead.

The white lead thus obtained mixes well with oil, and has also the advantage of not becoming blackened when exposed to the fumes of sulphuretted hydrogen, and of not peeling off in a saline atmosphere. As a basis for coloured paints, it is recommended on the ground that, being decomposed with greater difficulty, it can be mixed with almost any colouring substance; and, being free from acid, it does not change the tints of other substances.

The details of Lewis’s process, and the plant employed, are more fully set forth below.

Heretofore the manufacture of dry white lead from galena or the native sulphurets has been effected by roasting or desulphurising the galena, and then mixing the residue, after roasting, with carbon, and subjecting the mixture to the action of heat in a compound reducing or subliming and oxidising furnace, and collecting the resulting fumes in textile bags.

Lewis’s process, however, is based upon the discovery that by subliming unroasted galena, or the native or raw sulphuret of lead, and then oxidising the volatile products, cooling the fumes, and collecting them by means of a textile fabric, a superior basis of a pigment can be obtained. The admixture of carbon with the raw ore will facilitate the subliming process, or it may be carried on in a muffle or reverberatory furnace without the previous admixture of carbon.

The furnace, which has been found to answer well for the purposes above mentioned, is commonly known as the Wetherill furnace, and is represented in plan, in Fig. 21; in front and back elevations, in Figs. 22, 23; and in central longitudinal section, in Fig. 24.

_a_ represents the main chamber, the bottom _b_ of which is composed of iron bars perforated with small holes of about ¼ inch in diameter and about 1 inch apart, and preferably made slightly conical, with the larger diameter downward, that is to say, the said holes are of such a size as to prevent

Fig. 22.

Fig. 23.

Fig. 24.

LEWIS’S WHITE LEAD PLANT.]

the escape of the crushed ore and coal. These perforated bars are suitably sustained at the ends on the front and back walls of the furnace. The ash pit below the perforated bottom is of equal area therewith, and is provided with a door _e_ in front, and with a hole _f_ at the back for the reception of a pipe from suitable blowing apparatus.

The walls _g_, and arch on the top should be built of some refractory substance, such as fire-brick.

The front of the furnace is entirely open, and is provided with sliding doors _i_, by which it can be closed when working, and opened to remove the residuum.

At the back of the furnace there are two slides _j_, to permit access to the main chamber, for stirring the charge and for inspection.

At the back, near the arch, there is a hole _k_, governed by a sliding damper leading to a chimney, for carrying off smoke and impure gases at the commencement of the operation upon a new charge.

In the centre of the roof there is an aperture _l_, governed by a damper or sliding door _m_, the said aperture leading to a suitable apparatus for the collection of the oxidised vapours of lead.

The exterior walls _n o p q_ may be built up to form two feeding troughs _r_, one on each side of the arch or roof, and each provided with an aperture or passage _s_, communicating with the inside or main chamber, and each aperture or passage is provided with a cover to be put on after the furnace has been charged.

In working with this furnace, crushed ore (native sulphuret of lead) and carbon, preferably in the state of pea or dust anthracite coal, are mixed in equal proportions; the mixture is ignited, and the fumes are oxidised by the blast through the mixture, which also promotes the combustion. Dense white vapours or fumes pass off, and are conveyed to a separate chamber, where they are strained by passing through a screen or series of screens of muslin or other textile fabric. Lime may also be employed in the furnace, in the proportion of 200 lb. of lime to 400 lb. of galena, although the addition of the lime is not necessary in all cases.

Instead of the furnace above described, a muffle furnace may be employed, in which the heat is applied indirectly to the ore, with the precaution of constructing the sole or bottom of this furnace of a material not rapidly acted on by the constituents of the ore; or a reverberatory furnace may be used in which the heat is directly applied. In both of these two cases reducing carbon may or may not be mixed with the ore.

Sometimes, generated gas is employed in place of coal, to effect the same result.

In a later specification Lewis remarks that when white lead pigment is manufactured from galena or other lead ores, in the raw state, or even in the roasted state, by subjecting them to the joint action of heat and air, either with or without reducing means, according to the quality of the lead ores used, the fumes are discoloured by particles of carbon or sulphuret of lead, or both, when they are caught in bags of textile fabric, and are unfit when in this state for use as a white pigment.

The fumes which are produced by this action of heat and air on galena or other lead ore are cooled and then collected in bags, and Lewis prefers to expose the so-collected products to the joint action of heat and air, to destroy or to burn out all the particles of carbon or sulphuret of lead, or both, by either throwing the said fumes on a bright clean anthracite or coke fire, with a blast from the sides or from below, or by throwing them over such fires or into a cupola furnace, or by throwing or blowing them into a generator gas flame, or through externally heated retorts. He then in either case collects the escaping fumes from the furnace or retort in bags or screening chambers.

