WHITE EARTH COLOURS
The white earth colours are important for the purposes of the
colour-maker, because not only are they used by themselves as paints,
but also serve in the production of light shades of other colours.
The white colours containing clay or lime are the most abundant and
important of all, and will therefore be described first. The lime
colours comprise caustic lime, carbonate of lime (chalk or powdered
limestone), gypsum and bone ash.
CAUSTIC LIME
Though this product is not used direct as a painters’ colour, it is
employed in the preparation of compounds that are so used. It is made
on a large scale for the preparation of mortar, and there is therefore
no need for the colour-maker to manufacture it himself, since it
can always be bought from a lime-burner. It must be borne in mind,
however, that lime for the colour-maker’s purposes must possess certain
properties, failing which it is of no use to him. What these properties
are and how the product is made will now be briefly described.
When carbonate of lime, _i. e._ native limestone, is exposed to strong
heat it parts with carbon dioxide and is transformed into burnt or
caustic lime.
CaCO{3} = CaO + CO{2}
Carbonate of lime. Caustic lime. Carbon dioxide.
The limestone is burned either in kilns of very simple construction,
or else in more complicated furnaces in which a continuous process is
maintained. The ordinary limekiln, which can be found in many parts of
the country, consists merely of four walls, with a door in the front
one for the introduction of the fuel. Kilns of this kind are usually
set up in the vicinity of the limestone deposits, and are abandoned
when they get worn out.
The limestone is broken to lumps of fairly even size, about as large
as a man’s head, and these are piled up in a domed heap in the kiln,
sufficient space being left between the lumps for the passage of the
flame. A fire is then lighted under the pile, pine wood being mostly
used for this purpose on account of its high content of resin, which
gives a very strong flame. The fire is kept up until the top of the
pile has become white hot, and only a blue, smokeless flame is visible.
The appearance of this denotes that the burning is ended, the fire
being then allowed to die out and the lumps left until cool enough to
be taken out of the kiln.
This operation is performed with great care, particular importance
being attached to preserving the lumps as intact as possible and
preventing the formation of dust, which is of little value. The lime
made in this way is endowed with properties that render it valuable
for the purposes of the colour-manufacturer; but, on the other hand,
possesses certain disadvantages.
Owing to the use of wood as fuel, the caustic lime obtained in this
way is usually a very pure white, because the burning is continued
until the whole mass is glowing and the firewood has been completely
consumed. If this is not the case, the burnt lime is greyish in colour,
from the finely divided particles of carbon, which, of course, spoils
the lime for colour-making. The defects existing in lime burned in the
above type of kiln originate in the irregular character of the product.
It will be evident that the limestone lumps that are nearest the fire
will be far more strongly heated than those in the upper part of the
dome; and when calcined lime is kept incandescent for a long time, it
becomes so compact in texture that it quenches with great difficulty
when brought in contact with water. This condition is known as “dead
burnt,” and such lime is of little value.
The lumps at the top of the pile are least exposed to the heat, and
very often still contain carbonate, as is shown by the effervescence
produced on treatment with an acid. Such lime is imperfectly burnt,
and the lumps frequently still exhibit the crystalline structure of
limestone when broken. They quench rapidly, but when mixed with a
little extra water, the mass is no longer of the buttery consistency
typical of caustic lime, but contains gritty portions consisting of
unaltered limestone.
Owing to the defects of dead burning on the one hand and insufficient
calcining on the other, colour-makers now prefer lime that has been
burned in continuous kilns, because, when properly made, such lime is
very uniform in character, and is also cheaper than that burned with
such an expensive fuel as wood. In consequence of the greater capacity
of the continuous kiln, and the more uniform character of the product,
the old-fashioned kilns are more and more falling into disuse.
The arrangement of the continuous kiln is very simple. The kiln
consists of a fairly high shaft, open at the top, and provided at the
bottom with a small hole for the removal of the burnt lime. A coal fire
is lighted, and as soon as the kiln is heated up, alternate layers of
limestone and sufficient coal for burning it are introduced. The burnt
lime sinks to the bottom of the shaft and is pulled out, with iron
hooks, from time to time.
Given the right proportions of coal and limestone, the lime made in
these kilns is burnt to just the right degree, and is excellent for
builders’ use. In many cases, however, it is less valuable to the
colour-maker, and in some quite useless. For example, when the coal
is not completely consumed, carbon, even though only a very small
quantity, is deposited on the lime, and the burnt lime, instead of
being a brilliant white, as it should be, is grey; and colour made
therefrom is also greyish white and will spoil the shade of other
colours with which it is mixed.
The chemical composition of the original limestone also has an
influence on the character of the burnt lime. Limestone consisting
entirely of carbon dioxide and lime is so rare that sufficient is never
available for making burnt lime on a large scale. Even the purest
limestone found native in large quantities--namely marble--is not pure
carbonate of lime, but contains a certain proportion of extraneous
substances. At the same time it is too expensive to use for technical
purposes.
The ordinary impurities present in limestone are ferrous oxide, ferric
oxide, magnesia and organic matter. The presence of ferrous oxide can
usually be detected by the greenish tinge of the raw limestone, and
the reddish cast of the burnt product. Ferric oxide is revealed by its
reddish colour, in both the limestone and burnt lime.
Magnesia, which is present, for example, in dolomitic limestone, cannot
be detected by the colour, either before or after burning, this oxide
being itself perfectly white; but its presence is a drawback because if
in large quantity it makes the lime very difficult to quench, and such
lime is never of a fatty character.
Organic matter betrays itself by the colour, the lime being dark
tinted, varying from grey to black. Black limestones usually contain
carbon in an extremely fine state of division, and are quite useless
to the colour-maker owing to the impossibility of completely burning
off this contained carbon, which always imparts a greyish tinge to
the burnt lime. The behaviour of limestones in this respect varies,
however, considerably, and can only be ascertained with certainty by a
trial burning. Many that are rather dark in colour will, nevertheless,
burn perfectly white, whereas others, much lighter in shade, always
give a product that is not quite pure in tone. This divergent behaviour
seems to have some connection with the chemical composition of the
organic matter in question. If it consists of coal, or substances
analogous thereto, no really pure white lime can be obtained from
a light grey limestone, it being impossible to burn off the finely
divided carbon completely.
In addition to making a trial burning with a fairly large sample of
material, the behaviour of a limestone towards hydrochloric acid will
afford some information as to the nature of the grey colouring matter.
If the limestone dissolves completely when suffused with the acid, the
indications are favourable for its usefulness to the colour-maker.
