FUSION PYROMETERS
=General Principles.=—If a number of solids, possessing
progressive melting points, be placed in a furnace and afterwards
withdrawn, some may be observed to have undergone fusion whilst others
would be unaffected. The temperature of the furnace would then be known
to be higher than that of the melting point of the last solid melted,
and lower than that of the first which remained intact. Taking, for
example, a series of salts, the following might be used:-
══════════════════════════════════════╤═════════════════════
│ Melting Point.
Salt. ├──────────┬──────────
│Deg. Cent.│Deg. Fahr.
──────────────────────────────────────┼──────────┼──────────
1 molecule common salt + │ 650 │ 1202
1 molecule potassium chloride │ │
Common salt │ 800 │ 1472
Anhydrous sodium carbonate │ 850 │ 1562
” ” sulphate │ 900 │ 1652
Sodium plumbate │ 1000 │ 1832
Anhydrous potassium sulphate │ 1070 │ 1958
” magnesium sulphate │ 1150 │ 2102
══════════════════════════════════════╧══════════╧══════════
If, on inspection, it were found that the sodium sulphate had melted,
whilst the sodium plumbate had survived, the temperature of the furnace
would be known to lie between 900° C. and 1000° C. If a number of salts
or other solids could be found with melting points ranging between 900°
and 1000°, it would be possible to obtain a reading within narrower
limits. The accuracy of the method in all cases is decided by the
interval between the melting points of successive test materials.
Wedgwood, the famous potter, appears to have been the first to apply
this method of determining the condition of a furnace, his test-pieces
consisting of special clay compositions. The effect of the furnace
on these was noted, and the suitability of the temperature for the
work in hand deduced from the observations. Wedgwood in this manner
investigated the variations in temperature at different levels in his
firing-kilns, and was thus enabled to place the various wares at the
positions best suited for their successful firing. Modern potters still
use such test-pieces, as the information gained is not merely the
degree of heat, but the effect of such heat on the articles undergoing
firing. The fusion method, however, is now used to determine the
temperature of all kinds of furnaces, and the chief modifications will
now be described.
[Illustration: FIG. 67.—SEGER PYRAMIDS OR “CONES.”]
=Seger Pyramids or “Cones.”=—Seger, of Berlin, published in
1886 an investigation dealing with the production of silicates of
progressive melting points. By varying the composition, he was able to
produce a series of materials with melting points ranging from 1890°
C. to 590° C., the interval between successive compositions being 20°
between 1890° and 950°, and 30° from the latter temperature to 590°.
The highest member of the series has the composition Al_{2}O_{3},
SiO_{2}; and the lowest member 2SiO_{2}, B_{2}O_{3}. For convenience
in use the materials are made in the form of triangular pyramids, 5
cms. in height, and each side of the base 1·5 cms. long. Each pyramid
is stamped with a distinguishing number, and altogether 60 are made to
cover the range 1890° to 590°. When conducting a test, several pyramids
are selected with melting points known to be near the temperature of
the furnace, as discovered by previous trials. These are inserted in
the furnace standing on a slab of refractory material, as in fig. 67,
and may be watched through a sight-hole or withdrawn from the furnace
for examination after attaining the existing temperature. If the right
pyramids have been chosen, the appearance presented will be as in fig.
67, in which D is seen to have collapsed completely, C has bent over, B
has been rounded at the top, whilst A is intact. The temperature of the
furnace is then taken to correspond to the melting point of C, which is
found by reference to a table in which the melting points corresponding
to the different distinguishing numbers are given. The pyramids are
extremely cheap, and only those with melting points near to the working
temperature need be purchased. In cases where it is desired to increase
the heat to a specified point, and then to allow the furnace to cool,
these pyramids fulfil all requirements; an examination through a
sight-hole closed with darkened glass enabling the furnace attendant
to discover when the requisite temperature has been attained. The
procedure is more difficult when it is desired to maintain a steady
temperature, as this involves frequent renewal of pyramids already
melted. These appliances are sold under the name of Seger “cones,” the
latter word being evidently a misnomer.