The best process to be adopted depends upon the kind of fuel in the locality where the fumes are refined, and also upon the purity of the lead fumes. If they contain iron, clay, or the like, it is best to throw them into a coal fire; but if they are pure, one process is about as effective as the other, and the degree of purity of the fuel decides the kind of heating apparatus to be used.

There being such great difference in the purity of fuels, and this irregularity not allowing of uniform results, Lewis prefers to use a furnace in which the flame and heat are produced by burning gaseous fuel with air, which is forced into the furnace with the fumes which have been collected in a previous process.

Fig. 25 represents a furnace which may be advantageously employed for the purpose when gas is used as fuel.

_a_ represents a blower, into which the fumes are fed from the hopper _b_. The fumes, being thoroughly mixed with air in this blower, are forced into a chamber _c_, and then through a series of tuyeres _d_. At the same time, gas from a generator or producer is admitted by the flue _e_, and is burned by the incoming blast from the tuyeres _d_; the volatile fumes produced in the furnace _f_ pass through it and out of the flue _g_, and are collected in bags or screening chambers. By using gas fuel, which is easily and fully burned, and clean to handle, a fine white pigment is produced.

_MacIvor’s Processes._--The name of MacIvor is a familiar one in improvements in manufacturing chemistry, and not the least in connection with the subject of pigments, sometimes in conjunction with other inventors.

In 1889 MacIvor and others introduced some modifications in the process of producing white lead or carbonate of lead, by the treatment of oxide of lead (litharge or massicot) with a solution of acetate of ammonia, whereby the oxide of lead is transformed into hydrate and acetate, which are subsequently converted into carbonate of lead by the injection of carbonic acid. The hydrate and acetate of lead, in presence of free ammonia formed in the reaction, are quickly decomposed by

the carbonic acid, yielding the final product, namely carbonate of lead, and re-forming the acetate of ammonia.

The rapidity with which the conversion of oxide of lead into hydrate can be effected depends upon the strength of the acetate of ammonia solution employed, that is to say, the weaker the solution the slower will be the conversion. It has been found that, for commercial purposes, the solution of acetate of ammonia may be used with advantage of a strength which need not be more than 25 per cent. nor less than 5 per cent. A strength of 25 per cent, operates in a comparatively short space of time; but a strength as low as ½ per cent. will effect the hydration if there be a sufficient quantity of the weak solution, the lead oxide being in a fine state of division, and time of no object. This ½ per cent. strength, however, or any strength below 5 per cent., is not recommended for commercial purposes, having regard to the time required for completing the operation.

The conversion of the oxide of lead into the hydrate and acetate of lead is effected in the cold (heat may be used, but for commercial operation it is not recommended). The conversion is facilitated by employing a mechanical arrangement, similar in many respects to that adopted in a previous specification, but modified by connecting the digesting vat with a cistern or vat containing acetic acid, whereby any free ammonia carried over during the operation may be absorbed. Also by introducing carbonic acid to the mass of hydroxide and acetate of lead formed by the oxide of lead and acetate of ammonia, either by a series of concentric rings perforated with small holes in the bottom of the vat itself, or by passing carbonic acid down a hollow shaft to which are attached stirrers, and through perforated tubes attached to the blades of the stirrers, or by any other means that may ensure the thorough saturation of the hydroxide and acetate of lead so as to form carbonate of lead.

The solution of acetate of ammonia may be recovered from the white lead, and be repeatedly used, for the conversion of further quantities of oxide of lead into hydrate and acetate of lead, which hydrate and acetate are converted into carbonate of lead by the injection of carbonic acid. Theoretically, a given weight of acetate of ammonia in solution, used in conjunction with carbonic acid, should be capable of converting an unlimited quantity of oxide of lead into carbonate of lead; but during the manufacture of white lead by this process, it may be reckoned that there will be a loss of ammonia acetate varying with the strength of the solution employed, but it should not exceed 10 per cent. on each charge. This loss arises in the washing of the final product, and through the escape of some ammonia during the process.

Fig. 26 is a vertical section of the apparatus employed.

_a_ is a vat, which may be made of wood or other material capable of resisting the chemicals employed; it may conveniently be 6 feet in diameter and 4 feet deep, and it is provided with a closely fitting cover, _b_ is a cistern situate at a higher level, and intended to contain a solution of acetate of ammonia, _c_ is a pipe by which the solution can be drawn down from the cistern _b_ into the vat _a_; a cock is provided upon this pipe, as the drawing indicates. There is a man-hole _d_ in the cover of the vat, and the vat contains an agitator _e_, with a vertical shaft which can be turned by gearing _f_ as shown, or the agitator may be driven by any suitable motor. The shaft of the agitator _e_ is hollow, and pipes _g_, which stand immediately behind the stirring tines, are connected with the hollow shaft, to deliver the carbonic acid gas into the vat; or this may be effected by means of coils of pipe laid at the bottom of the vat, and pierced with small holes. The pipes _g_ are open at their lower ends. _h_ is a cock by which the liquor can be drawn off from the vat into the receiver _i_. A pump _k_ is provided upon the cover of the receiver, by which the liquor may be returned into the vat _a_. _l_ are outlets by which the white lead is discharged from the

vat into the washing cisterns _m_ and _n_. _o_ is a man-hole, which may be opened to facilitate the emptying of the vat. _p_ is a plug which is removed to let the white lead run out of the vat. _r_ is an ammonia catch box, charged with acetic acid.

