Chapter VII: Whites (2)

Could the corrosion of the blue lead be effected in such a way as to prevent any discoloration of the material by the tan and wood--could the corrosion be so produced as to be easily separated from the buckle or grating on which it has formed--could this separation be so effected as to prevent the breaking up of the lead skeleton, and the presence of small pieces of metal in the detached crust of white lead, two principal reasons for washing and drying are removed.

There is yet another consideration, that is, the presence of acetate of lead, always found in varying quantities in the incrustation produced, and remaining at the close of the operation and conversion. To remove this, careful washing, and after-stoving and drying must be accomplished. The amount of this salt present is found to differ with each operation, and in various portions of the same make.

The washing out of the acetate is never perfect, and it involves a large amount of labour.

Opinions differ as to the effect of this acetate if allowed to remain in the product. White lead makers on the “stack” principle aver that it should and must be washed out, lest it should damage the qualities of the paint. This is questionable, and not one can produce practical evidence of its being the cause of any damage if still contained in white lead. Facts seem to deny its harmfulness in this respect, inasmuch as the best prepared samples, those washed and dried from the most careful makers, will be found upon analysis to contain more or less of acetate of lead.

A large proportion of this salt in white lead may not be beneficial for many reasons, but a small percentage can do no harm; nay, for many purposes it may be good.

There is no substance used for driers for white lead that is more esteemed than this acetate of lead, commonly known as “sugar of lead.”

A small amount of this salt present in white lead would communicate drying properties, and this alone is what it could do.

Granting that we can discover a method of producing white lead of amorphous character, of good density, free from all discoloration, free from all particles of metallic lead, and free from all but a small percentage of acetate of lead, then washing will not be needed.

Stoving and drying become unnecessary. The work of women, their deadly occupation, so burdensome to the operatives and to all with whom they are concerned, is done away with.

_Condy’s Process._--An improvement in the manufacture of white lead was patented by Condy, of Battersea, in 1881, which, though giving perfectly satisfactory results when carefully conducted, necessitated special precautions, and led to his substituting in practice the following additions and modifications, which are of great consequence in rendering the process more certain in the quality of its product, and more valuable as a commercial manufacture on the practical scale, by virtue of its offering greatly increased facility and economy in production.

The results of numerous and repeated experiments on the larger scale induced Condy to qualify the recommendation contained in his first patent, viz. that of employing a solution of tribasic acetate of lead and bicarbonate of soda in proper proportion to precipitate nearly the whole of the lead, and further stating that he preferred to employ “a slight excess of tribasic lead salt rather than find carbonate of soda in excess.” Though, when carefully conducted, if the greatest nicety is observed, a satisfactory result is obtained; in practice, the least variation from the exact composition of the two substances is attended with the drawback that the white lead is liable to a slight uncertainty of tint after it is ground in oil, whereas by the process hereinafter described a positive and reliable result can be obtained, as the white lead produced will be of a uniform white colour, and not liable to turn when ground in oil. Though the earlier process was in itself complete for the manufacture of white lead from oxide of lead, it afterwards occurred to Condy that a great object would be attained in rendering the process more valuable and more practical, if a method were devised, worked out, and described for the manufacture of tribasic acetate of lead to be made entirely from metallic lead by the action of acetic acid or neutral acetate of lead on metallic lead, and not to be dependent on the employment in any way of previously manufactured oxide of lead.

This portion of Condy’s invention relating to the manufacture of tribasic acetate of lead may be described as follows: he melts, and, after skimming carefully, feathers the metallic lead by dropping it into water; he places this granulated lead in wooden vessels or vats previously fitted with perforated false bottoms under which are fixed taps for drawing the liquor off into other vats or tanks placed on a lower level. Having filled with granulated lead the vessels fitted with the false bottoms, he fills up the interstices with a dilute acetic acid composed of one part, by weight, of acid (specific gravity 1·045 at 60° F.) and 12½ parts of water, and after allowing the dilute acid to stand for two hours, draws it off through the taps into the lower tanks. This allows access of atmospheric air to the lead, which has the effect of heating the lead so that oxidation takes place.

After a time (about three or four hours), this oxidation begins to slacken, when he pumps up a second time the acid solution from the lower vat on to the granulated lead, and allows them to stand in contact for one hour; he then again draws off the liquid into the lower tank, and again exposes the metallic lead to atmospheric oxidation, allowing three or four hours for the latter operation; and if the solution of lead has not already attained the specific gravity of 1·040, at 60° F., he again repasses the liquor over the metallic lead partially oxidised, until it has attained that specific gravity, when he places the dissolved lead with fresh granulated lead and recommences the manufacture in the same way.

This operation succeeds much better on the large scale than on a laboratory scale. In vats containing upwards of one ton of lead, the result is all that can be desired, and can be obtained by passing the liquor from twice to three times over the metallic lead partially oxidised. A little practice enables the operator so to control the process that he can obtain the solution of the desired specific gravity with perfect ease.

This plan of manufacturing tribasic acetate of lead possesses the advantage of producing that substance wholly or nearly wholly free from the impurities contained in metallic lead, such as copper and silver, which are not taken up, or soluble, in the presence of metallic lead. In consequence of the circumstance that foreign matter is left almost untouched, it is practicable to make white lead of a fine quality from old lead such as lead piping, roofing, and worn out lead generally, which can thus be utilised to greater advantage than in any other way.

