Fire Assay Furnaces and Furnace Equipment

Fire Assay Furnaces and Furnace Equipment

The furnaces used in assaying are many in design, varying mainly with the kind of fuel used. The furnaces are classified as follows: (1) Pot furnaces, in which the assay is in direct contact with the fuel; (2) Muffle-furnaces, in which a muffle or receptacle containing the assay is externally heated.

As the muffle-furnace is practically essential for carrying on the operations of scorification and cupellation, and crucible fusions can be made satisfactorily in the muffle if it be large enough, muffle-furnaces have largely replaced pot furnaces for general assaying. In general, they are cleaner, more easily operated, better controlled as to temperature, and if large enough are of great capacity, which makes them especially desirable for smelter, mill and mine assay offices, where frequently a great number of assays are performed daily. The choice of fuel for heating the furnaces is usually dependent on locality. Bituminous and lignite coal, coke, anthracite, crude oil, gasolene or kerosene, wood, fuel and illuminating gas are all used. Of these, coke and anthracite are the fuels least desirable for muffle- furnaces, for burning without flame they must surround the muffle. This makes the firing difficult, requiring considerable attention. The best fuel, usually also the most easily obtainable, is bituminous or good lignite coal, yielding a long or reasonably long flame. One-, two- and three-muffle furnaces, constructed of fire-clay tiling, fire-brick, and common hard brick, tightly bound with stays and rods, are in common use, and for general utility, where much work must be performed, are very desirable.

Coal, Coke and Oil Furnaces

Fig. 1 shows such a two-muffle furnace in perspective, and Figs. 2 and 3, in cross-section. The essential parts of the furnace, as the tiling, A, B, L, K, etc., can be readily purchased, although the interior of the furnace may also be built of fire-brick. The tiling furnace, however, is more easily set up and is more durable. In the design of the soft-coal furnace, the essential dimensions are: area of fire-grate; distance from the grate to the bottom of the lower muffle; the

” fire space” i.e., the distance between muffles and the side and end walls of the furnace, and between the top of the upper muffle and the roof of the furnace, giving the proper space for combustion of the gases. These dimensions depend upon the nature of the coal. In Figs. 2 and 3 the grate dimensions are 17.25 x 21.0 in.; distance from grate to lower muffle, 18 in.; fire space, 2.5 in.; external dimension of muffle, 19 in. long, 12.25 in. wide, 7.75 in. high. The flue area should be from one-sixth to one-eighth of the grate area. The flue is best placed forward of a line through the center of the muffles to get the full sweep of the flame around them, although this arrangement with poor draft, is apt to cause smoky muffles.

The walls of the furnace are thick (13 in.) to prevent radiation. The front of the furnace above the muffle is arched. The arch tiling has in it a duct, leading to the flue, to carry off lead fumes. The muffles are supported by two sets of tiles, placed into the side walls and sometimes by an additional set in the rear end wall. These tiles frequently prove weak, and in falling away leave the muffle without support, causing it to be short lived. The supports are best made in such shape, of two pieces, that they will join under the center line of the muffle and arch over, supporting each other. The writer has used supports of this type, which were perfectly satisfactory and increased the life of the muffles greatly. A furnace of the kind described has a capacity of 25 to 30 fusions (20-gram crucible) per hour, including the necessary cupellations.

If the fusions are made in 30-gram crucibles or in 2.5-in. scorifiers, the capacity is from 20 to 24. With good draft, this furnace burns from 37 to 47 lb. of coal per hour, which at $7.00 per ton, makes the cost per assay for fuel amount to from 0.80 cents to 1.00 cent, when assaying continuously and somewhat more when the furnace is not charged to its maximum capacity. With a good grade of coal (6500 to 7500 calories), a maximum temperature of 1150° to 1200° C. can be obtained in this furnace after 4 hours firing. Figs. 4 and 5 show a three-muffle furnace of similar type.

Coal furnaces may also be readily modified to burn crude oil. This can be done by placing tiling in the fire-box, and making the necessary pipe and burner connections.

Fig. 6 shows such a furnace. The burner is a ¾ in. pipe connected by a T to the oil line, also a ¾ in. pipe. A ¼ in. steam pipe passes into the burner pipe at the rear through a packing nut which permits of the adjustment of the distance between the nozzle of the burner pipe and the nozzle of the steam pipe. By varying this distance the flow of oil may be affected independent of the steam and oil inlet valves. The nozzle of the burner pipe is an ¼ in. hole and that of the steam pipe an 1/8 in. hole.

The grate bars in the furnace are covered with fire brick as shown in the illustration. The placing of the fire brick is of importance as the successful working of the furnace is dependent upon their position.