All the metal-work of the apparatus with which the acetate solution comes into contact should be of such a character as to resist corrosion, or should be coated with a material capable of withstanding attack by the chemicals employed in or formed during the process.

The operation is by preference conducted in the following manner, but the details admit of variation. The charge of monoxide of lead for an apparatus of the dimensions indicated may weigh about 1120 lb.

The monoxide should be in fine powder, and may be either moist or dry. Having received this charge, which is introduced by the upper man-hole _d_, the vat _a_ is closed, and a solution of acetate of ammonia is let down upon the charge from the cistern _b_, or pumped out of the receiver _i_. The vat _a_ should be charged with the solution of acetate of ammonia in the proportion of three parts of said solution to one part of lead monoxide by weight. It is convenient to employ a solution containing 5 per cent. of acetate of ammonia, and the quantities above stated are suited to a solution of this strength; but the strength of the acetate solution may be varied within wide limits, as hereafter explained.

The charge of monoxide of lead and acetate of ammonia in the vat should be kept constantly stirred by the agitator until it becomes whitish in colour, when it will be found that the monoxide of lead has become converted into hydrate and acetate. The workman will know that this change is complete when the reddish or yellowish appearance of the monoxide of lead disappears, and the mass in the vat becomes whitish in colour.

The hollow axis of the agitator is connected with a pipe, by which carbonic acid gas is supplied to it under pressure sufficient to cause the gas to pass through the contents of the vat. This gas escapes at the lower ends of the pipes _g_, and ascends through the liquid in which the hydrate and acetate of lead are held suspended. A free flow of this gas should be maintained, and some excess allowed to pass away by a pipe on the cover into the ammonia catch box _r_. The pipe dips down into the acetic acid which this box contains, and any ammonia passing off with the carbonic acid gas is caught by the acid, and forms acetate of ammonia. When the hydrate and acetate of lead are completely converted into basic carbonate (and this the workman will know by its changed appearance), the motion of the agitator is caused to cease, and the white lead is allowed to settle.

There is a gauge glass on the side of the vat _a_, and in this glass the changes of appearance can be recognised by which the workman regulates the process.

When the white lead is deposited, the liquor is drawn off into the receiver _i_ by opening the cock _h_. The plug _p_ is then displaced, and the white lead is allowed to descend into the washing cistern _m_. Below the cock _h_ is another cock or cocks, in order that any further quantity of liquor may be drawn off into the cistern _i_ if required. The taper plug _p_ is then raised, by being pushed up from underneath the vat _a_, through pipe _l_.

The white lead requires to be well washed with clean cold water.

When a fresh charge of monoxide of lead has been placed in the vat _a_, the liquor is pumped up on to it from the receiver _i_, and then a further quantity of solution is drawn from the cistern _b_, and also from the ammonia catch box _r_, until the proper quantity has been supplied, which is determined by the gauge.

Quite recently, in conjunction with Mr. Watson Smith, Mr. MacIvor has introduced further improvements in the production of “white lead” or basic carbonate of lead, by the treatment of oxide of lead (litharge or “massicot”) with a heated solution of acetate of ammonium, in a closed vessel (called the digestor), with agitation and under pressure, so that the oxide of lead is transformed with exceeding rapidity into basic acetate of lead, chiefly consisting of the tribasic acetate, ammonia being set free; and the subsequent treatment of the basic acetate thus obtained, after filtering or otherwise removing insoluble impurities, and after cooling with carbonic acid gas in a separate vessel (called the carbonator).

The acetate of ammonium is by preference first charged into the digestor, and then the oxide of lead (litharge or massicot) in fine powder, and in the equivalent proportions calculated to be at least somewhat slightly in excess of the quantity necessary to form with the acetic acid of the acetate of ammonium the tribasic acetate of lead Pb(C_{2}H_{3}O_{2})_{2} + 2 PbO or Pb_{3}O_{2} (C_{2}H_{3}O_{2})_{2}, is added (this being in allowance for the certain amount of insoluble and unconvertible matters in the litharge), the acetate of ammonium liquor of a strength preferably not below 5 per cent., though it may be stronger, being first, before the addition of the oxide of lead, set in vigorous agitation and circulation by the pump, and heat having been applied by means of a steam heater as shown in the drawing and as will be explained later on.