Having obtained this solution of basic acetate of lead of the specific gravity of 1·040 at 60° F., Condy proceeds as follows:--To the solution produced by each 60 lb. of acid and 750 lb. of water previously pumped up into another vat or tank, he adds bicarbonate of soda in the proportion of 30 lb. for each 60 lb. by weight of acid originally employed, and agitates the mixture. This will generally precipitate all the white lead, but it is necessary to test the filtrate to ascertain the exact point when all the lead is thrown down. Sufficient bicarbonate of soda should be added to do this completely, and it would be better to use bicarbonate of soda in excess rather than leave any lead unprecipitated, as by this means greater certainty is obtained in securing on the large and practical scale a white lead capable of standing the effect of light and grinding in oil without changing. The white lead after precipitation can be washed, pressed, and dried in the usual way.

The following variation may be made from the method described of making tribasic acetate of lead, thus:--To each 60 lb. of acid and 750 lb. of water may be added sufficient of the tribasic solution to make neutral acetate of lead, with which to recommence the manufacture of tribasic acetate of lead by the process described. Vague reference has been made in works on technical chemistry to the possibility of using metallic lead in the manufacture of sugar of lead, but such references have been practically worthless, as they contain no information of a practical nature even for the manufacture of neutral acetate of lead, and no process at all has ever been described for the manufacture of basic acetate of lead from metallic lead acted on by acetic acid or acetate of lead.

_Gardner’s Process._--The conditions observed and fulfilled in the arrangements adopted by Prof. E. V. Gardner, are founded upon a study of the nature, properties and behaviour of the substances concerned, under certain methods of treatment.

There are several oxides of lead which may be formed under special conditions.--(1) Pb_{2}O, and the same oxide combined with water Pb_{2}H_{2}O_{2}; (2) PbO, and the same oxide combined with water PbH_{2}O_{2}. In the hydrated form these oxides combine readily with carbonic acid, but they do not combine readily with carbonic acid when dehydrated. The hydrates are most readily formed at about 120°-130° F., and are decomposed after they are formed if heated to 212° F.

These oxides and their hydrates combine with acetic acid to form acetate and sub-acetate of lead, and with nitric acid to form nitrate and sub-nitrate of lead.

Lead, submitted to the action of air, watery vapour, and acetic or nitric acid, or a mixture of these acids, with air or oxygen, with proper precautions, forms sub-acetate or sub-nitrate of lead, and this sub-acetate or sub-nitrate of lead readily absorbs carbonic acid and forms carbonate and sub-carbonate of lead.

Sufficient acetic or nitric acid, or a mixture of these and air or oxygen, and watery vapour, must be constantly supplied to form the subsalts of lead, to carry on the operation of converting blue lead into white lead; but an excess or an insufficiency of these agents will in either case prevent the formation of sub-acetate or sub-nitrate, and consequently of the sub-carbonates.

Thus--too little air, acetic acid and aqueous vapour prevents the formation of the hydrated sub-acetate and consequently sub-carbonate of lead. Too much acetic acid and aqueous vapour, and too little air forms an acetate on the surface of the lead, which, by the excess of water, dissolves and wastes, and washes the lead; it also varnishes the surface of the lead with a coating of acetate, and checks, if it does not completely prevent, the formation of sub-acetate of lead, and consequently the formation of carbonate and sub-carbonate of lead. Similar rules hold good in the case of nitric acid and the formation of nitrate and sub-nitrate.

The process of forming the hydrated sub-acetate or sub-nitrate of lead is most energetic at a temperature of 120°-130° F. In a lower temperature, a much longer time is occupied in carrying out the process of conversion; while at a higher temperature the delicate sub-acetate or sub-nitrate of lead first formed suffers loss of water, until, at 212° F., it is completely dehydrated. At a temperature about 135° F., the power of forming carbonate and sub-carbonates is lessened, and at 212° F. is considerably diminished. The carbonate itself is dehydrated at 212° F., and is decomposed at a higher temperature.

To obtain a white lead of excellent quality for its various uses, it is necessary to produce a substance which possesses sufficient body to cover surfaces to which it may be applied as paint, and it must possess sufficient base to combine with the oil of the paint to form a vehicle or varnish to retain and hold the body on the surface. This is found to be the case with sub-carbonate of lead, or especially with a compound constituted of two or three equivalents of carbonate of lead, with one equivalent of hydrated oxide of that metal.

Lead, to be attacked by chemical agents, should possess a clean surface. If the surface of the lead is _chemically_ clean, so much the better. The surface of the lead should be extended as much as possible, and exposed to the action of the gases and vapours at a certain heat for its conversion into white lead.

If the most favourable conditions are sought, the temperature should be between 120° and 130° F.; and to carry on the chemical action most satisfactorily, the lead during its conversion should present a granular coating--that is, the coating formed by the chemical agents should be granular and not smooth and continuous in character. It should be porous, and not of a continuous varnish-like or vitreous character.