=Watkin’s Heat Recorder.=—This arrangement consists of a small
block of fireclay, having a number of cylindrical holes in its upper
face. Pellets of materials of progressive melting points are placed
in the holes, in which they fit loosely. The block is placed in the
furnace, and afterwards withdrawn and examined, when those which have
completely melted will be seen to have sunk, and to possess a concave
surface; others which have been superficially fused, will show rounded
edges, whilst others will be intact. The melting point of the highest
member of the series which is observed to have rounded edges is taken
as the temperature of the furnace. The materials used in the
manufacture of the pellets are approximately the same as those employed
by Seger, being the same in number (60), and differing progressively
by similar intervals. It is not evident that the method of observation
is superior to the use of pyramids, although some workers may prefer
it, and the arrangement is merely an alternative plan of using the
Seger compositions. Watkin has also introduced a modification in which
straight bars of clay compositions are supported at the edges, the
temperature being deduced by observing which numbers melt, droop, or
remain intact.
=“Sentinel” Pyrometers.=—Under this name, Brearley, of Sheffield,
has introduced a number of compositions, chiefly of salts, which
possess definite melting points. These are made in the shape of
cylinders, about 1 inch long and ¾ inch in diameter, which collapse
completely when the melting point is attained. Compositions have
been found which melt at certain temperatures known to give the best
results in the treatment of different kinds of steel, and a cylinder
of correct melting point, placed in the furnace on a small dish near
to the steel, furnishes a simple and correct clue to the attainment of
the desired temperature. The existing condition of a furnace may be
discovered by taking a number of cylinders, having progressive melting
points, and making observations after the manner described under the
heading of Seger pyramids. A few “Sentinel” cylinders are frequently of
use in the workshop or laboratory for other purposes, such as a rapid
check of a given temperature in confirmation of the reading of an
indicating pyrometer, or in discovering whether a certain temperature
has been exceeded in a given case. “Sentinel” cylinders have been used
in such a manner as to give audible warning of the attainment of a
given temperature by means of a metal rod, which is made to rest on the
cylinder, and which, when the cylinder melts, falls and completes the
circuit of an electric bell. The upper range attainable by the use of
ordinary metallic salts is not so great as in the case of silicates,
but up to 1100° C. metallic sulphates, chlorides, etc., or mixtures of
these, give results quite as good as those obtained with Seger pyramids.
=Stone’s Pyrometer.=—This instrument is intended to indicate the
correct temperature at which a metal or alloy should be poured, and
consists of a silica tube at the bottom of which is placed an alloy
melting at the temperature at which the material operated on should be
poured. A silica rod rests on this alloy, and is connected at its upper
end to an iron extension, the extremity of which engages a pointer
moving over a scale. When the alloy in the silica tube melts, the rod
falls through the molten mass and moves the pointer over the scale,
thus giving a certain indication that the desired temperature has been
attained. Arrangements exist for adjusting the pointer to zero at the
commencement of a test.
=Fusible Metals.=—Instead of clays or salts, a number of metals
and alloys are sometimes used. These are placed in the form of short
rods in numbered holes in a piece of firebrick and inserted in the
furnace, and on withdrawal those which have undergone fusion will be
seen to have taken the form of the holes in which they were placed. The
temperature of the furnace is considered to lie between the melting
points of the last of the series to undergo fusion and the first which
remains unchanged. A series of metals of this description is more
costly than clays or salts, but is more rapid in action, owing to the
superior conductivity of metals.
=Fusible Pastes.=—These consist of salts incorporated with
vaseline or other suitable fat, and are used to detect the attainment
of a specified temperature by a piece of metal. If, for example, it
were desired to heat a piece of steel to 800° C. for a given purpose,
a paste containing common salt might be smeared on its surface before
placing in the furnace. On heating, the vaseline burns away, leaving a
white mark due to the salt, and this white mark will be visible till
the salt fuses. The disappearance of the white mark therefore indicates
that the required temperature has been reached; and the method is
simple and useful in cases where a number of articles are to be worked
at a uniform temperature.