The digestor is closed, and the temperature rises. As the temperature rises, and approaches 212° F., and as the pressure, due more especially to the tension of vapour of ammonia set free during the reaction with the litharge, increases, so is the rapidity of the conversion of the oxide into basic acetate increased, and more and more of this basic acetate becomes dissolved, whilst at or about 212° F. it is entirely in a state of solution. The degree of heat, or the prolongation of the heat, depend of course upon the state of dilution of the acetate of ammonium used. Something more specific will be said later on regarding this question of degree of heat.

The clear liquor, together with extraneous coloured particles, red lead, dirt and undissolved matters, is pumped out through a suitable filter to one of the carbonators, being carried en route through a cooling-pipe system, and being let cool further if necessary in the carbonator itself. After cooling thoroughly, the cold basic acetate, which may now have crystallised or separated out more or less, according to the strength of the solution, is treated with carbonic acid gas, whereby the basic acetate, in the presence of the ammonia set free, is converted into basic carbonate of lead of exceptionally white colour, and with high basicity and covering power, proper care being taken as specially indicated later on. There are several reasons why greater whiteness of the product is secured, the principal one being that filtration from solid and insoluble coloured impurities has taken place previous to the carbonating process.

The advantages of the employment of heat and pressure for the formation of the basic acetate of lead from oxide of lead, and the advantage of thus carrying out the conversion of oxide into basic acetate of lead separately from the conversion of that basic acetate of lead into basic carbonate or white lead, are--

Firstly--That separation by filtration from impurities and so forth left by the oxide of lead used, and insoluble in the acetate of ammonium, is made possible, and thereby a pigment of exceeding whiteness and purity can be obtained, besides high basicity, with corresponding body and covering power.

Secondly--That the rapidity of the conversion of oxide of lead into basic acetate of lead is immensely increased--it becomes in fact almost instantaneous, much time being saved.

Thirdly--That the conversion is effected quickly and in a perfectly closed vessel; no chance of the escape of ammonia occurs, and, in the carbonating stage of the process, the free ammonia present in the liquid assists in securing the formation of basic carbonate of lead, and the maintenance of this basicity throughout the conversion.

This free ammonia is converted by the carbonic acid into the less volatile but still alkaline carbonate, and, later on, this ammonium carbonate reacts upon the basic acetate of lead, converting it into basic carbonate of lead, acetate of ammonium, still less volatile, being simultaneously produced, ready for use over again.

It must be understood then, that as the carbonic acid passes into and through the ammoniacal mixture in the carbonator, it continually precipitates or forms basic carbonate of lead, in presence of more or less of the volatile alkali, which however continually diminishes in quantity as the conversion proceeds.

Were no ammonia present, but only tribasic acetate of lead, as in the case of the earlier methods of precipitating white lead, unless a very slow current of carbonic acid gas were passed through, some of the first formed basic carbonate of lead would be in danger of being over-carbonated and losing its basicity, being converted into mono-carbonate of lead. This danger is much lessened, and consequently a much greater possibility of rapidity of carbonating is conferred, in the case of the process as above described, and by virtue of the ammonia which is present.

Nevertheless the carbonic acid gas must not be passed, even into the ammoniacal liquid containing the tribasic acetate of lead, with such rapidity that distinct alkalinity to the usual tests ceases to be maintained, and the process must be terminated whilst the liquid is still alkaline. If a little lead salt on the one hand, and a little ammonia on the other, be left in the mother liquors ultimately obtained on filtering from the white lead, they will be returned and circulated.

Fig. 27 shows an elevation (partly in section) of the apparatus employed.

_a_ is an iron vessel made of boiler plate, and lined internally with lead. It may be here added that all the apparatus is thus lined, or is constructed of material invulnerable to the action of lead salts, ammonia, or acetate of ammonia.

The vessel _a_, which is termed the digestor, should be furnished with, a thermometer, pressure gauge, safety valve, charging hole _b_, and suitably arranged sampling pipe with cock.

Agitation of the contents of the vessel is effected by means of the circulating pump _c_, which, drawing off supernatant liquor along with air and ammonia vapour from the upper part of the digestor _a_, by the pipe _d_, forces it through the pipe _e_ to the heater _f_, consisting of a coil suitably heated by steam in a steam vessel _g_, or by similar means, and then is forced through a pipe _h_, leading to the bottom of the digestor _a_, where it terminates in a cone-spreader _i_, provided with a regulating valve _j_. By means of this spreader, the liquid is forced through the heavy and dense mixture containing the oxide of lead, which it vigorously agitates and keeps more or less in circulation as indicated.

The charges of litharge and acetate of ammonium will vary according to the strength of the latter. For a 5 per cent. strength of acetate of ammonium solution, however, it will be best to calculate the proportions as follows, viz. about 1200 gallons of 5 per cent. acetate of ammonium liquor for 1 ton of litharge, which should be finely ground.

