THE CHEMISTRY OF BRICK-EARTHS.
Introduction: THE BLOWPIPE.
It is not our intention to write an elementary treatise on chemistry;
but we know it is the custom for brickmakers to have chemical analyses
of their raw earths made, and we are aware also that the precise
meaning to be attached to these analyses is very little understood. Our
principal aim in introducing this subject, then, is to interpret, in an
elementary manner, certain typical analyses of earths and substances
used in brickmaking; but before doing so we shall explain some easy
methods of examining earths by means of the blowpipe, which will not
merely give some insight into their chemical constitution, but will
afford the intelligent brickmaker a means of investigation which he can
himself put into practice.
The results of a chemical analysis of a compound earth, as ordinarily
used by the brickmaker, widely differ from those obtained by a
mineralogical or petrological examination. The petrologist views
the earth as a mineral aggregate, the constituents of which may be
ascertained on appeal to a properly-constructed microscope--that is, in
the majority of instances. By noting the relative proportions of the
different minerals, he is enabled to state, with approximate accuracy,
what is the ultimate chemical composition of the whole. From this it
would appear that a rough chemical analysis could be drawn up by the
petrologist without having recourse to the ordinary methods of chemical
investigation. And in a limited sense that is true. But we should not
lose sight of the fact that there is, in too many cases, an amorphous
residuum in earths, the nature whereof the microscope is powerless to
reveal. It is upon this remnant that the chemist should direct his most
careful attention.
The mineralogist also can give a shrewd idea of the chemical
composition of a brick-earth by using a blowpipe and accessories. This,
in fact, may be regarded as a chemical means of investigation; but
it possesses this serious drawback, viz., the blowpipe only yields a
qualitative, and not a quantitative analysis. In other words, it can
tell us something concerning chemical compounds present in an earth,
but rarely informs us as to the relative proportions of them. Even
this, however, is of great service in many instances, though it does
not possess the value of a quantitative analysis. For example, we
have stated previously that certain ingredients are very undesirable
in a brick-earth, even in minute quantities; and that fact becomes of
increased value if we extend the field to earths used in terra-cotta,
and china and porcelain manufacture. Now, the blowpipe is a handy
instrument; it may be carried about by the prospector with its usual
accessories, and occupies but little space. Suppose he discovers a bed
of white earth which he believes to be good china-clay; he can prove
that fact, or at least obtain a great deal of information to that
end, by the mere use of that useful little instrument. Knowing, for
example, that fluorine is an undesirable constituent in such a clay for
many high-class purposes, he might test first of all for that; iron,
perhaps, may come next, and so in a few minutes he is enabled to arrive
at some valuable particulars that would take much longer to obtain by
chemistry in the wet way.
It will be profitable, therefore, for us to briefly describe the
blowpipe and the most common of its accessories, stating results
obtained in dealing with substances frequently met with in
brick-earths. With but little practice anyone can use the instrument,
though, as with most other methods of scientific investigation, it
requires expert knowledge to yield really excellent results. The simple
minerals and compounds to which we shall direct attention may be
detected with the greatest ease.
The essential constituents of a blowpipe outfit are as follow:--
1. Blowpipe.
2. Lamp.
3. Platinum-pointed forceps.
4. Platinum wire.
5. Charcoal.
6. Glass tubes.
7. Chemical reagents.
8. Miscellaneous articles.
[Illustration: Fig. 5.--Blowpipes.]
_1. The Blowpipe._--Common forms of blowpipe are shown in fig. 5. A
may be described as follows. It consists of three separate parts: a
tube _a b_ having a mouthpiece; an air chamber _c_ to retain moisture
caused by the breath of the person blowing; and a side tube _d_ ending
in a platinum-tipped jet. Another form of blowpipe, which, however,
does not differ essentially from that just alluded to, is shown in
fig. 5, B. It is not absolutely necessary to have the jet made of or
tipped with platinum, though certain examinations with the instrument
are facilitated by the use of such a tip. An essential point is, that
the hole in the jet should be of proper size, usually about 0.4 mm. The
trumpet-shaped mouthpiece shown in the diagram may be dispensed with.