To rapidly carry on the chemical action, the chemical agents should be well diffused and commingled throughout each other, and the blue lead should be so exposed as to be open to their attack on all parts of its surface equally, and all should be at, and kept at, a proper temperature and a proper degree of humidity. Too dry a heat prevents the process of conversion. Too moist an atmosphere wastes the materials and arrests their action. The conditions, therefore, which are most favourable will be a certain humid atmosphere of well diffused and commingled vapours or gases, acting on metallic lead exposed to them under the physical conditions described, and at a temperature between 110° and 135° F.

These favourable conditions can be further augmented by certain electrical arrangements in connection with them.

Again, in the sources from which the carbonic acid is derived, and in the arrangement of the apparatus for applying the carbonic acid, favourable and unfavourable conditions can be imported. Thus, by using paraffin, petroleum, benzine, or light oil of paraffin or petroleum, or similar carbonaceous substances free from sulphur, or mixtures of such carbonaceous substances either alone or mixed with air or other oxidising substance, comparatively pure carbonic acid may be furnished without at the same time producing any objectionable compounds.

As a result of his researches and experiments, Prof. Gardner has proposed (Eng. Pat. 1882, No. 731) certain improvements in the method of converting blue lead into white lead, in the apparatus employed, and in the method of making and applying the carbonic acid used in the process. Some of these improvements are applicable to the open stack or chamber process, while others relate to closed chambers.

Preferably, for the conversion of blue lead into white lead, Prof. Gardner adopts a closed, but not air-tight, chamber. This chamber is ventilated or relieved so as to enable the incoming gases and vapours to enter it without hindrance, and to escape by means of an exit valve and pipe, or an exit shaft, connected with the chamber, communicating with the exterior air, and regulated by a valve or damper. The gases and vapours within the chamber can find their way out by this exit shaft only, by slight pressure on expansion from within the chamber; thus the interior of the chamber is preserved from disturbance, by preventing the formation of currents within its atmosphere, and yet a perfect circulation of the gases and vapours is kept up.

To warm the interior of this chamber, which is constructed of any material that will resist the action of the vapours and gases, and which is not too absorbent, Prof. Gardner makes the bottom of the chamber of such a shape as to form a heating vessel to hold water or steam, this water or steam, or both, being kept at the required temperature by means of a steam coil. Matters are so arranged that the contents of this coil are protected from any excess of pressure, and consequently the temperature seldom or never exceeds about 212° F., unless for any special reason it is desired to raise it to a higher point. Sometimes the sides are constructed similarly to the bottom.

The materials of which these chambers are constructed must be capable of resisting the heated and acidified vapours within them. Cast or wrought iron faced with glass, slate, tiles, pewter, or glazed bricks; tinned copper, tinned brass, or pewter; timber, whether green or after carbonising by heat or sulphuric acid; all are more or less suitable. Means of observing the progress of the conversion, and means of lifting the contents in or out, must also be provided, as well as thermometers to indicate the temperature prevailing inside.

The gases and vapours which effect the conversion of the blue lead into white lead are generated outside the chamber just described, and are conducted into it by means of pipes, being first raised to such a temperature in excess of that which should exist inside the converting chamber as to allow for the cooling effect arising from the friction and loss of heat in passing through the various pipes and distributors attached to the converting chamber.

Owing to this extra heating, the gases and vapours are expanded, diffused, and commingled, and do not rob the interior of the converting chamber of any heat on entering it, so that the heat inside the converting chamber is kept constant, and can operate to further expand and diffuse the gases and vapours in contact with the blue lead inside the chamber. In practice it is found that the temperature of the converting chamber cannot be suitably controlled if any portion of the gases or vapours be generated within that chamber.

The blue lead is arranged in the converting chamber in trays, or on shelves or frames, so as to allow it to be completely surrounded and attacked by the vapours or gases, and thereby be converted into white lead, at the same time preventing the formation of direct currents or eddies. Framed supports resembling a dinner waggon serve well for holding the lead, and may easily be arranged to lift bodily in and out of the chamber with their burden of blue or white lead. The surface on which the blue lead is directly supported is made of graphitic carbon, hard coke, platinum, or carbonised or platinised material, such as is used for plates in electric batteries, or of other material standing in a similar electrical relationship to lead, or capable of generating with it an electric current.

In applying this development of electrical energy to the ordinary “stack,” as for instance the Dutch process, the pots containing the acetous liquid and blue lead are made of, or lined with, such electrical carbon, or contain a portion of it in suspension, by which the same effect is realised. Advantage may also be derived from furnishing a supplementary supply of carbonic acid to the stack beyond that due to the decomposing dung, &c.; as well as from injecting a current of air or oxygen at suitable temperatures, and from the admission of steam in a coil throughout the stack.

The generation or production of the acetic or nitric acid vapours is attained by heat in a vessel so arranged that its contents are kept at a certain temperature for vaporising, by having the boiling acid solution at about 1·003 sp. gr., procured by mixing water with vinegar or acetic or nitric acids.

The supply pipes conveying the air or oxygen and the carbonic acid, or either of them, can easily be made to emit their contents close to the boiling acid solution, whereby the vapours arising from the latter are mixed and intermingled with them, and all pass together through the pipes and distributors into the converting chamber or stack.