If, on the contrary, a black residue is left, the coloration is due
to finely divided carbon, and there is then little prospect of the
material furnishing a suitable product. In any event, a trial burning
is the most reliable guide. In addition to carbon, the presence of any
large proportion of ferric or ferrous oxide is objectionable, since,
in either case, the product will be tinged red with ferric oxide, into
which the ferrous oxide is transformed at calcination temperature.
In addition to comparing the colour of the product with a standard
sample, the suitability of a burnt lime for colour-making can be tested
by quenching. If a lump about the size of the fist be placed in a large
porcelain basin and suffused with a small quantity of water, preferably
poured in a thin stream, the lime, if properly burned, will continue
to absorb the water for a considerable time, like a sponge, and will
very soon give evidence of a brisk reaction by increasing in bulk and
generating such an amount of heat as to cause the immediate evaporation
of a few drops of water allowed to fall on the surface of the mass.
Finally, the entire lump will crumble down to a very delicate,
voluminous powder, consisting of slaked lime (calcium hydroxide).
This chemical reaction is expressed by the equation:--
CaO + H{2}O = Ca(OH){2}
Lime Water Calcium hydroxide.
When the amount of water added to burnt lime is no more than
sufficient to effect its transformation into hydroxide, this latter,
as already stated, forms a delicate white powder. The addition of more
water results in the formation of a homogeneous pulp, of a peculiar
fatty character. Since this fatty appearance is only possessed by pure
lime, it is a criterion of high quality in burnt lime, and contrasts
strongly with that of the less valued poor (or lean) lime.
Calcium hydroxide acts as an extremely powerful base, and therefore
must not be mixed with colours that are sensitive to the action of
strong bases. As a matter of fact, its direct use in painting is very
small. Of course, a thin milk of lime is used for whitewashing walls,
etc.; and if any colouring ingredients are added they must be such--_e.
g._ ochres--as are not affected by the lime. Nevertheless, quick
and slaked lime are very important in colour-making, as forming the
originating material for the preparation of a number of colours.
When slaked lime is mixed with sufficient water to form a stiff pulp,
and is left exposed to the air for some time, a change will be observed
to take place, the mass solidifying gradually (commencing on the
outside) and finally crumbling to a soft white powder. This change is
due to chemical action, the lime having a great affinity for carbon
dioxide, which it readily takes up from the atmosphere--a fact which
explains the solidification mentioned. It would be erroneous to assume
that the lime is again completely converted into calcium carbonate in
this way; for, though such conversion does ultimately take place, it
requires a very long time for completion.
The resulting compound is, actually, a double compound of calcium
oxide and carbonate. Although this compound has fairly strong basic
properties, they are, nevertheless, far weaker than those of caustic
lime, being partly neutralised by the carbon dioxide absorbed. If the
superficial area of the slaked lime be increased by spreading it out
thinly, so as to offer greater opportunity for the action of carbon
dioxide, the formation of the double compound in question will be
greatly accelerated.
This double compound is prepared artificially in special works, and
the resulting colours are put on the market under various names. They,
too, must not be mixed with colours that are sensitive to alkali, and
on this account they cannot be used in fine paints. If applied as a
white priming to the walls of rooms, care must be taken to cover the
coating with some substance that will protect the topping colour from
the action of the lime. For this purpose, painters use a wash of milk,
soap and water, etc.
An important property of lime is its behaviour towards casein, the
substance forming the curd of milk. With this body it combines to
form a mass which sets hard and is highly resistant, viz. calcium
caseate, and is formed when limewash is stirred up with milk or freshly
precipitated casein. Weatherproof distempers for outside use are
prepared in this manner.
PEARL WHITE
The preparation frequently met with in commerce under this name is
nothing more than a burnt lime of great purity. It is prepared in the
coastal districts by burning oyster shells, the resulting burnt lime
being easily transformed into a fine powder, the pure white colour of
which is due to the absence of iron. It is used in the same way as pure
burnt lime, and is mainly of interest in seaside towns where oyster
shells are often accumulated. It may be pointed out that the name pearl
white is often applied also to pure white grades of white lead.
VIENNA WHITE
This colour is prepared from any kind of burnt lime that is
sufficiently pure; that is, free from ferric oxide. The method of
preparation is simple, requiring no special apparatus, and can
therefore be carried out wherever suitable lime is available.
Operations are commenced by carefully slaking well-burnt lime with
water, a sufficient excess of which is added to produce a fairly thick
pulp. To accelerate the absorption of carbon dioxide, the mass is
exposed to the air in thin layers, by spreading it out on boards, so as
to present a large surface to the air. As soon as the pulpy character
has disappeared, the mass is detached from the boards, and is pressed
and kneaded, with wooden paddles, into prismatic cakes which are left
exposed to the air--being, of course, protected from the wet--until
the absorption of carbon dioxide is complete--a condition that can be
recognised by the earthy character of the product. The cakes are then
dried, an operation entailing great care, since lightness is a sign of
good quality, whereas a damp product is very heavy.
In forming the cakes they must not be touched by the bare hands,
because the lime is so caustic that it would soon destroy the skin. The
foregoing method of manufacture is capable of many improvements, which
can be introduced without adding much to the cost of production.
If the lime is formed into large blocks, it will evidently take a
long time for the mass to acquire, all through, the earthy character
indicating combination with carbon dioxide. This drawback can be easily
remedied by forming the mass into small cakes, which will become ripe,
owing to their larger surface, much sooner than the bigger blocks.
A very good plan to adopt in moulding is to form the burnt lime into
a stiff paste with water, preferably by adding enough water to make a
viscous mass, and leaving this in a lime-pit for several weeks, the
prolonged storage enabling the lime to acquire the already mentioned
fatty character, and at the same time to become highly plastic.
Lime treated in this way can be forced through a nozzle, forming a
cylindrical rope, which can be cut by a knife into convenient lengths
and left on boards for a few days until they have become firm enough to
stand up without breaking. Cylinders made in this manner, with a length
of about four inches and a diameter of two inches, will absorb carbon
dioxide very quickly.
The absorption can be still further accelerated by setting up the
cylinders in an atmosphere highly charged with the gas, for instance
in the vicinity of a manure pit, as they will then avidly take up the
carbon dioxide abundantly liberated from the rotting manure. Similar
acceleration will take place if the boards carrying the cylinders are
placed in a stable, or in a room where wash for making spirits is
fermenting, because large quantities of carbon dioxide are liberated in
both places.
Working the caustic mass by hand is accompanied by so many
inconveniences that it seems highly desirable to employ some mechanical
moulding device which will render contact with the wet lime entirely
superfluous. It may be pointed out that such a device can also be
advantageously used for moulding all earth colours in paste or pulp
form, and in particular for shaping ferric oxide colours into rods or
small cylinders.