[Illustration: Fig. 6.--Blowpipe Lamp, &c.]
_2. The Lamp, or Candle._--A convenient form of lamp is a Bunsen
gas-burner furnished with a special jet (fig. 6, A). For certain
purposes, however, this flame cannot be employed, as when testing a
substance for sulphur, as coal-gas frequently contains sufficient
sulphur to vitiate results. Moreover, in country districts and in
the field coal-gas is not always procurable. A convenient form of
lamp, though rather too large for transporting purposes, is known as
Berzelius’ blowpipe lamp. This, as improved by Plattner, is shown in
fig. 6 B. This consists of an oil vessel on a stand provided with
two openings closed with screw-caps, the one opening being used for
charging the lamp with oil, the other being fitted with a burner
bearing a flat wick. The lamp may be adjusted to any required height
on the stand by means of a screw. Olive oil, or refined rape oil,
is usually burnt. A spirit lamp with a flat wick is sometimes used.
In countries where neither coal-gas, alcohol, nor oil are readily
available, the prospector may use a small grease lamp. This consists
of a cylindrical box of thin metal having a wick-holder soldered on
one side, through which a flattened wick is drawn. The box may then be
filled with grease, solid paraffin, old candle-ends, or fat of similar
description. Professor Cole describes[6] it as follows:--When brought
into use the wick is lighted, and the flame directed with the blowpipe
upon the surface of the solid tallow or fat, until this is melted to
a depth of about a quarter of an inch. The lamp will then become hot
enough during use for a continuous supply to be maintained; but it is
still better to hold the lamp with the pliers over a spirit lamp until
all the contents become fluid. When about half or three-quarters empty,
it is well to drop in extra lumps of fuel--a single candle-end or
so--during use, and this additional material becomes melted up slowly
with the rest. The wick must be freely supplied with fluid fuel, or it
will char and waste away. If the lamp is kept sufficiently hot, the
wick will not require raising during a day’s work; but it can be easily
thrust up with a knife point after the flame has been at work a few
minutes. A cylindrical cap fits down upon the lamp when put aside. For
many ordinary purposes a good carriage-candle may be employed to give a
blowpipe flame, but candles have the disadvantage of not remaining at a
constant level--an important point when one is comfortably at work.
_3. Platinum-pointed Forceps._--At least one pair of forceps is needed,
and it should preferably be made of steel, nickel-plated to prevent
rusting. One end has platinum points self-closing by means of a spring,
so that the piece of mineral to be heated, placed between them, may
be firmly supported. At the other end are other forceps of ordinary
pattern for picking up small fragments; this end, however, should never
be placed in the flame. A pair of common self-closing forceps might
also be at hand for holding test-tubes, etc., in the flame.
_4. Platinum Wire._--A few inches of thin platinum wire are
indispensable, and lengths of an inch or so may be fixed into suitable
handles. A convenient method is to have a small glass rod for a handle,
and by fusing the tip of one end of the rod the glass may readily be
made to hold the piece of wire. Pieces of platinum foil are useful,
also, as will presently be seen.
_5. Charcoal._--The outfit should comprise several pieces of charcoal,
and a convenient form for each piece is a circular disc about an inch
in diameter, flat at the top and convex beneath. Long prisms of the
same material, square in section, are occasionally required; these may
be up to 6 inches, or so, in length.
_6. Glass Tubes._--These should be of hard glass, small, of several
diameters, the bore being large enough to place fragments of minerals
or earthy substances within. Closed tubes, such as test-tubes, are
always requisite.
_7. Chemical Reagents._--These are, for the most part, used as fluxes,
and those most commonly employed are borax (sodium tetraborate),
soda (sodium carbonate), and salt of phosphorus or microcosmic salt
(phosphate of soda and ammonia). Small quantities of potassium
bisulphate (in a glass bottle), as also small bottles of hydrochloric,
nitric, and sulphuric acids, and a solution of cobalt nitrate, are also
useful in certain cases. It is hardly necessary to remark that the
chemicals employed must be of the highest degree of purity.