In preparing the carbonic acid it is necessary to observe certain precautions, especially that it shall be pure and that no carbonic oxide shall gain admission to the chamber. If the ordinary chalk and carbon method be employed, the furnace must be provided at the uptake with a series of air inlets, so as to ensure that the gases passing from the furnace to the delivery tube shall be most thoroughly oxidised.

The purification of the carbonic acid may be effected in the usual way by passing it through a vessel containing water; or, if requisite, it may be cleansed, expanded, and heated in one operation by passing it through or over hot water, or over a hot aqueous solution of carbonate of soda, all of which, however, incur considerable cost in plant and manipulation.

Owing to these difficulties in purifying carbonic acid, Prof. Gardner prefers to prepare a practically pure carbonic acid in the first place, and this he does by allowing petroleum, benzine, paraffin, or other hydrocarbon or carbonaceous liquid to gradually fall into a retort containing chalk or other suitable carbonate at a high temperature, whereby relatively pure carbonic acid gas is generated; and while still in a highly heated state it encounters a stream of air or oxygen, ensuring its complete combustion before entering the converting chamber.

Another method of procuring fairly pure carbonic acid is by the combustion and oxidation of any of the liquid hydrocarbons mentioned above, in suitable lamps; and when carbonic acid gas of exceptional purity is required, it may be obtained by heating bicarbonates to a sufficient degree to drive off one molecule of carbonic acid, reducing the bicarbonate to carbonate, from which it can be reproduced by treatment with carbonic acid gas obtained from cheaper sources.

The _modus operandi_ adopted by Prof. Gardner is as follows: The lead is granulated and prepared for conversion in one operation, thus--an iron or a slate slab about 2-3 inches thick is placed in a tank containing acetic or nitric acid solutions rising about 3 inches above the upper surface of the slab. The lead is melted at low red heat and poured from a height of 4 to 6 feet into the acid solution, through which it falls till it encounters the slab, and thereupon passes away into the surrounding solution, being thus converted into a spongy condition. In this condition the lead is spread on frames or trays, which are then lifted bodily into their places in the converting chambers. The latter is closed and heated to 120° F. for 3-4 hours, or until the whole contents have assumed a uniform temperature. Thereupon the acid vapours and air are admitted and distributed, taking care that the temperature is not thereby reduced below 110° F. nor increased above 125° F., steam being temporarily shut off if necessary. This treatment is continued for 24 hours.

The admission of aqueous vapour should be so regulated that while a dry atmosphere is avoided, yet there is no appreciable condensation of moisture in the chamber. While ensuring this condition, the temperature may reach as high as 130° F. for a second period of 24 hours, but must not overstep the limits of 120° F. minimum and 135° F. maximum.

After 48 hours’ treatment, the supply of duly warmed carbonic acid gas is admitted for a period of two hours, without discontinuing the introduction of acid vapours, air and steam; and this addition of carbonating gas is repeated for two hours at a time with intervals of four hours during which it is cut off. When, after four or five days, efflorescence or exfoliation appears on the lead, the supply of carbonic gas is increased to two hours in every four, or four in every six; and the admission of acid vapours, air and steam may also be augmented so long as the temperature is not allowed to exceed 130°-135°F. The whole operation is completed in seven to fourteen days.

Of this process, Mr. Carter Bell, in his paper before referred to, speaks in the highest terms. No washing or drying is necessary. No women are engaged in the manufacture, and but few men. The white lead thus produced by the aid of electricity is deposited in a peculiar state of disintegration, it is perfectly amorphous and non-crystalline, of the purest quality; its density is 5·8. When ground in oil and made into paint, it possesses great body and a covering power inferior to no other paint, if not superior to them all.

Painters who have used the paint, practical men, and amongst these we may observe coach painters, have pronounced its excellence and superiority to the best ordinary white lead paint.

By this electrical process of manufacture, not only is the time consumed in the making and in the preparation of this material greatly shortened, but the cost of preparation is reduced, and added to this is the important fact, the vital factor in our consideration. The labour of women is unnecessary. No lives are sacrificed to its working requirements.

Prof. E. V. Gardner, who has been for some years occupying his attention with the subject of white lead making, with a view especially to remedying its existing evils, has invented his electrical chamber process of manufacture, and an entirely new course of after treatment. He has for the last seven or eight years been more or less occupied perfecting his conception, and accommodating it to practical and commercial claims.

Chamber processes are not new, there have been several patents enrolled for making white lead in closed chambers, but none has proved commercially convenient or practically successful in its adaptation, and none has survived to the present time.

In Germany, white lead is made in chambers at the present day. The lead in gratings or sheets is supported on wooden rods, saddle fashion, the chamber is then filled, and its contents are submitted to currents of acetic acid vapour, air, steam, and carbonic acid gas; the time needful for conversion is six or seven weeks. The after steps in the separation of the incrustation, and its preparation for trade purposes are much the same as in the “stack” product preparation. It is washed, stoved, dried, and ground. The white lead made on the German plan does not differ in any material degree, save it be in price, from the best English commodity. We may assume that the washing and drying in Germany consumes a like period of time to that process in English works, viz. two or three weeks. We then see that the German plan of making white lead cannot be perfected in less than eight or ten weeks’ time from the commencement of the corroding action in the chamber. To compare these facts and the efficiency of the different plans described:--

The time required to complete the corrosion in the stack is at least 14 or 16 weeks.