[Illustration: FIG. 26.]
Such a machine (Fig. 26) is composed of a rectangular box with
semi-cylindrical bottom, a detachable shaft carrying a sheet-metal
worm being arranged in the box so that the worm is in contact with
the rounded bottom and is continued into the cylindrical extension of
the box. This extension terminates in a hollow cone, to the mouth of
which nozzles of varying aperture (square, rectangular or round) can
be attached. A knife, operated by hand or mechanical means, enables
the extruded soft mass to be cut into convenient lengths, which drop
on to a series of easy running rollers in front of the nozzle, and are
thereby delivered to an endless-belt conveyor from which they can be
transferred to the drying-boards.
When the box has been charged with the lime pulp and the worm is
rotated, the latter forces the soft mass into the cone and extrudes it
through the nozzle, so that, as long as there is any material in the
box, it is discharged as a continuous rope, of square, rectangular or
cylindrical section, on to the guide-rollers, where it can be cut off
into lengths by the knife.
A fundamental condition for the preparation of a good Vienna white is
the employment of pure raw material, which must be free from ferric
oxide or earthy impurities, and fully burned. An excellent material
for this purpose is calcined mussel shells, which furnish a loose,
and at the same time very pure, lime, and are very largely used for
lime-burning in places such as Holland, where they are available in
large quantities.
Vienna white is not much used as a paint colour, owing to its powerful
alkaline properties which have a destructive effect on many colours.
It is, however, largely employed as a polishing agent, for which
purpose it is powdered and is put on the market--mostly in bottles--as
Vienna lime. Its very handsome white colour and low price render it
particularly suitable for coarse painting, for example as a prime
coating for painted interior walls. To guard against the danger of
the painted decoration being destroyed by the alkaline nature of the
white, it is advisable to coat the dried ground with alum solution, the
alumina of which combines with the lime to form an insoluble compound
to which organic colours adhere well. The sulphuric acid also enters
into combination with the lime, the resulting gypsum having no effect
on the paints subsequently applied.
CHALK
The name chalk is used for a number of commercial substances which
differ considerably in both the mineralogical and chemical sense.
French chalk, for instance, is a mineral belonging to the steatite
group and, apart from its name, has nothing in common with true chalk,
except the white colour, and even this differs altogether from that of
chalk properly so called. It is therefore necessary, in the interests
of proper nomenclature, to differentiate the various kinds of chalk,
commencing with the mineral known by that name to the chemist and
mineralogist.
In chemical composition, true chalk is calcium carbonate, but of a
fossil character, for if chalk dust be examined under a high-power
microscope, it will be seen to consist of the shells of minute animals,
and is therefore to be regarded as fossil. The organic matter of the
animals has long disappeared, leaving the inorganic material, a very
pure calcium carbonate, behind.
[Illustration: FIG. 27.]
Such progress has been made that the zoological status of the animals
which inhabited the shells--many thousands of which are present in a
lump of chalk--has been identified; and it is known that these animals
were of marine type. Fig. 27 shows the appearance of the animal remains
in Meudon chalk when highly magnified, the upper half being viewed by
transmitted light and the lower by reflected light.
Notwithstanding the extremely minute dimensions of the chalk
animalculæ, their remains form rocks of great thickness in all parts of
the world. In Europe we find, for example, extensive chalk formations
in England, whose Latin name Albion was bestowed on account of the
white chalk cliffs occupying long stretches of the coast. The hills of
Champagne consist almost entirely of chalk; and Rügen, together with
many other islands, is nearly all chalk cliffs.
It is only in very rare cases, however, that chalk occurs in
sufficient purity to be immediately suitable for use as a pigment
or writing-material. For the most part it contains other minerals,
or large fossils, from which it has to be separated by mechanical
treatment. Nodular flints are often met with in chalk, and many
deposits contain such large numbers of the petrified shells of the sea
urchin that the chalk really cannot be used as a pigment at all, by
reason of the high cost of purification. The only places where chalk
can be advantageously worked for the preparation of pigment is where
the mineral is in a high state of purity, and also contains only very
few sandy particles. Such chalk deposits are worked on a mining scale,
and, as a rule, in the state in which the chalk comes from the quarry;
it is in the form of a soft mass, easily scratched with the finger-nail
and of fairly high density, owing to the considerable quantity of water
with which it is ordinarily impregnated.
In order to convert this crude chalk into a product that can be
used as a pigment, it is first left to dry until the lumps can be
easily broken, and then crushed into small pieces, from which all the
extraneous minerals, which occur as large lumps, are sorted out and
removed. This picking process is important, especially when the chalk
contains flints, because these latter are very hard and would injure
the millstones in the subsequent grinding.
The lumps of chalk are reduced by mechanical means, such as a
stamp-mill, or, more frequently, in a mill of the same type as for
grinding flour, since it is impossible to get the lumps so dry as to
produce the degree of brittleness necessary for a thorough reduction
in a stamp-mill. The millstones are enclosed in a wooden casing, and
the chalk is ground in admixture with water, the ground mass escaping,
through an opening in the casing, as a thick pulp which is stored for a
considerable time in large tanks.
Experience has shown that this method of prolonged storage in contact
with water greatly improves the colour. The only explanation of this
fact is that the chalk still contains a very small amount of organic
matter, which gradually decomposes in presence of water. The evidence
in favour of this is the peculiar smell given off during storage.
Even with the most careful grinding, chalk cannot be transformed into
such a fine powder that is directly fit for all purposes; and the only
way to obtain the requisite fineness is by levigation. Owing to the
large quantities that are usually handled in this process, the milky
liquid coming from the mill is mostly run into large brick tanks,
where it is left to settle until all the chalk has deposited and
the supernatant water is perfectly clear. Tapping-off being usually
impracticable, the water is generally drawn off by careful syphoning,
so as not to disturb the fine sludge at the bottom of the tank.
The deposit in the settling-tanks is shovelled into wooden boxes,
perforated at the sides to enable the water to drain away, the chalk
being prevented from escaping by lining the boxes with linen cloths.
The pulp soon loses its liquid character and shrinks considerably,
the boxes being then filled up with more sludge, and so on until the
contents have ceased to shrink. When the mass is so far dry that it
will no longer run when lifted, the boxes are covered with boards and
inverted, discharging the contents on to the boards, on which the mass
is left to become quite dry. Filter-presses are also used.
Large prismatic masses of chalk never dry so uniformly as to prevent
the formation of cracks, and if the chalk is to be sold in this form
the cracks are plastered up with thick pulp; this operation, however,
being superfluous when the chalk is to be sold as powder.