_8. Miscellaneous Articles._--Strips of test paper, both turmeric and
blue litmus, a small hammer, a steel anvil about an inch cube, a bar
magnet, a pair of cutting pliers, a three-cornered file, and a few
small watch-glasses are very desirable, though not absolutely essential.
The reader, on glancing at the foregoing formidable list of articles,
may possibly imagine that some considerable outlay is requisite, and
that they must occupy much space. But that is not the case. An ordinary
blowpipe, a grease lamp, a small spirit lamp, and all the articles
mentioned in paragraphs 3 to 8, both inclusive, occupy but a small
space. They may be packed in a box specially fitted, and one in the
writer’s possession, containing all of them, measures only 10 inches by
5 inches by 3¼ inches, and is less than 3 lbs. in weight.
Now, as to the use of these various things. First of all, let us
examine the flame, as produced by a candle, which is typical of flames
obtained by other means described, except the Bunsen lamp. A candle
flame (see fig. 7) consists of the following parts:--
1. A dark core (_a_), which contains the gaseous products of
decomposition given off by the melted tallow drawn up by the wick.
2. A highly luminous zone (_b_), in which only partial burning of the
combustible gases takes place. In this, oxygen from the air combines
chiefly with the combustible hydrogen, whilst the carbon is separated
in a highly heated state, which causes the luminosity.
3. An outer mantle of blue tint (_c_), where the oxygen of the air is
always present in excess, so that the separated carbon is here burnt.
The highest temperature is found in this part of the flame.
[Illustration: Fig. 7.--Candle and Gas Flames.]
Technically, the outermost zone (_c_) is known as the _oxidising_
flame, and the inner luminous zone (_b_) the _reducing_ flame. The
two portions of the candle flame act in different manners on specific
mineral substances, and the blowpipe operator may use either of them
at will. The method of doing this is illustrated in the same figure.
To obtain the reducing flame, the blowpipe jet is brought to the edge
of the flame a little distance above the burner, or wick. The operator
then produces a gentle blast, which deflects the latter (upper figure)
without altogether passing into it, so that the flame is still charged
with glowing carbon. A yellowish luminous flame is the result, the most
active part of which lies at a short distance from the end.
On the other hand, the oxidising flame is utilised by passing the
blowpipe jet a little farther into the flame (lower figure) and blowing
more strongly. A pointed non-luminous flame is the result. This will
be seen to possess an inner blue cone, before the point of which the
hottest part is situated. Substances to be fused are placed in this
part of the flame, whilst those to be oxidised are placed a little
farther away, in order that they may be exposed to the air at the time
they are being highly heated.
The “platinum wire” is an absolutely indispensable adjunct to a
blowpipe outfit, and is employed as follows:--A short piece of
the wire, an inch or so in length, being attached to a handle, as
previously described, the free end of it is bent into a loop about the
size of this O. This may be heated in the flame employed, or, better
still, in the flame of a spirit lamp, and, when hot enough, it may be
dipped into a small quantity of the powdered borax or microcosmic salt,
some of which will be found to adhere to the wire. On further heating
the borax it will swell out and form a number of irregular bubbles,
which (heat still being applied) will subsequently settle down into a
clear, colourless bead in the loop of the platinum wire. A satisfactory
bead having now been made, a portion of the mineral substance to be
analysed (in the shape of small grains) is taken up by dipping the
heated borax bead therein.
The actual operation of determining the nature of the substance then
commences. Using the blowpipe, and directing either the _reducing_
flame (R.F.), or the _oxidising_ flame (O.F.), on to the substance on
the borax, according to circumstances presently to be detailed, the
operator notes the change in colour (if any) of the flame yielded by
the process. At this point a very annoying thing sometimes happens;
for, in liquefying the borax bead, it is apt to fall off the wire, and
another bead has then to be made. To avoid this, great care should
be taken not to blow too vigorously at first. With the microcosmic
salt especial care and dexterity must be exercised in this connection.