The Gardner’s electric process requires for the same purpose only 14 days.

As to point of time, the German plan excels the stack, and can be carried out in one-half the time required in the stack method of conversion.

The Gardner’s electrical method excels the German, and can be perfected in one-third the time needed for the German chamber operation, and one-sixth the time required the stack.

These figures open out a most important matter when we regard the capital invested and lying dormant in stack lead works.

It is well known that we have in electricity a most powerful agent by which to effect the chemical combination of various substances on the one hand, or on the other, by its means to break up and disrupt a chemical compound. Professor Gardner’s main principle of action in his new process is founded upon these facts, and he takes advantage of electrical power to cause the combination of the lead with the necessary elements to build up white lead in his chambers. He either employs electrical discharges to energise and render active in their chemical affinities the various materials engaged, or he so disposes of them as to form an electric or galvanic combination in the chamber. In the latter arrangement the chamber and its contents represent a gas battery on an extensive scale. In practice he prefers the latter plan; it is more simple, more manageable in the hands of the ordinary workmen. The original plan was to have graphite or graphitic carbon plates or supports for the buckles or gratings of lead within the converting chamber. These carbon plates and the lead to be converted, were so placed as to form a collection of galvanic “couples” or “pairs,” and in this condition were submitted to the gases entering the chamber from without. It will be understood that various modes of arrangement would occur without departing from the principle concerned.

In practice this method answered very well, but presently a difficulty arose; not only were the graphitic carbon or graphitic plates expensive, but they were easily broken, and became friable in use. Carbon plates of an especial kind were manufactured to meet these failures and remedy these defects, not without success, but still open to objection. In looking round for some substitute to replace the carbon, two points were to be kept in view, to seek some electro-negative to lead like the carbon, and some electro-conductive like it, and some material that would bear rough handling such as workmen give, and be practically convenient in its adaptation to a working chamber.

Gold or platinum would excel in these particulars, but there are considerations which debar their use. Pure tin presented itself, and tin was tried with great success. Tin would at first be thought, on account of its close electrical relationship to lead, far from favourable to the purpose. Graphite or carbon would appear far more suitable. Practice pronounced the tin to act as efficiently as carbon. This may at first seem contradictory and strange, but if we consider that while the carbon is certainly more highly electro-negative to lead than tin, yet the tin is the more conductive, and offers the less resistance to the electric current of the two; in this manner the tin compensates by its conductive power all it may lack, as compared with carbon, in electrical energy when coupled up with lead.

Tin plate is now used as the electric-negative element in the chamber of the Gardner plan. Tin plate means pure tin. Ingot tin is rolled out into plates, the bottom of the chamber is covered with this pure tin plate, so are the bearers and the shelves, or supports which are to hold the lead during its conversion. Tin pipes and tin fittings, which resist the action of acetic acid, are also used to conduct the gases to the chamber to carry on the converting operation, and to preserve the product from any source of discoloration.

When a chamber is prepared for the converting operation, the whole of the lead it contains will be in metallic communication with the tin supports, and these with the tin covered bottom of the chamber.

The chamber when working is kept at a certain temperature by a steam coil beneath the floor of the chamber. The process is simple. The lead buckles or gratings are placed on tin-covered stands, somewhat in form and make like a dinner-waggon. The whole is hauled up and dipped into a bath of acetic acid and acetate of lead; it there remains for one or two minutes, it is then hauled out, drained and lifted bodily through the top into the chamber. Other stands filled with buckles are so dipped and so placed till the space of the chamber is fully occupied.

This dipping cleanses the surface of the metal, and when it is exposed to the air it is speedily coated with a hydrated oxide of lead. This is the first step in the process of conversion. The chamber when filled is closed, and its temperature is brought to about 100° F.; then vapour of acetic acid and vapour of water and air are supplied from without to the interior of the chamber. This is continued for 15 or 20 hours. The lead buckles within the chamber will now possess a whitish coating, consisting of subhydrate and subacetate of lead, and they will present a uniform colour. Carbonic acid generated in any convenient manner is next passed into the acid generator; it mixes with the other gases and vapours, and with them goes on its way to supply the chamber. Speedily the action of the carbonic acid is observed, the surface lead becomes quite white and presents the appearance of a snow shower having fallen within the chamber. The formation of white lead is now speedily effected. This treatment is continued throughout the space of 13 days; at the end of this time the supply of acetic acid vapour is stopped, and the supply of air, steam and carbonic acid is continued, according as it is desired to obtain white lead rich in oxide or in carbonate.

After a short further period, steam and air only are sent into the chamber, which is varied in temperature to 120° or 130°F., and lastly the steam supply is stopped; air alone enters the chamber, which is kept heated by the coils beneath the floor. The contents of the chamber are now in a dry state, and the operation is terminated.

It will occur to most readers that these terminal proceedings amount in effect to a convenient method of washing and drying the white lead while it is still attached to the parent lead, and this it is in fact.