In order to obtain a more compact product and accelerate the drying of
the moulded lumps, some makers use presses, in which the fairly dry
chalk is subjected to progressive heavy pressure.
Owing to the fineness of the component particles of chalk, they adhere
so firmly together, without any bind, that a fair amount of force is
necessary to break down a piece of perfectly dry levigated chalk.
Sometimes, however, chalk exhibits the unpleasant property of losing
its cohesion almost completely when dry, and in such cases it can only
be shaped into prisms with great trouble. This peculiarity is specially
accentuated when the chalk contains magnesia; and in order to mould
chalk of this kind into blocks, a binding agent, such as ordinary
glue, must be added to the water used in grinding, care being taken
not to use too much, or the chalk will become too hard, when dry, for
certain purposes, _e. g._ as drawing or writing chalk.
For some purposes, chalk is sold in powder form, and very high purity
is not then essential, an admixture of magnesia or clay being harmless.
Gilders, for instance, use large quantities of chalk for priming
picture frames, and stir the chalk up with a certain amount of bind
(mostly size), to give the particles the desired cohesion.
The chief requirement exacted of a good quality chalk is a handsome
white colour; and this depends entirely on the quality of the raw
material, not on the method of preparation. It is known that a
substance quite devoid of colour will furnish a perfectly white powder,
because the colourless particles reflect the light in all directions
without breaking it up into its constituent yellow, red and blue rays.
Chalk, too, is in reality a colourless substance, and reflects light
with greater uniformity in proportion as the fineness of the particles
increases. Consequently, when one has a chalk that is not perfectly
white, it can, nevertheless, be made to furnish a very handsome product
by bestowing great care on grinding and levigation. Properly prepared
chalk should be as fine as the finest flour.
When the colour of the best grades of chalk are compared with what may
be termed pure white--such as that of white lead, zinc white, permanent
white--a skilled eye will always detect a greyish or yellowish tinge in
the former, even if obtained from the whitest Carrara marble.
The grey tinge is due to the presence of organic matter, which cannot
be eliminated by any known means, but which can be shown to exist by
the fact that when such chalk is heated to incandescence in the air
for a short time, the resulting burnt lime is pure white, the organic
matter having been burned off. A yellow tinge is caused by minute
traces of ferric oxide, which--as also ferrous oxide--almost invariably
accompanies calcium carbonate; and limestone free from determinable
quantities of these oxides is of rare occurrence. Ferrous oxide does
not reveal its presence in limestone unless in large proportion, its
pale green colour being of low tinctorial power, whereas ferric oxide,
which is a very strong colouring agent, can be more readily detected.
To those who are engaged in the manufacture of white earth colours,
however, it is quite immaterial whether a limestone or chalk contains
ferrous oxide, because that oxide quickly changes into ferric oxide in
the finely divided product, and a chalk which was originally pure white
will become decidedly yellow in a short time.
Fortunately, such a yellow-tinged product can be rendered perfectly
white by simple means and at small cost, all that is necessary being to
add a suitable quantity of a blue colouring matter. When this has been
done, the chalk will seem pure white to even the most skilled eye.
This result of adding a blue pigment is based on the well-known
physical fact that certain kinds of coloured light produce white
light when combined, the colours that give this effect being termed
“complementary.” A pure blue is complementary to a yellow with a
reddish cast--_e. g._ ferric oxide--and therefore a chalk that is
tinged yellow by a small quantity of ferric oxide can be changed into
a seemingly pure white substance by the addition of a blue pigment.
The only pigments of use in this connection to the colour-maker are
such as have very intensive colouring power and at the same time
are low enough in price. Such substances are ultramarine, smalt and
coal-tar dyes. Smalt is the best because its colour is unalterable.
In point of chemical composition, this substance is a very hard glass
coloured blue by cobaltous oxide. For improving the colour of chalk
or any other white, the smalt must be in an extreme state of fine
division, and levigated to an impalpable powder. Ultramarine can be
used for the same purpose, but is not so permanent.
To ascertain the correct proportion of blue pigment, it is advisable
to make a systematic experiment, which is easily performed. Exactly
90 parts of the chalk in question are triturated with 10 parts of
blue pigment in a mortar until the entire mass has become a perfectly
uniform pale blue powder, which contains 10% of the blue ingredient.
Several samples, each representing one hundred parts of the white
pigment to be corrected are carefully weighed out, 1 part of the blue
powder being added to the first sample, 2 parts to the second, 3 to the
third, and so on, and the mixtures are compared with a standard white
substance, such as best white lead or zinc white, to see which most
nearly approaches the standard colour. It is then easy to calculate
how much of the blue requires to be added to 100 or 1000 lb. of the
material to be corrected.
The correction can be effected in several ways; for instance, by
grinding the blue pigment directly with the bulk, by adding it at the
levigation stage, or mixing it with the dry, finished product. The
first two methods are attended with certain drawbacks which render
it difficult to obtain a perfectly uniform product, owing to the
specific gravity of the blue pigments being higher than that of the
whites. Consequently, when the two are mixed in presence of water--as
is always the case in grinding and levigation--the heavier blue
pigment settles down more quickly, and several strata can be clearly
distinguished in the sediment. The upper layers will still have a
decided yellow tinge--the proportion of blue being too small for proper
correction--whilst the next in order will be pure white--accurately
corrected--and those at the very bottom will be decidedly blue, because
they contain the largest proportion of the blue substance.
The most satisfactory results are obtained by dry mixing; and this can
be successfully practised when the colour-maker has a cheap source of
power (such as water power) available. Where, however, costly power
plant has to be provided, only the finest grades of white pigments can
be improved in this way, the expense of labour being too high for cheap
materials.
As a pigment, chalk possesses many valuable properties. The organic
structure of chalk gives it high covering power as a wash, a thin layer
applied to a surface sufficing to mask the colour of the underlying
ground completely. The lime in chalk being combined with carbonic acid,
its basic properties are so extensively weakened that chalk can be
mixed with even the most delicate colours without fear of their shade
being affected. A coating of pure chalk paint on any surface will never
change colour in the air; and on this account, chalk is extensively
used both as an indoor wash and by wall-paper manufacturers.
PRECIPITATED CHALK
Many chemical processes furnish soluble salts of lime that constitute a
by-product of little value. These salts, however, can be advantageously
utilised for the preparation of an artificial chalk which is preferable
to the native article in many respects. For instance, where large
quantities of calcium chloride solution are available, and soda can
be purchased at a sufficiently cheap rate, they can be converted into
artificial chalk, because these two substances react on each other,
forming, on the one hand, calcium carbonate, which is precipitated as a
very delicate, insoluble powder, and on the other, sodium chloride, or
common salt, which remains in solution, according to the equation:--
CaCl{2} + Na{2}CO{3} = CaCO{3} + NaCl.