If all goes well, the powdered mineral substance (if fusible in the
borax) readily melts down, and becomes incorporated with the borax. On
permitting the latter to cool, which it very rapidly does, the bead
should now be carefully examined, and any change in tint noted. Most
beautiful transparent colours, pregnant with meaning, are often seen to
have formed with the borax as flux.
The operator may test his skill by making the following brilliant
experiments. Take up a few small fragments of the mineral malachite (a
carbonate of copper) by means of the clear, colourless, heated borax
bead, and then introduce them to the _oxidising_ flame. They slowly
dissolve in the borax, and, whilst doing so, the tip of the blowpipe
flame becomes emerald-green in colour. After applying this flame for a
minute or two, the whole of the mineral will have become incorporated
with the borax, and, when the bead is still hot, note that it is also
of a rich green tint, but that, on cooling, it turns blue. If too much
malachite has been taken up in the first instance, a very dark green
tint is imparted, which still remains when the bead is cold, and it
appears to be quite black. Its true colour, however, may be ascertained
by flattening the bead out before it is quite cold. It is always well
to begin by using a small quantity of the mineral substance at first,
and adding to this as may be required.
Assuming that a fine rich green bead has been produced, and that it
contains a relatively large amount of copper, the operator may now
hold it in the _reducing_ flame and re-melt the bead; if the operation
has been conducted carefully, the bead will then show red, and be
practically opaque when cold. The red bead may now be re-heated in the
_oxidising_ flame, when it will be found once more to return to a green
colour. The student will find this easy operation excellent practice,
as proving to him, in the absence of a demonstrator, that he is really
able to recognise and use the oxidising and reducing flames at will.
Many mineral substances yield a distinctive colour in this way--a
useful factor in a qualitative analysis.
Before using the platinum wire, be careful to ascertain that it is
quite clean; a borax bead made thereon should be perfectly white and
transparent.
The “platinum foil” is employed as a support during fusions; pieces
about an inch and a half long, by half an inch in width, are generally
used. A small platinum spoon is sometimes adopted when fusing
substances with acid, potassium, sulphate, or nitre.
Minerals may be tested to see whether, in the ordinary blowpipe flame,
they are fusible or not. To do this, a fragment of the substance to be
tested is held in the flame by means of the “platinum-pointed forceps.”
If the mineral is found to be fusible, then its “degree of fusibility”
may be ascertained according to the following table. The “degrees of
fusibility” are six in number:--
1. Fusible in ordinary gasflame, even in large fragments. Example:
_Stibnite_, or grey antimony.
2. Fusible in fine, thin pieces, in the ordinary gasflame, and in
larger fragments in the blowpipe-flame. Example: _Natrolite_, a
hydrous silicate of alumina and soda.
3. If very thin splinters be used, fusible without difficulty with
the blowpipe-flame. Example: _Almandite_, or iron-alumina-garnet.
4. In thin splinters fusible to a globule. Example: _Actinolite_, a
non-aluminous variety of hornblende.
5. Thin edges may be fused and rounded without great difficulty.
Example: _Orthoclase_ felspar--already described.
6. Fusible with great difficulty on the finest edges. Example:
_Bronzite_, one of the augite group of minerals.
Now, it is highly probable that many of our readers will not
understand, or be able to recognise the six minerals above enumerated;
and we recommend those who may be sufficiently interested, to purchase
them from a mineral dealer--such as Damon, of Weymouth, or Russell, or
Gregory, or Henson, or Butler, in London. A set, comprising the six,
should cost from two to three shillings. With these, as a standard
for comparison, the operator readily grasps the method of assigning a
fusible mineral to its proper degree in the scale.
