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

[Illustration: Fig. 18.

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.

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

[Illustration: Fig. 21.

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

[Illustration: Fig. 25.--LEWIS’S WHITE LEAD PLANT.]

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

[Illustration: Fig. 26.--MACIVOR’S WHITE LEAD PLANT.]

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.

[Illustration: Fig. 27.--MACIVOR’S WHITE LEAD PLANT.]

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_.

[Illustration: Fig. 28.--APPARATUS FOR MAKING ZINC OXIDE.]

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.

The sulphide of zinc is in the form of white extremely light powder,

which is carried along by the current of sulphur into the collecting

chamber, such as is used for the manufacture of oxide of zinc. This

allows of the separation of the different constituents of the products

into different qualities; those parts that are carried the farthest

are the whitest and best generally. Those portions nearest to the part

of the apparatus where combination takes place may sometimes contain

metallic zinc, if the sulphur supply has not been carefully attended

to, but this may be separated from the sulphide by levigation.

The collecting apparatus should be kept at a temperature slightly

superior to that of the boiling point of sulphur, in order that

sulphur, which is necessarily in excess, may not be deposited with the

sulphide of zinc, but may pass on in the vaporous form to a suitable

condenser, where, after condensation, it may be collected and used

again.

Before the sulphur vapour reaches the condenser, the last traces of

sulphide carried along with it are collected by the interposition of

metallic screens or sieves, placed between the sulphur condenser and

the apparatus.

In carrying out the process, care must be taken to keep the collecting

apparatus as cool as possible consistently with the fulfilment of the

conditions above mentioned, viz. that no sulphur be condensed therein.

In practice, this object can be effected with little difficulty.

Impurities in the zinc and sulphur are of little consequence provided

they are not volatile, and not of such a nature that they would detract

from the whiteness of the sulphide of zinc formed.

[Illustration: Fig. 29.--APPARATUS FOR MAKING ZINC SULPHIDE.]

The accompanying diagram, Fig. 29, is given as an example of a plant

that may be employed with good results.

_a_ is the sulphur melting pan with its steam jacket _b_, and steam

pipe _c_; _d_ is a cast-iron still, arranged within a gas furnace _e_;

_f_ is a crucible for melting and volatilising zinc, the said crucible

being contained in a gas furnace _g_; _h_ is an automatic apparatus for

freeing the mouth of the zinc vessel from deposited sulphide of zinc;

_i_ is the collecting apparatus; and _k_, the condensing apparatus.

(3) _Mixtures._--There are a number of compound pigments commonly known

as zinc whites, which only deserve the name in so far as they contain

a proportion, greater or smaller, of some zinc salt. At the same time

it must be admitted that some of these combinations possess very good

qualities, and that the foreign ingredients largely correct the weak

points of the zinc compounds.

Freeman’s.--This pigment, when ground with oil in the customary way,

forms a paint equal in body and covering power to the best white lead,

while it is superior in colour, permanence, and density, and is free

from odour and noxious qualities.

It is produced by grinding together “zinc white” (either oxide or

sulphide), lead sulphate and barium sulphate, in certain proportions,

in the dry state, in an edge-runner mill. By thus grinding the several

pigments together, their particles become intimately incorporated and

undergo changes in character. The barium sulphate not only cheapens the

product by reason of its low cost, but also imparts a distinct feature

in rendering the paint more free working. The proportions generally

adopted, calculated by weight, are 5 parts lead sulphate, 2 of zinc

white, and 1 of barium sulphate. The duration of the grinding will

necessarily vary in accordance with various governing conditions, but

it should be continued until the mixture has a density of about 200 lb.

per cubic foot.

Orr’s.--The pigment known as Orr’s enamel or Charlton white, is a

compound of oxide and sulphide of zinc and sulphate of strontia, or of

baryta. It is prepared in two ways.

(_a_) Barytes is calcined for some hours at white heat with charcoal,

and the calcined mass is lixiviated with water to wash out the barium

sulphide; to one-half of this solution is added zinc chloride, which

produces a precipitate of zinc sulphide, leaving barium chloride in

solution. To this mixture of zinc sulphide and barium chloride is added

the remaining half of the barium sulphide and some zinc sulphate, the

result of which is a double precipitate of zinc sulphide and barium

sulphate. This is water-washed, filter-pressed, dried, calcined at red

heat, thrown immediately into cold water, ground very fine, and finally

dried.

(_b_) The second process closely resembles the first, but celestine

takes the place of the barytes.

Either form of Orr’s enamel is a good useful white pigment, very

permanent, mixing well, of excellent covering power, and pure in colour.

_Characters._--Zinc oxide, being an expensive pigment, is liable to

adulteration; fortunately, all such adulterations are easily detected,

and their nature ascertained by a few simple tests. Zinc oxide, if

pure, should dissolve entirely without effervescence in nitric acid;

any residue would indicate adulteration with barytes or china clay;

the former may be distinguished by its weight and the yellowish green

colour it imparts to the Bunsen flame, the latter is lighter and gives

no colour to the Bunsen flame. Boiled with strong sulphuric acid,

barytes is not acted on, while china clay is. If, after cooling, the

mass be diluted with water, and ammonia be added to the liquor, if

barytes is present no precipitate will be obtained, while if china clay

is present a white precipitate is produced.

If the zinc oxide dissolves with effervescence, white lead or whiting

may be present; the solution should give no precipitate of black

sulphide of lead on passing sulphuretted hydrogen through it. On

neutralising the solution in nitric acid with sufficient ammonia, and

adding ammonia sulphide to precipitate all the zinc (the precipitate

should be white, any other colour would show some impurity), filtering

off and adding a little oxalate of ammonia, no white precipitate of

calcium oxalate should be obtained; such a precipitate would show

presence of whiting or gypsum.

The white pigments having as a base the sulphide of zinc, also contain

barytes, oxide of zinc, sulphate of strontium, &c. They can be

distinguished by evolving sulphuretted hydrogen gas, recognisable by

its odour, on treatment with an acid. They are not entirely soluble in

acids, the residue being mostly barytes, but may also be sulphate of

strontium; it is immaterial whether the two be distinguished or not.