The contents of the converting chamber are lifted out through the opened top, and the buckles or gratings with their crust of white lead are turned into the agitator. This agitator is an iron cage revolving inside a closed chamber of the same material. During the revolution of this cylinder or cage, the contained lead gratings fall from side to side, and the incrustation on their surfaces becomes detached and broken up. It falls in, this broken state through the bars of the cage or cylinder into a receptacle beneath. The denuded buckles or gratings are retained in the cylinder and are removed. These gratings or buckles are cast of such a thickness as to withstand two or three converting operations in the chamber before they are recast.

This crude white lead is carried by an elevator, or it falls into the hopper of a pair of granite crushing rolls, also enclosed; and from these it passes into the mixer or incorporator from which it can be removed in a dry state or mixed with oil.

The incrustation of white lead will be found upon examination to be possessed of some peculiarities, the result of the electrical action which has been going on within the chamber. It is quite white. It falls from the lead buckle or grating which it coats, if the grating be struck against a piece of wood with but a slight blow. It is easily friable, and can be rubbed to the finest powder between the thumb and finger, or on the palm of the hand.

Now, we may explain, as we conceive it, the philosophy of its production in this state of disintegration.

We know that a feeble and prolonged current of electricity will in time deposit metals from their solutions in a crystalline condition, and that if we quicken the current of electricity and cause it more energetically to act on the same solution, we can precipitate the metal from that solution in a state of powder.

It is to similar action of electricity as that to which we last refer that we ascribe the formation of the crust on the gratings of lead in the non-adherent and disintegrated condition in which it is produced, and by reason of which it is so easily detached from the lead and broken up to powder. No edge runner grinding, such as is required by the stack process, is in this case necessary.

The crude white lead and crushed material, whether in a dry state, or incorporated with oil, is finished and ground in a granite roller paint mill, from which it issues as dry white lead, or as white lead in oil.

Paint made from this electric white lead has been sent to America, to France, to Belgium, to Germany for trial, and has also been largely tested in this country by painters, engineers, and others unacquainted with its precise nature, and it has been productive of good results.

Of its density, body, and covering power, there can be no doubt, and never once have these qualities been called in question.

The cost of the manufacture of white lead by the stack process is about 3_l._ to 3_l._ 10_s._ per ton. By the German method, the cost is about the same as by the stack. By Gardner’s electric process, the cost of conversion is 10_s_. a ton. To this 10_s._ must be added the cost of labour expended in preparation, an item which cannot be well estimated on the present limited scale of manufacture; it could not exceed an additional 15_s._ a ton. This would bring the cost of manufacture of electric white lead to 25_s._ a ton.

In this electric process inferior lead can be operated upon with success. Brands of that metal such as white lead makers by the stack method dare not employ, may be successfully converted in an electric chamber, and with fair results as to the quantity and quality of the white lead produced.

By the use of Gardner’s electric process it would appear that we not only preserve health, but save lives; we not only save time, but interest on large capitals, which lie idle for long periods at a time; and we can economise and simplify the whole manufacture and preparation of white lead, divesting it of all its present cumbersome and unhealthy stages. Gardner’s process, we believe, must take a prominent position as one of the most necessary, valuable, and scientific inventions of modern times.

_Hannay’s Process._--Mr. J. B. Hannay, whose name is well known in connection with various chemical and engineering inventions and processes, has recently brought out a process for the manufacture of white lead. The old method of producing white lead or carbonate of lead is one involving much time and labour, together with no small risk to the health of the workpeople. By the new process brought out by Mr. Hannay, sulphate of lead is manufactured direct from galena or lead ore, without the necessity of the intermediate process of the reduction of the ore and the extraction of metallic lead. It is said that the sulphate of lead produced by the new process is whiter and more permanent than the carbonate.

The process is described as follows:--A furnace, 36 in. by 30 in., and 48 in. deep, contracting to a narrow chamber about 36 in. long by 14 in. wide, communicating with the main flue, is charged with coke and brought to a red heat. The bed of coke is so thick as to be almost up to the level of the sill of the furnace door. The lead ore is not charged in large quantity and left for an indefinite time, but is thrown in in quantities of a few shovelfuls at a time, the object being to effect extremely rapid volatilisation and consequent oxidation. The proportion of ore to fuel is 1 ton to 1 ton, and the result is said to be the conversion of 95 per cent. of the charge into its equivalent in white lead. In the first or wide chamber, the coke is oxidised, carbonic oxide being the resulting gas, the rapid volatilisation of the galena or sulphide of lead also taking place in the same chamber. On entering the inner portion of the furnace, the volatilised sulphide of lead and the carbonic oxide are converted into carbonic acid and sulphate of lead.

Forced blast has been employed to cause the high temperature necessary for the volatilisation of the ore; but it has been found unnecessary, admission of air at atmospheric pressure through tuyere holes being quite adequate. After leaving the inner chamber of the furnace, the gases pass into a flue, level with the furnace, and about 40 feet long by 16 square feet in sectional area. From this flue the gases pass into a tower about 20 feet high, and from thence into wrought-iron flues 3 feet in diameter. These flues terminate in a wrought-iron chest, in which are fitted two steam injectors. The gases are forced by these injectors into the central chambers of the condensers. These condensers are two in number, and contain a central chamber, 16 feet by 12 inches, into which the gases are forced as described. The gases escape through interstices into the outer chambers of the condenser, and are there condensed, a continuous stream of water occupying the lower part of the condenser. The waste gases escape by a downcast leading to a tall chimney. The temperature of the gases as they enter the condenser is about 840° F.