If, however, these solutions were mixed together in a crude state, the
resulting product would be of only low value as a pigment, being of a
yellow tinge and never pure white. This is due to the fact that the
impure lime salts, being waste products from chemical works, frequently
contain fairly large amounts of ferric oxide, and the soda also is
often so high in that impurity that the colour of the precipitated
chalk is considerably impaired.
Fortunately, there is no difficulty in eliminating this ferric oxide
by chemical means, and obtaining a product of superior colour to
the best native chalk. This is effected by treating the perfectly
neutral lime-salt solution with calcium carbonate, which causes the
precipitation of the iron, a corresponding amount of lime passing into
solution.
In order to eliminate the ferric oxide from the lime-salt solution so
completely that not even the most delicate chemical test known will be
able to reveal any trace remaining, the solution is placed in a vat
and stirred up with finely powdered chalk. If the solution contains
any free acid, effervescence, due to the liberation of carbon dioxide,
will take place; and in such event the addition of chalk is continued
until the free acid is all neutralised, and the added chalk sinks to
the bottom undissolved. The chalk should be in slight excess, so that a
decided sediment is visible at the bottom of the liquid when at rest.
This deposit is stirred up again at intervals with the liquid for
several days. When ferric oxide is present, the colour of the deposit
will gradually change to a yellowish brown, through the precipitation
of ferric hydroxide by the chalk; and in this way the final traces of
iron can be removed.
The liquid is then carefully drawn off, without disturbing the
sediment, and the soda solution is run in so long as a precipitate of
calcium carbonate continues to form. The completion of the reaction
can be ascertained by pouring a small quantity of the liquid into a
tall, narrow glass, leaving it to clarify, adding a little more soda
solution and observing whether any further precipitate is produced.
On the other hand, it may be that an excess of soda has already been
added in the precipitating tank; and this can be determined by testing
a sample with turmeric paper--blotting-paper soaked in a solution
of the colouring-matter of turmeric root--which is turned brown by
alkaline reagents. Even in very dilute solution, soda will give this
colour change, and the test is therefore very accurate. The complete
precipitation of the lime in the solution can be ascertained by passing
a small quantity through blotting-paper and treating it with a little
acid potassium oxalate solution, which, if lime be present, will at
once produce a strong crystalline precipitate of calcium oxalate, which
is only very sparingly soluble in water. If the oxalate gives merely
a slight turbidity, the residual amount of lime is so small that the
process may be regarded as complete.
Since carbonate of soda is usually much dearer than the lime-salt
liquor, it is preferable to leave a small quantity of the lime
unprecipitated. Given sufficient care in effecting the precipitation,
and especially when fairly strong solutions are used, a brilliant white
precipitate of calcium carbonate is obtained, which is in such a finely
divided state that the minute constituent crystals can only be detected
under a high magnifying power.
This precipitated chalk being already in an extremely fine condition
needs no further preparation, and, when washed, is ready for immediate
use, forming a handsome pigment with excellent covering power.
When precipitation is ended, the deposit is allowed to settle down,
and the clear supernatant liquid is carefully drawn off so as not to
disturb the delicate sediment, which is then stirred up thoroughly with
clean water, left to subside, washed again, and then spread out to dry
on cloths which are suspended by the four sides. The surplus water
drains away and the residue gradually assumes the consistency of paste,
in which condition it can easily be moulded to any desired shape. If
left long enough to dry completely, it forms a very delicate powder,
furnishing a pigment of excellent quality.
If this precipitated chalk be moulded into prisms for sale, the blocks
are laid on one of their broad sides until firm enough to turn over
on to one of the narrow faces, slabs of gypsum being used as the
supporting material, in order to ensure uniform drying. The gypsum
absorbs water with avidity and thus dries the prisms evenly.
A defect of these prisms is their great fragility; but their strength
may be improved by mixing a little very weak solution of dextrin to
the mass after the last washing-water has been completely removed. In
drying, the dextrin binds the material of the prisms sufficiently to
keep them from breaking except under the influence of a fair degree of
force.
CALCAREOUS MARL
As already mentioned, calcium carbonate rarely occurs in a perfectly
pure condition in Nature; and chalk, also, is frequently contaminated
by other minerals. A variety of limestone occurring as extensive
deposits in many places is that in which calcium carbonate is
associated with clay. Sometimes the clay predominates, and the mineral
is then known as marl, being really a clay contaminated with chalk. If,
on the other hand, the chalk forms the chief constituent, the mineral
is termed calcareous marl.
Calcareous marls are used in much the same way as limestone, some
modification, however, being necessitated by the presence of the clay.
Although limestone containing a certain amount of clay can be burned
in the kiln, it yields an inferior lime that is of little use to the
builder owing to its low binding power. Marl of a certain composition
finds an important application in the manufacture of hydraulic lime or
cement.
The only kind of marl suitable for pigment is that containing clay
with very little colour; and this is of somewhat rare occurrence,
because most marls contain sufficient ferric oxide to give them a
yellow shade. Marl that is fairly free from ferric oxide, however, can
very well be used as pigment; and many white pigments sold as “chalk”
are really finely ground marl. In accordance with the general practice,
in the colour industry, of giving colours a great variety of names, and
suppressing the real names, which, so far as the artificially prepared
colours are concerned, should bear some reference to their chemical
composition, numerous white earth colours bear fancy names, though
really consisting of chalk, lime (generally marl), or white clay.
In France, where both chalk and clay are of frequent occurrence--the
soil of Champagne, for instance, being all chalky--the manufacture of
the white earth colours is extensively practised, and a large number
are put on the market, usually named after the place of origin, and
consisting of either calcium carbonate or marl.
The trade names of the white earth colours include Cologne chalk,
Bologna chalk, Briançon chalk, Champagne chalk, Blanc de Bougival,
Blanc de Meudon, Spanish white, Blanc d’Orleans, Blanc de Troyes, etc.
All are either more or less pure chalk, marl, or a fairly white clay,
pipeclay--which is also used for making clay pipes and for removing
grease spots.
GYPSUM
The mineral known as gypsum, or alabaster, consists of calcium
sulphate, or sulphate of lime, its composition being expressed by
CaSO{4} + 2H{2}O. In gypsum the crystalline structure is just
discernible, whilst other varieties, such as the so-called “marine
glass,” occur in considerable quantities as large, perfectly
transparent masses. “Russian glass” consists of large, transparent
lumps possessing the specific property of gypsum, viz. that of cleaving
in two directions, in a high degree. Alabaster is composed of finely
granular masses, which are either quite white, or else yellowish, or
traversed by grey veins. This variety of gypsum is very abundant in
central Italy, and the best blocks are employed for the production of
works of art.