Another object of examination in the forceps is to see what colour
(if any) is imparted to the flame by the divers minerals experimented
upon. It is a good rule not to permit the specimen, when being fused,
to touch the forceps in the neighbourhood of the actual part fused. For
a mineral containing antimony or arsenic would tend to form a fusible
alloy with the platinum points, and so ruin the forceps.
The pieces of “charcoal” alluded to in our inventory, are used for
placing the mineral substance upon in certain parts of the blowpipe
operation, which may be briefly described. Essentially the charcoal
forms a support to the substance during fusion; but the glowing carbon
has also a kind of reducing effect. Taking a long prism of charcoal,
such as that described, page 63 _ante_, the mineral to be dealt
with should be placed near one end of a flat surface and the prism
so held that the flame from the blowpipe, will sweep down its full
length. The object of so doing is to give a chance to any volatile
substance (derived by the operation from the mineral) to deposit on
the comparatively cool surface, which deposit is often indicative of
the chemical nature of the mineral. To carry this point home, the
following experiments may be conducted by the student. Taking a piece
of _stibnite_ (sulphide of antimony), which, as we have just learnt,
is a most fusible mineral, we place it on the charcoal in the manner
indicated. Whilst melting, and the blowpipe flame be continued to be
directed upon it after it has become fused, it will be noticed that a
yellowish-white deposit is taking place on the length of charcoal; this
is called a _sublimate_.
Mineral substances may also be assisted in fusing on the charcoal by
using the reagents described in our list of chemicals, &c., included in
a blowpipe set.
In regard to the use of the “glass tubes,” it may be remarked that they
are used principally for the examination of minerals which yield a
volatile substance on being heated therein, and to detect the presence
of water and the like. It is important to make a distinction between
the closed and the open tubes. When a mineral fragment is placed in a
tube, closed at one end, whatever takes place will be in presence of
very little air, or oxygen; on the other hand, when the tube is open at
both ends, and is inclined during the experiment, a constant stream of
oxygen passes through the tube, and the mineral is being dealt with in
presence of that. The employment of this oxygen makes a great deal of
difference in the results obtained, as a few elementary experiments
will show. If we place a piece of sulphur in a tube, closed at one
end, and heat it gently, we notice that a yellow coating takes place
inside the tube; but if we now employ a tube open at both ends and
heat it very slowly indeed, we notice that the sulphur goes off as an
invisible gas, and if the experiment has been properly conducted, there
should hardly be a trace of the sulphur left on the glass. A number of
experiments of a similar nature might be quoted, but enough has been
said for the present to show the utility of the tubes.
The “chemical reagents” alluded to have already been sufficiently
described to render any further discussion on them unnecessary for our
immediate purpose.
In regard to the “miscellaneous articles” mentioned, it may be remarked
that the test papers are employed in the detection of certain acids
and bases; whilst a strip of brazil-wood paper is for the detection
of fluorine. The hammer and anvil are for breaking the substance to
be tested into small fragments; the magnet for withdrawing particles
of iron from the pulverised material; the three-cornered file for
assisting in determining the relative hardness of minerals, &c., &c.
In examining substances before the blowpipe, it is highly desirable
that the various operations should be carried out in some definite
order. The following has been found convenient:--
_a._ In a glass tube closed at one end.
_b._ In an open tube.
_c._ On charcoal.
_d._ With borax and microcosmic salt.
_e._ As to flame colouration.
_f._ With other reagents.
The size of the fragment to be dealt with in an examination, depends
on circumstances, but for ordinary purposes a piece of the size of a
small rabbit-shot will be found sufficient.
It is convenient in this place to describe a few chemical reactions
without the use of the blowpipe; that will render the effects on
certain minerals, presently to be mentioned, clearer to the reader.
