THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXII.

FRIDAY, DECEMBER 15, 1916.

No. 2920.

Refractory Materials in South Yorkshire.*

By Prof. W. G. FEARNSIDES, M.A.

In dealing with refractory materials, it is usual to
classify them, according to the percentage of silica
which they contain, under the heads of acid and basic,
with, if we like, an intermediate subdivision which may
be spoken of as neutral. “Acid ” refractory materials
are used for the making of “ acid ” steel, “ basic ”
refractory materials for the vessels used to contain
molten “ basic ’’ steel, and the terminology used to
describe the acidity or basicity of refractory materials
is the same as that made use of by the geologist, who,
in speaking of the “ acid ” of a rock, means merely the
percentage of silica present. Many good ultra-acid
silica bricks carry as much as 95 per cent, of silica,
and are the most acid refractory materials which we
can well get hold of. Magnesite or magnesia bricks are
at the other end of the scale, and often carry less than
5 per cent, of silica. By going round the yards of the
Sheffield dealers in refractory materials, one could
probably find material on offer, for purposes of furnace
lining or building, which would match every single unit
of percentage between 98 at the top and about 2 at the
bottom. Materials such as these could be mined at
various places, all within 20 miles of Sheffield.

To the industries of Sheffield the question of refrac-
tory materials i*s one of special importance. One cannot
maintain a fire without a fireplace in which the fuel
can be burnt; one cannot melt, in any considerable
quantity, metals which have a high melting point,
unless one has a vessel which will contain the molten
metal without itself losing its rigidity. The neighbour-
hood of Sheffield is particularly fortunate in its native
supplies of materials, suitable both for the building of
furnaces and for the making of vessels which will con-
tain the molten steel. The clay ironstones inter-
stratified among the measures above the Silkstone and
below the Barnsley coals in South Yorkshire have their
outcrop by Tankersley, Thornclifie, and Parkgate, and
are continued by Renishaw,. Staveley, Sheepbridge, and
Clay Cross, in the district to the south.

Yorkshire Pot Clay.

As a source of refractory materials in the modern
sense, the importance of the Sheffield district originated
with the coming of the cutlery trade, and it was a
fortunate circumstance that that most refractory of
Yorkshire fireclays, the Black bed of Stannington, has its
outcrop in the immediate neighbourhood of the places
where the tilt hammers for power forging of steel were
set up. This Stannington Black bed, better known as
the pot clay, is the lowest bed of the Yorkshire coal
measures, and rests directly upon the millstone grit.
At its outcrop it is a clay comparatively easy to manu-
facture, and for more than a century has been found
very suitable for lining iron furnaces wherein intense
and lon'g-continued heat is required, whilst for almost
•as long a period it has been used as an essential con-
stituent for the making of crucibles or “ pots,” for the
famous Huntsman process.

As won from present mines, the Stannington or
Loxley pot clay has not always the refractory qualities
which it had when originally found. Probably, by
careful searching along the outcrop, equally pure clay
might be obtained, but it does not pay to get it, and
crucible pots are now made by mixing several clays
imported from a distance. The blending of these clays
is generally maintained secret, but nearly all the makers
use some 25 per cent, of pot clay from the Stannington
Black bed. A bed of fireclay, the equivalent of the pot
clay, underlies the coal measures of Yorkshire, from the
north-east corner of the coal field near Leeds, westward
by Halifax, and southward past Sheffield, and west of
Chesterfield, almost to Nottingham in the south.
Throughout the coal field the lowest seam of coal
which belongs to the coal measures has as its under-
clay this celebrated pot clay, which, when sufficiently
weathered, is highly refractory.