The temperature of the liquor may vary between 140° F. and 212° F.; but within these limits, the lowest temperature consistent with the sufficiently rapid conversion and solution of the litharge is preferable. The reason of this is simply that the less the heat employed, the less is the tension of the ammonia, and the chances of the loss of ammonia are thus minimised; in addition to this, less delay is involved and less refrigeration or cooling is needed before carbonating.

The object of the violent agitation of the litharge amongst the heated acetate of ammonium in the digestor _a_, is that caking of the former may be prevented, and a most rapid conversion of the oxide of lead into basic acetate of lead, with minimum expenditure of heat, be secured.

The effect of the heat in the closed space is greatly aided by that of the pressure due to the tension of the ammonia and aqueous vapour at the increasing temperatures.

When the litharge is converted into tribasic acetate of lead, and brought into solution, the liquor and sludge of insoluble matters is preferably pumped through pipe _k_, by means of the filter-press pump _l_, and forced through the filter-press _m_, which removes and retains the insoluble matter, allowing the clear liquor to pass to the cooler _n_, and through this to the carbonator _o_, similar in construction to the digestor _a_.

Here the cooled liquid is circulated by means of pump _p_ (in a similar manner to that adopted in the digestor process already described), carbonic acid being simultaneously pumped in through the pipe _q_, which is perforated as shown, or introduced in any other way.

When carbonated to the desired extent, the white magma, consisting of basic carbonate of lead and mother liquor, is drawn through the pipe _r_, by the pump _s_, into the filter press _t_.

The clear liquid flows through _u_, into the covered mother liquor tank _v_, whilst the press-cakes of white lead, after sufficient washing with water, are removed and dried in a suitable manner.

The washings are run off to a weak liquor tank (not shown) for concentration for use over again. Any inert gases accompanying the carbonic acid, or the latter alone in excess, pass from the carbonator through pipe _w_, into the catch-box or other condensing and absorbing apparatus _x_, containing either dilute acetic acid or cold water, in order to retain any ammonia carried over, and furnished with perforated trays or baffle-plates.

This catch-box is, if necessary, also connected with a further condenser, so as to remove all ammonia from the displaced air, or inert gases (if impure carbonic acid has been used) of the carbonator. It is a lead lined vessel preferably. The ammonia carried from each charge thus is tested by measuring the volume of the solution from the catch-box or other condensing and absorbing apparatus, and estimating the ammonia present in the solution. This amount of ammonia in a sufficiently concentrated form is then added to the charge in the carbonator, when cold, so as to produce a completely neutral solution. The ammonia in the catch-box is either added to the ammonia stock in the ammonia department, used for dilution of strong ammonia in making fresh acetate, or strengthened up to further absorption in the catch-box or condenser, until strong enough to add to a freshly run and cooled charge in the carbonator, to replace any ammonia driven off by heat.

The sludge in the filter-press _m_ is washed with water to remove acetate liquors and ammonia, the weak liquors being run to a separate closed vessel similar to the carbonator _o_, but smaller, and not shown in the drawing.

Here white lead is precipitated by carbonic acid, and the product is passed into the filter-press _t_, the weak liquors associated with it serving to give a preliminary washing to a freshly received charge of white lead already in the press. The weak filtrate liquors thus obtained are preferably sent to the weak liquor tank already mentioned, for subsequent concentration, instead of to the stronger liquor tank _v_.

The sludge-cakes from the filter-press _m_, connected with the digestor, are suitably treated for the recovery of lead therefrom.

The mother liquors contained in tank _v_ from the white lead filter-press _t_, are directly returned to the ammonium acetate tank _y_, by the pump _z_, for use over again.

The carbonators may be operated singly as described, or two or three may be connected together so as to be worked in rotation, the partially absorbed carbonic acid from one carbonator being completely, or more or less completely, absorbed in the one, or two, with which it is connected. The last of such series of carbonators would of course be connected with the catch-box arrangement previously described, or other condensing and absorbing apparatus.

_Characters._--The advantages and disadvantages in the employment of white lead have been described pretty fully by Prof. Barff in one of the Cantor series of lectures which form such an important feature in the publications of the Society of Arts.

Probably the fact that white lead possesses the body it has is the reason why it has been so largely used, and why so many paintings which have been painted with it have come to a most untimely end. Prof. Barff says he knows of no pigment so liable to change of colour as white lead. In saying this he expects that there are many who will not agree with him. They know that white lead works well and easily, and they like it because it covers down well; but then he points out some of the great defects under which it labours.