In the first place it may be ascertained whether the mineral is soluble
in water, and if so, to what extent. Then as to whether it becomes
soluble in certain acids, such as hydrochloric or nitric acid. The
former acid is generally used, except for metallic sulphides, and
those minerals containing heavy metals, such as lead, silver, &c.; the
latter is employed for the exceptions named. Several minerals, even
when in a powdered state, are hardly, if at all, affected by acids. The
results to be noted during the test with acids, commonly fall into the
following three groups.[7]
A. The mineral may dissolve quietly with or without colouring the
solution; this holds good, for example, with hematite (a variety of
iron), also of many of the sulphates and phosphates.
B. There may be a bubbling off or effervescence of a gas, which gas is
usually carbon dioxide; but may be hydrogen sulphide. Chlorine may be
liberated, or reddish fumes of nitrogen.
C. There may be separation of some insoluble substance as sulphur,
silica, &c.
We will close this chapter by stating the behaviour under blowpipe
examination of various minerals, given in preceding pages, as being
common in clays and earths used in brickmaking:--
_Quartz._--This is infusible, and remains undissolved, even in a
microcosmic salt bead; but it fuses readily with soda, on charcoal.
In the flame it splinters into fragments, which fly off with great
rapidity. It is soluble in hydrofluoric acid. _Flint_, when pure,
behaves in a similar manner.
_Orthoclase Felspar._--Fusibility, 5; flame colouration brilliant
yellow, when much sodium is present; not decomposed by hydrochloric
acid. It may be distinguished from other common felspars by its high
degree of fusibility.
_Oligoclase Felspar._--Gives a sodium yellow flame; fusibility, 3.5;
not decomposed by hydrochloric acid.
_Biotite Mica._--With fluxes gives a strong iron reaction of yellowish
red colour; decomposed in concentrated sulphuric acid, leaving a
residue of siliceous matter.
_Muscovite Mica._--When heated in a tube closed at one end, yields
water which often gives fluorine reaction with brazil-wood test paper
by colouring it straw-yellow; it is not decomposed by acids, and
whitens and fuses only on thin edges.
_Kaolin._--Is infusible; gives off water when heated in a closed
tube; and with cobalt nitrate on charcoal, a fine alumina reaction is
obtained.
_Aluminium._--On charcoal, this becomes blue with cobalt nitrate,
though if the surface is fused the reaction is not so clear. Prof. Cole
advises that the soda-residue be dissolved in dilute hydrochloric acid,
then evaporated to dryness, re-dissolved in that acid water, filter
off any silica, and neutralise with ammonia; alumina is precipitated
together with any iron present. The precipitate, if white, or nearly
so, may be tested with cobalt nitrate, and the result is a fine blue
colour.
_Limonite Iron._--Fusibility about 5; yellow and reddish beads;
water given off in closed tube; in reducing flame magnetic residue on
charcoal; soluble in hydrochloric acid after a short time.
_Iron Pyrites._--Fusibility about 2; yellow and red beads; in closed
tube yellow precipitate due to sulphur; magnetic after reduction on
charcoal; insoluble in hydrochloric acid.
_Rock Salt._--Intense yellow sodium flame; fusibility about 1;
microcosmic salt with copper oxide shows strong chlorine reaction--a
fine blue flame surrounding the bead when re-introduced into the flame.
It is soluble in water.
_Selenite_ (_Gypsum_).--Fusibility about 2.5; brilliant flame; in
closed tube it becomes white and opaque and much water is given off;
with soda, on charcoal, sulphur reactions are obtained; soluble in
hydrochloric acid.
_Calcite_ (_Carbonate of Lime_).--Flame glows very strongly; infusible;
effervesces freely in cold hydrochloric acid.
_Dolomite._--Flame, with hydrochloric acid, like calcite; infusible;
effervesces in hot hydrochloric acid.
_Magnesite._--Infusible; with cobalt nitrate a fair magnesia reaction
on charcoal, _i.e._, turns into a dull pink; effervesces in hot
hydrochloric acid.
_Manganese._--With borax in oxidising flame a red-violet bead is
obtained, but with the reducing flame it is colourless.
The above are commonly met with in brick-earths; for other minerals
and substances also found, the reader may be referred to special works
dealing with blowpipe analysis.