Ordinary gypsum, which frequently occurs in the vicinity of dolomitic
limestones, is found in a great variety of colours, bluish-grey,
yellowish or reddish tints being the most common. Pure white lumps,
which are plentiful in some deposits, can be used as white pigment,
the method of preparation being simple, viz. merely reducing the mass
to powder. This is easily effected, the specific hardness of gypsum
being only 2; and in many cases it is soft enough to scratch with the
finger-nail.
If the original gypsum is white, the powder forms a dazzling white
flour which, notwithstanding, is of comparatively little value as
a pigment, on account of its low covering power. For this reason,
powdered gypsum is chiefly used for making plaster of Paris (calcined
gypsum) for plaster casts and stucco. Gypsum may also be employed to
advantage for lightening various colours, since it is inert towards
even the most delicate.
KAOLIN, PIPECLAY
Large areas of the earth’s surface are covered with clay, which often
attains a considerable thickness. Nevertheless, the kind of clay that
is suitable for use as pigment is comparatively scarce. The principal
requirement for this purpose is a pure white colour, but by far the
great majority of clays are either yellow or of a shade between blue
and grey (for example the clay of the Vienna basin).
The character of clay is just as varied as its colour. In some places,
large deposits of extremely fine clay are found, the material, when
mixed with water, forming a highly plastic mass which, when dried and
subjected to slight pressure, furnishes a very soft powder. On the
other hand, some clays are so interspersed with large quantities of
sand, large stones and the debris of mussels, that they cannot be used
until they have been put through very careful mechanical treatment.
This great divergence in the physical character of clays is due to
their method of formation. Clay originated in the weathering of
felspar, which chiefly consists of a double salt, a compound of the
silicates of alumina and potash. Under the influence of air and water,
this compound is decomposed, the potassium silicate passing into
solution, whilst the aluminium silicate, being insoluble in water, is
carried away by that medium. When the water can no longer carry the
particles of aluminium silicate in suspension--for example when it
reaches a sea or lake--the silicate settles down to the bottom, and a
deposit of clay is formed.
If the original felspar was very pure, and in particular very low
in iron, the resulting clay will be of a handsome white colour. An
example of this is afforded by kaolin, or porcelain earth, which is
preferably used for making china. If, however, the felspar contained a
considerable proportion of ferric oxide, the resulting clay is yellow;
and if stones or mussel shells became incorporated with the clay prior
to deposition, these bodies will be found as inclusions in the deposit,
and such clay will require much troublesome preparation--grinding and
levigation--before it is fit for use.
For the purposes of the colour-maker, the most suitable clay is one
that is pure white, free from inclusions, and does not change colour
when exposed, in a finely divided state, to the action of the air.
Many clays that were originally white gradually assume a yellow tinge
on prolonged exposure to air and moisture, because the clay contained
ferrous oxide, which changes, in the air, to the stronger pigment,
ferric oxide.
Many kinds of clay merely require a simple levigation to fit them for
use as pigment. The lumps of freshly dug clay are placed in large
tanks, etc., filled with water and stirred up continuously in order
that, instead of forming a plastic mass which is very difficult to
distribute in water, the particles detached from the lumps may pass at
once into suspension. This turbid water is then transferred to another
tank, etc., where the minute particles of clay are allowed to settle
down, and the water becomes quite clear.
Where this work is carried on on a large scale, it is advisable to put
the freshly won clay into large pits close to the clay deposit, and
to leave it there, covered with water, during the winter season. The
freezing of the water breaks down the larger lumps of clay, by the
resulting expansion, and this facilitates the subsequent levigation,
the cohesion between the particles being destroyed.
If the clay contains larger proportions of lime or magnesia, a little
experience will enable their presence to be detected at once by the
way the clay behaves on being placed in contact with water. Pure clay
quickly forms a fatty and extremely plastic paste, and sticks closely
to the tongue when applied in the dry state. On the other hand, clay
containing much lime or magnesia is far less plastic when mixed with
water, and the dry clay hardly adheres to the tongue at all.
These latter clays are classed as poor or lean, in contrast to the fat,
plastic kinds. For certain purposes for which clay is used as pigment,
these admixtures are not harmful; whereas others, especially quartz
sand and mica, not infrequently present in white clays, constitute a
serious drawback.
As already mentioned, clay is formed by the weathering of felspar,
which is a constituent of granite and gneiss, both rocks composed
of quartz, mica and felspar. When the clay has been derived from
the weathering of such rocks, it is easy to understand that it may
contain admixtures of quartz and mica, which are frequently visible
to the naked eye, or at any rate under the microscope. Whereas clay
forms a white, amorphous mass, the grains of quartz sand are decidedly
crystalline, transparent and of vitreous lustre; the scales of mica,
on the other hand, appearing as thin tabular crystals, mostly of a
green or brown colour and exhibiting, when viewed at certain angles, a
brilliant metallic sheen.
Quartz sand can be eliminated from clay without any special difficulty,
quartz being of higher specific gravity and therefore settling down
quickly, leaving the delicate particles of clay in suspension in the
liquid. The scales of mica are harder to get rid of, their tabular form
retarding deposition from the suspending liquid; and on this account,
several washings are often required to separate them completely.
In all cases where clay is to be used as a white distemper, the
presence or absence of lime is immaterial; but where it is to be
employed for removing grease, lime is a drawback. This is also
sometimes the case when the clay is wanted for the purposes of the
colour manufacturer. The author has found, by experience, that
perfectly pure, white clay forms a good paint, in a vehicle of oil or
varnish--a purpose to which it has, so far, been seldom applied, if at
all. Such paint is of good covering power, and possesses the valuable
property of remaining quite unaffected by atmospheric influences.
If, however, the clay contains even but a small quantity of lime, it
cannot possibly be used as an oil or varnish paint, for though the
freshly made paint has a very good appearance, its character soon
changes, turning viscous and suffering a considerable diminution of
covering power. Thinning with turps or boiled oil results in the
formation of small lumps, so that it is quite impossible to obtain a
uniform coating on even a small surface.
This behaviour is apparently due to the presence of the lime, the
explanation being that the fatty acids always present in the oils and
varnishes used for the paint combine with the lime to form compounds
which, from the standpoint of the chemist, must be regarded as soaps.
The small lumps already mentioned really consist of lime soap, and
the formation of these colourless compounds accounts for the lessened
covering power.