If you take some oil, and if to that you add lime-water, the oil will mix with the lime-water, and form a kind of emulsion. Again, if you boil oil or fat with soda, a kind of soap is formed, and the process of manufacturing soap is termed the process of saponification. Now if, instead of boiling fat with soda or alkali, we boil it with plumbic oxide or oxide of lead, we shall form a soap, and that soap goes by the name, amongst medical men, of _emplastrum plumbi_, or lead plaster. This is a substance made by the saponification of oil with the oxide of lead. Because this oxide and carbonate of lead have the power of saponifying oils, you get in white lead that peculiarly smooth easy working which you do not get with any other white pigment; and it is on this account, for one reason, that it is liked by artists and painters. Taking a piece of paper coated with some of this lead plaster, if you throw a light upon it, you will see that the substance is semi-transparent. This is a peculiarity of lead that it will saponify and form this sort of transparent substance.

The famous landscape painter, Mr. Wilson, made an addition to a room in his house. The old part of the room had been painted a dark colour; the new part, of course, when it left the workman’s hands, was perfectly white, and therefore the painters painted down the dark colour with white lead, until the whole room displayed one uniform tint. After a while, however, it was found that the part which was originally painted dark became dark again; the dark paint, in fact, showed through the white lead. Sometimes, possibly, when an artist wishes to put in figures upon a dark background that he has painted, he uses white lead, and the figures will stand out well and brilliantly at first, but after a time the dark colour upon which they are painted strikes through the lead, and the figures of course recede. Now, this striking through is owing to a slight process of saponification, no doubt owing to an interchange between the carbonic acid of the lead carbonate and the stearic and oleic acids of the oil with which the lead is mixed; so that, in time, the white lead, which has a body which makes it so great a favourite with artists, loses that body, and becomes a transparent or semi-transparent substance, something like lead plaster. Here is a reason why white lead should not be used unless the ground has previously been brought to a light colour.

There is another objection to the use of white lead, and really a very valid one it is. Persons go on year after year laying out sums of money for having their houses painted with white lead, when other pigments which will keep their colour might well be employed. A house painted with white lead after some time darkens in tint considerably; the colour is changed by some influence that is acting upon it through the air, and that influence is sulphuretted hydrogen gas. If you paint with white lead doors placed near a drain from which this gas escapes, those doors will become browned and blackened. White lead is very often, particularly that procured at ordinary shops, adulterated with a substance called sulphate of baryta, or, commonly, barytes. This is much more transparent when ground with oil than white lead itself, and it will materially impair that property for which white lead is valued, viz. that of covering down well and solidly. White lead adulterated with barytes has, generally speaking, a bluish sort of look; it is semi-transparent. It has not that opacity that pure white lead has. If you take a small piece of white lead and put it into a test-tube, and add to it a little nitric acid, or aquafortis, and some water, if the lead is pure the whole of it will dissolve in the liquid, and you will have a pure solution. If it does not dissolve there is a white precipitate, which will fall down to the bottom of the tube, and that precipitate is sulphate of baryta. Sulphate of baryta is insoluble in aquafortis, but carbonate of lead, and most lead salts, are soluble in it.

There is another excellent test for the purity of white lead, which is this. If you take a small portion, and grind it up with a little carbonate of soda into a small pellet about the size of a pea, and then put it upon a piece of charcoal and hold it in the middle flame of a blow-pipe for some short time, the sulphate of baryta becomes decomposed, and you get sulphide of sodium formed. If this sulphide of sodium be acted upon by an acid liquid, sulphuretted hydrogen is given off, which could not be formed from carbonate of lead, for in it there is no sulphur at all; and inasmuch as sulphate of baryta is the impurity for which we have to look, the presence of sulphide after this treatment indicates that it was with the white lead which was examined.

LIME WHITE.--A name sometimes given to the white pigment prepared from sulphate of barium. See baryta white, p. 170.

LITHOPHONE.--This is a fancy title for one of the several varieties of white pigment having the metal zinc as a basis, and described under zinc whites on p. 247.

MAGNESITE.--The mineral known by this name is a natural carbonate of magnesia, just as limestone is a natural carbonate of lime. Where sufficiently abundant it is quarried, ground, and levigated much in the same manner as barytes, which it greatly resembles in its qualities as a pigment, and for which it constitutes a suitable substitute. It is very white, heavy, and opaque; permanent in ordinary situations; neutral with other pigments, mixes equally well with oil or water, and possesses good covering power.

MINERAL WHITE.--One of the names applied to the pigment prepared from gypsum, see p. 183.

ORR’S ENAMEL WHITE.--A name derived from the maker of a certain variety of the zinc sulphide pigments, described under zinc whites, p. 254.

PARIS WHITE.--Another name for the best brands of whiting, see below.

PERMANENT WHITE.--This name is often bestowed upon baryta white (see p. 170), on account of its durability as compared with white lead.

SATIN WHITE.--There is a certain amount of confusion in the application of this term, for while it is sometimes referred to baryta white (p. 170), it is also a synonym for fine gypsum (see p. 183).