Pot clay, as first found, was at its outcrop, and
weathering had done a good deal to it. About Loxley
the workable thickness of the bed at the outcrop is
about 7 to 8 ft.; but if that particular bed be followed in
from the outcrop downwards along the dip, the work-
able thickness of the bed is found to become gradually
less, from, say, 8ft. to about 5 ft. If a lump of clay
from the outcrop be compared with a lump from the
deeper parts of the workings, and the water content be
measured, a little calculation suffices to show that much
of the apparent swelling of the clay at its outcrop is
simply due to absorbed water. A certain amount of
salts has been removed, but the great reason for the
original success of the outcrop pot clay was that it had
had opportunity to take up water, and had swollen to

* From a paper read before the Midland Institute of
Mining, Civil and. Mechanical Engineers.

very nearly twice its bulk. When people began to
make pots from the outcrop clay, they never had the
black-heart troubles which now they had to meet. The
sulphur, which was the chief cause of black-hearting,
had been dissolved out as sulphates by percolating
water. The probable explanation of the continued
deterioration of fireclays from particular makers was
that the supply of -surface clay was becoming exhausted,
and that the people who were mining the clay were
gradually getting under an increasing depth of cover.
There was no place in Yorkshire where the pot clay
was being worked with more than 200 yds. of cover. So
far as mining was concerned, that was a small cover,
but in respect of the chemical weathering of the clay, it
was a very considerable cover, since the clay there had
a chemical constitution which it owed to conditions
which had nothing whatever to do with the present-day
surface conditions. If in boreholes or exploratory
levels a sample of clay were found possessing chemical
characteristics identical with those of the clay which
was now being won from the deeper levels of the
Stannington pot clay workings, and that sample were
reported upon by a chemist who had no special know-
ledge of the changes which could be wrought in clay by
weathering, that chemist would undoubtedly turn it
down. Speaking to an audience of mining engineers,
the author took the opportunity to emphasise that
point, believing that, if people who needed refractory
clays wrere prepared to take trouble in the preparation
and manufacture of the clays which were to hand, many
of the materials which had to be brought out of the
pits along with coal could, if properly treated, be made
to yield clays from which refractory vessels of the fire-
clay type might be made, quite equal to those- from the
original pot clay.

The characteristic of the pot Clay was its high per-
centage of alumina. All refractory clays, in fact all
refractory materials, were so, generally, because their
content of fluxes was rather small. Whilst there was
certainly some connection between coal seams and their
associated refractory materials, he could not at all
accept the orthodox text-book view that it was the
roots of the plants that sucked out the fluxing salts. The
soluble salts present in clay from the deeper levels had
been leached out in the case of the clay found at the
outcrop, the salts having been enclosed in the clay at
the time of deposition, in a condition such that modern
conditions could bring about their removal.

Black-Heart Bricks.

With regard to clays which had a habit of becoming
black-hearted during the process of burning them to
bricks, and of running like treacle when the resulting
bricks got into the furnace, he felt that, if proper care
were taken at the brick works, black-hearting could be
prevented, and the whole of the material made into
satisfactory bricks; in fact, this had been done in the
case of a sample exhibited. The trouble was almost
wholly due to sulphur, which, up to a temperature of
about 1,100 degs. Cent., remained in the form of pyrites
or iron sulphide, but, above that temperature, reacted
with silica, fluxed, and gave off a certain amount of
sulphur dioxide within the sticky mass inside the brick,
making the body of the brick swell and warp. It was
a peculiarity of clays that, in making bricks, some water
was needed to hold the bricks together, and therefore,
during the early stages of burning, the atmosphere
inside the bricks was steam. This steam prevented the
access of the air requisite for burning cff the carbon-
aceous material in the brick substance. Moreover,
until the carbon was completely consumed, the carbon
dioxide or monoxide formed also helped to retard the
oxidation of the sulphur in the pyrites. By burning
bricks in such a way as to consume all the carbon at a
temperature lower than the temperature at which silica
fluxes with iron pyrites, the bricks would remain per-
meable to air, thus allowing the iron pyrites to oxidise
to iron oxide, and giving the russety colour or biscuit
tint preferred by the consumer, as an indication of
proper burning.

These remarks on brick burning were applicable to all
clays containing carbonaceous material and sulphides as
intrinsic impurities common to nearly all classes of
fireclays.

Passing upwards in the succession from the millstone
grit, one came next to the coking coal, also a tremendous
asset to the Sheffield metallurgical trades. The coking
coal, which lay about 40 yds. above the pot clay, was
almost unique in the Yorkshire coal field, by reason of
its low sulphur percentage. The author had never
beard of anything being done with the under-clay of that
seam as a refractory material, but he thought that the
sulphur percentage in that material would also be low,

as in the coal itself. Unfortunately, near Sheffield the
coking coal was only about 9 in. thick, and, though it
increased very considerably northwards, to more than
2 ft. about Stocksbridge, it was not a constant seam.
It might be worth while for those who were working the
coking coal to investigate the refractory properties of
the seat-earth upon which it lay.