Given a fine white clay, otherwise capable of forming a valuable
pigment, it is sometimes possible, by simple means, to eliminate
accompanying lime, provided the amount of the latter is not too great,
and also provided that very cheap hydrochloric or acetic acid is
available. The acid need not be pure, and the impure but very strong
pyroligneous acid, which is very cheap on account of its empyreumatic
smell, may be used.
To eliminate lime from the clay, the still moist levigated mass is
introduced, in small quantities, into a vat containing the requisite
quantity (see later) of hydrochloric or acetic acid, the addition being
continued until the liquid gives only a faintly acid reaction with
blue litmus paper. When the clay is run in, effervescence is produced
by the liberation of the carbon dioxide displaced by the stronger acid
employed.
The amount of lime present in a clay may be determined by very simple
means. A small sample of the clay is dried by artificial heat, until
of constant weight, and exactly 100 parts by weight of the dry mass
are placed in a glass and suffused with hydrochloric acid, sufficient
of the latter being used to make the liquid still strongly acid after
effervescence has ceased.
The contents of the glass are transferred to a filter, and washed
with pure water so long as the washings continue to redden blue
litmus paper. The residue is then dried until of constant weight, and
the difference between the initial and final weights will give the
percentage of substances soluble in hydrochloric acid.
After performing this simple test on a clay, it is easy to calculate
the quantity of acid needed to extract all the soluble constituents
from a given weight of the material. All that is necessary is to
measure the volume of acid required to extract a small quantity of
the clay completely. Thus, if one pint of the acid at disposal is
sufficient to treat one pound of the clay, the amount needed for a
given quantity of clay is a simple matter of calculation.
Since, on account of the cost of pure hydrochloric acid, crude acid
will always be used, it will be necessary to remember that this crude
acid always contains ferric oxide in solution--this being the cause of
its yellow colour. If the amount of acid taken is barely sufficient to
combine the whole of the lime, leaving the latter slightly in excess,
the ferric oxide--which would otherwise tinge the clay yellow--will be
precipitated.
If, on the other hand, the acid is in excess, the clay is obtained free
from all constituents soluble in the acid. The purified clay must then
be freed from the calcium chloride, formed by dissolving the lime, by
a thorough washing, since the clay would otherwise always remain moist
on account of the hygroscopic properties of the chloride in question.
Moreover, any small residuum of free acid would constitute a drawback
on the clay being mixed with other colours.
Calcium chloride is very soluble in water, and therefore can be
completely removed from the clay by washing. The purified clay is left
to settle down as completely as possible, and after drawing the liquid
off from the sediment, the latter is suffused with pure water and left
to settle once more. As a rule, two such washings will cleanse the clay
of calcium chloride and free acid sufficiently to render the product
suitable for any purpose.
When large quantities of clay have to be treated in this manner,
considerable amounts of calcium chloride solution will be obtained,
which can be advantageously utilised for the production of
precipitated chalk, all that is necessary being to collect the liquor
in a large tank and treat it with a small quantity of slaked lime, to
transform the surplus free acid into calcium chloride and precipitate
the ferric oxide present in solution. At the end of a few days the
liquor in the tank will consist of a very pure solution of calcium
chloride which will furnish an excellent precipitated chalk when
treated in the manner already described under that heading.
BARYTES, OR HEAVY SPAR
This mineral--chemically, barium sulphate, BaSO{4}--occurs native, as
extensive deposits, in many places--England, Bohemia, Saxony, Styria,
etc. It sometimes forms handsome tabular crystals, but more frequently
compact masses, which may be pure white, grey yellow, etc., in colour,
and are distinguished by high specific gravity (usually 4·3–4·7),
to which the mineral owes its name. This high density also limits
the application of the mineral, and it cannot be used as a pigment,
in the true sense of the term, being only suitable as an adjunct to
artificially prepared colours.
The employment of barytes in the colour industry is often regarded as
adulteration, which, however, it is not when the case is considered
from the right point of view. For instance, the only preparation which
can properly be termed white lead consists of basic lead carbonate.
This, when pure, is a rather expensive pigment, whereas, for certain
purposes, the consumer requires a product that can be obtained at a low
price. In order to satisfy this demand, the only course open to the
colour-maker is to mix the white lead with a cheap white substance,
which enables him to turn out different grades of white lead, which,
although low in price, are far inferior to the pure article in covering
power. Pure white lead being itself a very heavy substance, the only
bodies suitable as adjuncts are such as are also of high specific
gravity; and of all the cheap pigments known, heavy spar is the only
one endowed with this property. Consequently, this substance is
extensively used in making the cheaper grades of white lead and the
pale kinds of chrome yellow.
The only cases in which the addition of heavy spar to a colour can be
regarded as an intentional fraud on the consumer is when he is sold,
as pure white lead, chrome yellow, etc., a product really composed
of a mixture of such colour and barytes. Moreover, the presence of
barytes in white lead can be easily detected by a simple examination,
pure white lead readily dissolving, with considerable effervescence,
in strong nitric or acetic acid, whereas barytes is insoluble in all
acids, and therefore remains, as a heavy white powder, at the bottom
of the vessel. In this way both the presence and amount of barytes
contained in a sample of white lead or chrome yellow can easily be
ascertained.
The preparation of barytes for the purposes of the colour-maker is
entirely a mechanical operation. The barytes, which though fairly hard
is easily reduced, is crushed with stamps, ground in a mill and finally
levigated, it being impossible to obtain a sufficiently fine powder
even by repeated grinding.
Native barytes must not be confounded with the artificial barium
sulphate sold as permanent white or blanc fixe, which is an extremely
finely divided barium sulphate obtained by precipitating a solution
of a barium salt with sulphuric acid or a soluble sulphate, and is
a painters’ colour that is highly prized for certain purposes. Both
the native sulphate and the artificial variety have the property
of remaining completely unaltered by exposure to air, and they can
therefore be mixed with any kind of pigment without fear of the colour
deteriorating.
As a rule, barytes is first roughly crushed in edge-runner mills or
stamps, and then ground to the extreme degree of fineness obtainable in
ordinary mills. Even with the greatest care, however, it is impossible
by this means to obtain sufficient fineness of division for mixing with
fine colours, the only way in which this can be accomplished being by
levigation.
Given a fairly pure white barytes to begin with, levigation furnishes a
handsome white pigment that can be mixed with colours of any kind; but
when used by itself in association with oil or varnish, its covering
power is very low and the colour never perfectly white. Native barytes
is therefore unsuitable, as such, for paint.