The product is pumped from the condensers to a settling vat. Here it settles for an hour, the deposit being pumped into a second vat and washed with dilute sulphuric acid, in order to remove impurities. The resulting product is washed several times with water, and is then passed through a filter press. From the press it drops into bogies, which carry the pulp to the drying house, where it is dried by hot air. The process would thus appear an extremely simple and practical one. Several chemical authorities of repute, as well as manufacturing firms and others who have used the new white lead, have expressed themselves strongly as to its merits. One point immensely in favour of sulphate of lead as opposed to carbonate, is that the former is almost entirely innocuous, whereas the poisonous properties of the other are well known. It is claimed that the sulphate is not acted upon by coal gas or by the atmosphere of towns, which is always more or less impregnated with gases such as sulphuretted hydrogen, whose reactions with various metals have only too good reason to be known.

According to the patent specification bearing the names of French and Hannay, they employ lead ores, lead fume, or lead slags containing sulphur; and when these materials do not contain sufficient sulphur to form a sulphate with all the lead which sublimes in the process, they add to them pyrites or other sulphur-yielding substance to make up the deficiency. They heat the materials mixed with a suitable proportion of coke in an air-blast cupola furnace, which is by preference of an improved and special construction shown in Figs. 18 to 20, and hereinafter described; and they thereby produce sulphite of lead as a sublimate, provided that there are no chlorides such as common salt present in the charge, in which case sulphate and chloride of lead will be formed.

The sublimate is carried forward with the current of gases through flues to a fume condenser, which is by preference of the kind known as Wilson’s and French’s. As the gases and sublimate pass through the flues, hydrochloric acid is mixed

Fig. 19.

Fig. 20.

FRENCH AND HANNAY’S WHITE-LEAD FURNACE.]

with them, being by preference formed in a chamber in connection with the flue, by introducing a solution of chloride of sodium in spray, and by providing a sufficient excess of sulphurous acid beyond that required for forming the sulphite of lead. Air is also present, and a well known reaction takes place, yielding hydrochloric acid and sulphate of soda, the operation taking place at a part of the flue near enough to the furnace to be always at a red heat. The hydrochloric acid thus mixed with the gases and sublimate causes the formation of chloro-sulphite of lead, or other combinations of lead, sulphur, oxygen, and chlorine of variable constitution, depending on the proportions of the several constituents, but in most cases the product is a body which forms a white pigment of extremely good quality.

Fig. 18 is an elevation of their improved cupola furnace; Fig. 19 is a corresponding vertical section; and Fig. 20 is a horizontal section as at the level of the principal tuyeres.

This cupola furnace is formed with a deep hearth _a_ (like the American lead smelting cupola) having a siphon outlet _b_, for withdrawing molten lead when necessary. At a level a little above the siphon outlet _b_, an outlet _c_ for slag and scoria is provided, and the main blast tuyeres _d_ enter at about the same or a slightly higher level. With this arrangement, a considerable depth of melted lead is constantly maintained, and the choking of materials is thereby avoided at the level of the tuyeres. For a short distance above the main tuyeres _d_, the interior of the furnace is made with the sides _e_ moderately and gradually widening upwards, and is afterwards continued upwards of uniform diameter or width. The charging door _f_ is between 3 and 4 feet above the main tuyeres _d_, and the space above the charging door is crossed by two or more arched diaphragms _g h_, of brickwork, having irregular openings in them, there being doors _i k_ in the side of the furnace above each diaphragm, for inspection and cleaning. The purpose of these diaphragms is to cause the air, gases, and sublimate to become thoroughly intermixed and to ensure the complete combustion of any black smoke and the oxidation of any sulphide of lead that may be present.

An upper series of small tuyeres or jet pipes _l_ is provided for admitting air a little below the lowest diaphragm _g_, these jet pipes _l_ being supplied by a branch pipe _m_ from the main air blast pipe _n_, it being found that the introduction of the air and the production of heat can be best regulated by supplying a portion at the upper part of the furnace in this way, in addition to that supplied by the main tuyeres _d_; and a tap or valve _o_ is fitted on the branch pipe _m_ for adjusting the supplementary supply thus admitted. Above the highest perforated diaphragm _h_, the interior of the furnace communicates with a lateral flue _p_, through which the gases and sublimate pass; and at a part of this flue _p_, sufficiently near the surface for the gases to be still hot enough, an enlargement or chamber _r_ forming a descending part of the flue, is constructed. Into this chamber _r_ a solution of chloride of sodium is introduced as a spray from a number of jet pipes _s_, for the purpose hereinbefore explained. This chamber _r_ is provided with a door _t_ at its lower part, for periodically removing matters that become deposited in it. The continuation _u_ of the flue communicates with the lower part of the chamber _r_.