Proceeding upwards, the next was the middle band
or clay coal, with which, about Wadsley and Ought! -
bridge, a certain amount of good fireclay, and on occa-
sion ganister, was associated. Material won from -near
the outcrop of this seam was used in silica brick works,
and, in another neighbourhood, in making up fireclay
blocks. The composition of the under-clay of the clay
coal probably varied from place to place; when
weathered, the material might be suitable for manufac-
ture into a refractory material.

Ganister Coal.

The next coal seam in the series was the Halifax hard
bed or ganister coal, which was the most persistent of
all the seams in that coal field, and could be traced in
all the borings going down to the millstone grit. It was
known also in Staffordshire and Lancashire. Though
as a seam of coal it was not very good round about
Sheffield, its seat-earth in the neighbourhood of
Deepcar and Oughtibridge afforded a supply of siliceous
refractory material which was second to none. All
along the outcrop, from about as far south as Amber-
gate, and at least as far north as Halifax, workings
more or less systematic were spreading down the dip
from the outcrop. All along the outcrop numerous
firms, who had worked their pits in down the dip from
the.outcrop, more or less on the pillar-and-stall system,
were winning, along with, say, 1 to 2 ft. of coal, from
6 in. to 6 ft. of first-rate ganister. Since the Mushett
development of the Bessemer process of steel making,
about 1856, people had been “ picking out the eyes ” of
the outcrop, and, although they had not got all the best,
they had, in many places, spoiled what they had left,
and those who now had to mine what remained would
have to expend a great deal more per ton of material
won than would have been the case if the outcrop had
remained untouched until a suitable systematic method
of working had been invented; but if the available
labour were concentrated on a part of the district which
could be worked systematically on the longwall plan,
they would be able to get as much of it as the trades
of Sheffield needed.

The siliceous seat-earth of the ganister coal was a
satisfactory refractory material, because, when suitably
fired, it could be burnt to the condition required in
acid ’ ’ steel furnaces in a rather shorter time, and
with less expenditure of fuel, than other rocks con-
taining equally high or perhaps higher percentages of
silica. Silica, the most important refractory material
of the acid type, was present in all sandstones. Fine-
grained, angular quartz sand was the basis of ganister
rock. Ordinary quartz, when heated to a temperature
of 850degs., began to swell, and to invert to tridymite,
the change and expansion becoming more and more
rapid the higher the temperature rose above 850 degs.
Cent. However, in none of the brands of bricks on
the market in this country had time been allowed dur-
ing the burning for the inversion to tridymite to become
complete. The author exhibited a brick which had been
re-fired. It was originally a 3 in. thick brick, but had
now expanded to nearly 3|in. thick in re-firing.

Firing Silica Bricks.

Now, it ought to be possible to fire silica bricks in
such a way that almost the whole of that expansion had
taken place before the bricks came to be built into a
steel furnace; but, made as they were at present, a
large proportion of that expansion had to take place
when the furnace was first made hot, and, seeing that
the lower part of the furnace was usually built of fire-
bricks which continued to shrink somewhat on approach-
ing their failing temperature, whilst the upper part was
of silica bricks which went on expanding, the difference
of behaviour of the two materials occasionally caused
furnaces to collapse unexpectedly. The apparent
reason for the success of the deeper Halifax hard bed
ganister was that, between the grains of sand com-
posing the ganister there was some small quantity of
impurity, some catalytic agent, as it were, which trig-
gered off the change from quartz into tridymite. Cer-
tainly, in all classes of burnt siliceous materials the
expansion process began from the outside of the indi-
vidual grains, and worked its way inwards. And if this
something continued, as it were, to scratch the outside
of individual grains, and stirred up the physical con-
ditions within the grains so that the metastable equili-
brium of the low-temperature quartz was quickly
destroyed, then one could understand why ganister
bricks at Oughtibridge or Deepcar could, when kept a
very few hours at 1.200 or 1,250dogs. Cent., accom-
plish the expansion change more completely than sand-
stones from other districts when fired to a similar
temperature, but lacking the catalytic agent, could do