Varieties that are not pure white are sometimes corrected with
ultramarine, added in the grinding-mill. If the yellow tinge is due
to iron compounds, this can often be remedied by treating the finely
ground material with hydrochloric acid, which dissolves them out, this
treatment being followed by a thorough washing with pure water.
As already mentioned, white lead is most frequently mixed with barytes,
this being usually added when the white lead is being ground, by
feeding the two materials to the mill and grinding them together.
The crudeness of mechanical methods of reduction is clearly
exemplified by comparing the most carefully ground and levigated
barytes with that obtained by artificial means. The permanent white
largely used in the production of wall-paper, and quite unalterable
in air, is, chemically speaking, identical with native barytes, viz.
barium sulphate. The two also seem to be identical in crystalline
habit, as is usual in the case of one and the same mineral, whether
native or prepared by artificial means. Artificial barytes is obtained
by treating a soluble salt of barium with sulphuric acid, or a solution
of sodium sulphate (Glauber salt), so long as a precipitate continues
to form.
This precipitate is barium sulphate, which subsides completely on
account of its extreme insolubility, this being greater than that of
any other salt known. The rapid rate of deposition results in the
formation of extremely small crystals, which, being colourless and
reflecting the light completely, appear to be perfectly white. Even
when permanent white is applied in very thin layers to any surface, its
covering power is very considerable, by reason of the extremely fine
subdivision of the material.
This behaviour of artificial barytes in comparison with that of the
natural product, affords an important hint in connection with the
preparation of earth colours, namely, that in order to obtain products
of specially good quality, the endeavour should be to reduce the raw
materials to the finest condition possible. This result is accomplished
most securely by bestowing the greatest care on grinding and
levigation; and it is therefore highly important that the manufacturer
should select, from the various apparatus used in reducing the
materials, those that are best adapted for the purpose.
CARBONATE OF MAGNESIA
Although carbonate of magnesia is seldom used alone as a pigment, it
can be advantageously employed as such when circumstances permit.
It is met with not infrequently, in Nature, in a crystalline form,
as magnesite or bitter spar, the latter name arising from the fact
that the soluble salts of magnesia have a bitter taste. Still more
frequently, magnesia occurs in association with calcium carbonate, in
the mineral dolomite, which contains up to 20% of magnesia.
A less abundant native mineral is hydromagnesite, which consists
of basic magnesium hydrocarbonate. Hydromagnesite is a very light,
chalk-white mass, with a non-greasy feel, which, when reduced to a soft
powder, forms an excellent material for paint. It is highly inert, in
a chemical sense, and can therefore be mixed with the most delicate
colours, having no other effect thereon than to render them lighter in
shade.
This product can also be prepared artificially, by treating a dissolved
magnesium salt with a solution of carbonate of soda, the result being
the formation of a pure white precipitate, which is very brilliant when
dry, and is characterised by unusually low specific gravity. In some
places, conditions are such that this preparation can be made on a
large scale at very low cost. For instance, there is a spring at Bilin,
in Bohemia, the water of which contains large quantities of alkali
carbonates in solution; whilst in the vicinity of Saidschütz is a
spring fairly rich in magnesia salts. The waters from these two springs
are concentrated by evaporation, and mixed in large tanks; and when a
sufficient deposit of the resulting basic carbonate of magnesia has
accumulated, it is taken out of the tanks, placed on linen filters and
washed with water. The residue is dried slowly, without the employment
of a high temperature, and then forms a white powder, which is very
light and can be used for a number of purposes, chiefly medicinal,
though it is also well adapted as a material for paint.
For this latter purpose it is, however, far too expensive; but since
the conditions obtaining at Bilin are certain to occur elsewhere, we
have included carbonate of magnesia among the earth colours.
On account of its specific lightness, carbonate of magnesia is
specially adapted for making pale shades of certain delicate lake
colours, which, if toned with even perfectly pure chalk, would undergo
alteration in course of time. Carmine, for instance, can be graded,
by the addition of carbonate of magnesia, into every possible variety
of shades between the pure red of carmine itself and the palest pink;
and the resulting colours are quite permanent whether mixed with gum
solution or any other vehicle.
TALC
Although this mineral is not used as a pigment by itself, it must be
mentioned here because it is not infrequently employed for mixing with
other colours, and is also used in the wall-paper industry. It also
serves to distribute certain pigments in a state of fine division, the
“rouge végétal” of the perfumer, for example, usually consisting of
talc and a small quantity of very fine carmine.
In commerce the name talc is sometimes applied to two separate
minerals, true talc and steatite or soapstone. The former is rarely
met with native as well-defined crystals, mostly occurring as scaly
masses in primitive rocks. Thin pieces exhibit a certain degree of
flexibility. The hardness of this mineral is so small that it can
be scratched with the finger-nail; and its sp. gr. is 2·9–2·8. Talc
is easily scraped, and the powder remains sticking to the knife, a
property which renders the substance difficult to reduce to powder,
because it balls together and takes a very long time to convert into
a fine flour. The process is facilitated by calcining the talc and
quenching it in cold water, this treatment increasing the hardness
and at the same time making it more brittle, and thus more easy to
pulverise.
A characteristic feature of all the talc minerals is their peculiar
greasy appearance and feel. The colour varies, white pieces alone being
of any use to the colour manufacturer. The yellow- or green-tinged
varieties owe their shade to the presence of ferric and ferrous
oxides. In chemical composition, talc consists of a combination of
magnesium silicate with hydrated silica, the supposed formula being:
4MgO . SiO{2} + H{2}O . SiO{2}, and the percentage composition: silica,
62·6%; magnesia, 32·9%; water, 4·9%.
STEATITE OR SOAPSTONE
Steatite so closely resembles talc in most of its properties, that
the two minerals were long regarded as identical. Whereas, however,
talc is scarcely acted upon at all by the strongest acids, steatite is
completely decomposed by prolonged boiling therewith, although both
minerals have exactly the same composition.
As a pigment, steatite is far more important than talc, and, as French
chalk, is largely used for drawing or writing. To prepare it for this
purpose pure white steatite requires no preliminary treatment, beyond
cutting the large lumps up into quadrangular prisms, which are mounted
in wood, like lead pencil, and used for writing on the blackboard. The
powder produced in the cutting process is made up into pastel crayons.
With this object, the powder is mixed with a sufficient quantity of
some mineral pigment to produce a mass of the desired shade, and is
kneaded to a stiff paste with water containing an adhesive such as gum,
glue or tragacanth mucilage. The mass is shaped into prisms, which,
when dry, are cut into pencils and mounted in wood. Steatite being like
talc, without action on even the most delicate colours, can be used as
a diluent in the preparation of light shades.