Preferably the spray of chloride of sodium is fine enough to allow of all or nearly all of the water being instantly evaporated, so as to leave the salt in fine particles, and in a favourable condition for being acted upon by the sulphurous acid, steam, and oxygen present in the gaseous currents. The temperature of the chamber _r_ should not be allowed to fall below red heat. The proportion of chloride of sodium used will depend on the amount of chlorination of the lead that may be desired, in addition to what is necessary for converting the zinc and other metals into chlorides. In practice a proportion of salt 2½ to 5 per cent. of the weight of the sublimate formed answers the purpose and yields a good product, but a larger proportion may be used without injury.

It is of great importance to keep the temperature of the upper part of the furnace steadily at a red heat and flaming, as the colour of the sublimate will be inferior if the temperature is either too high or too low. To facilitate the proper regulation of the temperature, a pyrometer (which may be similar to the kind used in ironworks) is employed, which pyrometer is placed in the flue at a distance from the furnace where it cannot be injured; and by a few trials is ascertained what temperature should be indicated by the pyrometer when the temperature in the furnace is what it should be. This point having been ascertained, a glance at the pyrometer will at any time show whether the furnace is working properly or not. The inventors also provide for rapidly cooling the upper part of the furnace without interfering with the lower part, in the event of the heat becoming too great, by arranging a water pipe _w_, with a set of jets, round the top of the furnace, so that on turning a tap on the supply pipe a spray of water may be applied to the outside of the furnace; and as it is desirable that water applied in this way should not run down to the lower part of the furnace they build gutter plates _x_ into the sides of the furnace just above the charging door _f_, to lead off any surplus water to a drain pipe.

Sufficient hydrochloric acid may be formed or introduced, as hereinbefore described, not only for forming chloro-sulphite of lead in the condenser, but also for saturating all the free oxide of lead, and for combining with and rendering soluble any iron, zinc, antimony, silver, or other metals. The chlorides thus formed become dissolved in the water of the condenser, and the solution, separated from the insoluble white pigment, may be treated by known processes for recovery of the metals. When the lead ores or other lead-yielding materials contain silver to a greater extent than 5 oz. per ton, a notable quantity of the silver is volatilised, and if it is left in the white pigment it renders the latter sensitive to sunlight; whereas if rendered soluble in the manner hereinbefore described, and separated by any of the known processes, it becomes a source of profit.

The white pigment is washed in the ordinary way; and when chloride of zinc is not completely removed by washing, a small quantity of sulphuric acid may be mixed with the pigment, by adding the same to the last washing water, to convert the chloride of zinc into sulphate, which is not hygroscopic.

The white pigment made as hereinbefore described is a very good and economical material for manufacturing into chrome yellow, this being done by mixing a solution of any suitable chromate or bichromate with the wet pigment; whilst the chrome yellow thus obtained may be converted into chrome orange or red by treating it in the usual way.

_Italian Process._--The precise period of the introduction of white-lead manufacture in Italy is unknown, but it was certainly previous to the beginning of the present century. Prior to 1881, the Dutch process was exclusively used in Italy. In 1881 the so-called Brumlen and Dahn process was introduced into Liguria. Somewhat later the Rhenish process was introduced. The Rhenish process is one in use in nearly all the Italian white-lead manufactories. It is employed in a large manufactory at Cogoleto, as follows:--Lead in thin sheets of about 3 feet in length, and 4 inches in width are placed in a clay chamber having the form of a cube, of about the capacity of 5800 cubic feet. In this chamber there is a wooden framework, upon which are hung the sheets of lead. Three of these sheets weigh together about 4½ lb., and as the capacity of the chamber is about 20 tons, it can hold about 30,000 sheets of lead.

On the floor of the chamber are placed twenty-four copper receptacles, each having four circular apertures. These receptacles are all in direct communication with a large pipe of masonry, which, by means of a copper tube, receives the gas coming from a boiler and furnace placed under the chamber. In the boiler, which is also of copper, is placed a mixture of 900 parts of water, and 80 parts of acetic acid concentrated to 40°, and the capacity of the boiler is about equivalent to 25,882 gallons. The furnace serves the purpose of producing carbonic acid.

The gaseous mixture, consisting of volatilised acetic acid, carbonic acid, and aqueous vapour, is admitted from the boiler and furnace into the chamber in quantity and proportions best adapted to each stage of the work. The chemical reactions resulting are analogous to those which take place in the Dutch process, and the Brumlen and Dahn process. Each operation lasts six weeks, and gives a product of about 20 tons of white lead, with a consumption of 7 per cent. of acetic acid, and 9 tons of coke. The residuum of lead is about 10 per cent. of the quantity placed in the chamber. At the end of each operation the white lead taken from the chamber is washed and purified in large tubs, some of which are furnished with filters. Finally, it is packed in earthen vessels, and dried, when it is ready for the market, and is sold either in cakes or powder.

The establishment at Cogoleto, above referred to, is able to produce annually about 2000 tons of white lead, of which 1200 tons are produced by the Rhenish process, and 800 tons by the Brumlen and Dahn process. There is also in a manufactory in Milan, the process of revolving heaters. The process of precipitation has not yet been used in Italy. The Rhenish process, as above described, furnishes the greater part of Italian white lead. The smaller manufacturers still use the Dutch process, but the Brumlen and Dahn process is generally regarded as unsatisfactory.