SPANISH WHITE.--The most carefully prepared samples of whiting (see below), are often known by this name.

STRONTIA WHITE.--Though much less common than the closely similar sulphate of barium, the natural sulphate of strontium is equally suitable for employment as a pigment, and is prepared in exactly the same way as baryta white (see p. 170). The artificial product is also used. Both possess qualities remarkably akin to those of baryta white.

TERRA ALBA.--An old-fashioned name for levigated gypsum (see p. 183).

WHITING.--This material is simply prepared chalk. It should be soluble in hydrochloric acid with effervescence, leaving at the most but a small residue. Sometimes samples of whiting are found which are more or less alkaline or caustic in their properties. This is a serious defect for many purposes. It can be detected by treating the sample with water, and adding to the liquor a little phenolphthalein. If a brilliant red colour is obtained, caustic lime is present, and the sample should be rejected, if to be used for mixing with chromes or Brunswick greens, where a neutral product is required.

Chalk itself is too familiar to need any description beyond saying that it essentially consists of carbonate of lime, with always a small percentage of silica associated with it.

Its preparation consists in hand selection to exclude the silica which, occurs in the more pronounced form of flints, then grinding in several stages, levigation and drying. The levigation is effected by having a series of settling pits into which the ground material flows with water, and deposits according to its degree of fineness. The drying is performed in chambers provided either with pipes carrying steam or heated air, or by fires beneath the floor, and thoroughly ventilated so that the moist air can escape as fast as it is saturated. Finally the dried whiting is again ground very fine. The drying must be done with great care and at a low temperature, so as to ensure avoiding calcination, whereby the carbonate of lime is changed into oxide (quicklime).

Whiting is a permanent and useful pigment mixed with water in distempers, but is not applicable as an oil colour.

ZINC WHITES.--Originally and properly the term zinc white was reserved for the white pigment consisting of zinc oxide; but latterly many kinds of white pigment have been introduced containing a large proportion of sulphide of zinc, sometimes associated with more or less oxide, and sometimes without any oxide, and these are also by many people called zinc whites, to which name they are perhaps as well entitled as the original zinc oxide. It will therefore be convenient to arrange them all under the same general heading of zinc whites.

(1) _Oxide._--Under the influence of a white heat metallic zinc is volatilised, and if the vapour is thus brought into contact with oxygen, either in the pure state or as air, combustion takes place, and the oxygen unites with the metal to form zinc oxide. On this very simple principle is based the manufacture of zinc oxide white.

The operation is conducted in plant similar to that shown in Fig. 28, which consists essentially of two departments, that in which the zinc is volatilised and that in which the oxidised vapour is deposited for collection.

The volatilising process takes place in a series of oblong fire-clay retorts _a_, varying somewhat in form but always with a contracted and rising neck. Ordinary dimensions are about 2 feet long and 9 inches in diameter each way, with walls about 1½ inches thick. These are heated to whiteness and then charged with ingots of metallic zinc.

The retorts are arranged in double rows in reverberatory furnaces _b_, two furnaces being arranged back to back so as to economise heat. The furnaces are fired at the side, and the heat is conveyed around the retorts by means of the flues _c_, the products of combustion of the fuel finally escaping by the chimney stack _d_.

As the vaporised zinc is emitted at the mouths of the retorts _a_ in a partially ascending current, it immediately encounters a plentiful supply of air, and thereupon takes fire (undergoes combustion or oxidation). In this condition it enters the lower and funnel-shaped end of the sheet-iron flue _e_, by which it is conveyed into the series of settling compartments _f_.

While the bulk of the zinc oxide thus formed passes into the settling chambers, a portion of it is too heavy to do so; its specific gravity is such that the force of the draught is not sufficient to carry it up. This portion falls at once into a receptacle placed beneath the mouth of the flue _e_.

In order to obtain the necessary draught, the conduits _h_ are open to the outside atmosphere, and introduce a supply of air just below the mouths of the retorts _a_, so that it impinges against the current of escaping zinc vapour. After passing through the settling chambers _f_, the superfluous air finds an outlet at _i_ into a sufficiently capacious flue _k_, which communicates with the chimney stack _d_. Thus the draught created by the fuel consumed in the reverberatory furnaces is made to assist the current through the settling chambers.

These settling chambers _f_, are constructed of wood, and are usually about three in number, intercommunicating of course. The zinc oxide enters the first compartment through an aperture in the top of the side to which the discharge end of the iron flue _e_ is attached. After traversing the first chamber, the stream of air and such oxide as has not yet settled passes into the second chamber through the orifice _l_, near the bottom of the partition dividing the two chambers. To reach the next compartment, the stream has to ascend again, the aperture being at the top of the partition, and this alternation is carried on to the end of the series, thus checking the through draught and facilitating the settlement of the zinc oxide. The floors of all the chambers are made funnel-shaped, with a door at the lowest point, so that the discharge of their contents may be as automatic as possible. The flue _k_ contains screens hung at intervals for the purpose of hindering, as far as possible, the escape of minute particles of zinc oxide into the chimney, and thence into the outer air, whereby they would be lost.

A description of the process as conducted in Belgium, says that ingot zinc is placed in a series of retorts within one furnace, and the oxide is formed in an exhaust chimney, and then passes through a long series of passages and condensing chambers, in which are ranged tanks of sheet iron or cloth to collect deposits. At a certain hour of the day it is collected into casks, and after being tested as to quality it is ready for delivery. According to the purity of the metal various qualities are produced. The best is called “blanc de neige,” or snow white, and is of a very superior quality; No. 1 white is the most used, the ores for this quality being selected and purified by remelting; No. 2 white is the common variety. In the process of manufacture there is more or less waste material, imperfectly oxidised, deposited in the retorts and passages. This residuum is carefully ground, washed and dried, and is employed in painting in the place of lead.

In the American method of making zinc white they use the ore direct. This is cheaper than the Liège method, but its product is of inferior quality to that produced by sublimation. There are but two works in Belgium for making zinc white, the Vieille Montagne Company (at the Valentine Cocq works), which produce yearly 3000 tons by sublimation; the other is at Ougree, near Liège, where the American method is employed, but at present it is idle.

There are many other modifications in detail in different works. One may be noticed here as it is claimed for it that the pigment is gifted with greater covering power or body, the limited degree of which is the only drawback to zinc oxide whites. The plan consists in this, that the oxide is allowed to collect in the condensing chambers till it is of such a depth that a man entering stands waist deep in the pigment. The latter is gathered in pieces of sacking, which are drawn together and squeezed up tightly, so that the oxide, when newly prepared, is pressed into hard dense masses.

(2) _Sulphide._--Prof. Phipson, in a paper read before the International Health Congress, at Paris, remarked that for several years efforts had been made to discover some white substance to replace white lead for painting buildings, ships, &c. He himself had devoted several months to this important subject, but without success. There has been found, it is true, in oxide of zinc a substance less poisonous than lead, and serving very well as a white pigment in oil painting; but its production is very expensive, and its mechanical properties as a colour in oil are not pronounced enough to allow it to compete in commerce with white lead. Such is not the case, however, with an invention of Mr. Thomas Griffiths, of Liverpool, who has succeeded in obtaining a very interesting product. This new preparation, which is being manufactured at the present time on a pretty extensive scale, has for its base sulphide of zinc (or an oxysulphide of that metal), the properties of which as an oil colour are of the most remarkable character. It is prepared by precipitating one of the salts of zinc by a soluble sulphide, and washing and drying the precipitate. The latter is then calcined at a red heat, with some precautionary measures, then taken from the furnace, and, while still warm, thrown into cold water. It is afterwards levigated and dried. The result is a white pigment, very fine, and of great beauty. Regarded from a hygienic point of view, Griffiths’ new white is infinitely superior to white lead, as it also is in its practical bearing; it possesses no injurious qualities; its manufacture and use do not affect the health of workmen; its durability in climates of the most diverse kinds is, so to speak, illimitable; it is altered neither by gaseous emanations nor by dampness; and its price is comparatively low. The most remarkable thing about this new white is that it covers as well as white lead, while it withstands the effects of all kinds of weather, so that its use is not only deprived of all danger to health, but it is much more economical than white lead. Prof. Phipson stated to the Congress that he regarded this new chemical preparation as being among the most ingenious and useful products that have been discovered in our time.

A later method, introduced by Griffiths and Cawley, consists in making an artificial sulphide of zinc by bringing the vapours of zinc and sulphur into intimate contact.

In carrying out this process, sulphur is melted in a jacket pan heated preferably by high pressure steam. The melting vessel is connected with a cast-iron still by means of a jacketed pipe, and the connection is regulated by means of a valve in the bottom of the melting pan. The latter should be at such a height above the still that the pressure due to the column of sulphur in the conduit pipe may be greater than the tension of the sulphur vapour in the still, so that when the valve is opened, the sulphur in the melting pans may descend into the still. The still is kept during the process at a temperature of incipient redness, so that when the sulphur reaches it, the sulphur is immediately vaporised and the resulting current of vapour passes to the chamber described below.

Metallic zinc is melted in a retort or crucible, heated preferably by means of a furnace on the Siemens or a similar principle, and raised to such a temperature that it begins to volatilise freely. When this takes place, the resulting zinc vapour is met by a current of sulphur vapour obtained as above described, and in excess of that required to form with the zinc sulphide of zinc. The reaction takes place according to the chemical equation Zn + S = ZnS.