In starting the fire a piece of oiled waste is lighted in the fire box just back of the burner. When the waste is burning well, oil and steam are turned on simultaneously. The oil and steam valves are then set to give the proper flow. Plenty of waste should be used to furnish a blaze until the fire box is hot enough to ignite the oil, otherwise explosions are apt to occur. The steam used should be dry, and to insure this the steam pipe leading to the burner may be passed around the flue as shown in the figure. The valve DV at the end of the steam line is kept slightly open during working to permit the escape of water of condensation. A small steam coil may also be placed in the oil tank to keep the oil more fluid. The furnace may be heated to a red heat 15 to 20 minutes after starting. The furnace has a capacity of 25 to 30 assays, including cupellations in 1¾ to 2 hours and 50 to 60 assays in 3 hours.

The amount of oil used varies from 4.2 to 5.3 gals, per hour. With oil at 8½ cents per gal. the cost per assay for fuel is 2.2 cents to 2.8 cents. Figs. 7 and 8 show a wood-burning muffle-furnace. In some districts wood is the only available cheap fuel. If the fire-box and fire spaces are properly designed (i.e., of larger size than in the coal furnace) and a deep bed of fuel is provided for (i.e., the distance from the grate surface to the bottom of the fire-door is from 8 to 10 in.), sufficient temperature for ordinary assaying can be attained in this type of furnace. Almost any wood may be used.

In the furnace shown in Figs. 7 and 8, the grate is 18 in. wide and 26 in. long; the distance from the grate bars to the bottom of the muffle is 26 in. and the fire space is 2.5 in. wide at the sides and 3.5 in. at the top. Fig. 9 shows a wood burning furnace of somewhat different construction. With pinon pine or fir wood at $4.50 per cord, the cost of fuel is 65 cents for a daily run of 30 to 40 assays.

With a poor grade of wood at $6.50 per cord, in another instance, 30 assays cost 93 cents, or 3.1 cents per assay, including cupellations.

Coke and anthracite muffle-furnaces when used are usually smaller, although large furnaces may be specially designed and built of the general type of the coal furnaces described.

Fig. 10 shows a small coke or anthracite furnace. The fuel is fed in at the top and kept well heaped around the muffle. A furnace of the kind shown in Fig. 10 will consume from 32 to 38 lb. of coke per hour, according to draft. With a muffle 11 x 16 x 7 in., 10 assays per hour, including cupellation, can be made

The method of support of the muffle in the furnace has great bearing on the life of the muffle. Muffles inadequately supported soon crack and fall to pieces. It is perhaps better to support muffles by one substantial rather broad support near the middle and across the whole bottom, and by resting the front end on the furnace wall and the rear end on two replacable clay supports, than to have more numerous supports extending short distances only beyond the walls on the bottom. Muffles should be stored in a dry warm place to prevent their absorbing moisture and when new muffles are placed in a furnace, it should be fired lightly with wood chips for an hour to anneal the muffles, before heavy firing is begun. The spilling of slag and lead in the muffle rapidly leads to corrosion and softening of the bottom and consequent destruction. To avoid this deterioration in part, muffle bottoms should be covered with a layer about ¼ in. thick of bone ash, silica sand, or Portland cement, to act as an absorbent. Muffles are also subject to destruction from the fluxing action of the ashes of the fuel burnt on the grate. Ashes high in iron oxide are the worst in this respect.

In setting muffles, it is essential for the attainment of the best heating conditions to thoroughly lute up the space around the edge of the muffle and the arch opening in the front of the furnace into which it fits. If this space be of considerable size, it is best filled in roughly with chips of broken crucibles, etc., before applying the luting material. Luting material may be of fire clay and crushed fire brick or crucibles, one-fourth of the former to three-fourths of the latter, mixed with sufficient water to make a plastic mass. The fire brick may be crushed to pass an eight mesh screen. Raw fire clay has too great a shrinkage to be used alone. One-quarter fire clay, one-quarter shredded asbestos, and one-half crushed fire brick makes a good luting material.

Figs. 11 and 12 represent a combination of crucible pot furnace and muffle furnace, such as is used in England. The furnace may be built of ordinary fire brick and is dimensioned in such a manner as to avoid cutting brick as much as possible. It is fired by coke. The two crucible furnaces S connect with the main flue, 9 x 9 in. in size, by the flues N. The crucible furnace nearest the muffle also connects with the muffle furnace by the extra flue O. By these means the hot gases of combustion may be diverted to the muffle furnace, instead of directly to the stack.

The pot furnaces are 14×14 in. in section, and on account of the sloping top, are 17 in. deep at the front and 23 in. at the back. The top of the furnace is best made of a sheet steel plate cut as required. The doors are made of two tiles, each 20 x 10 x 4 in., held together by two pieces of 1.5 in. channel iron clamped by two 0.5 in. rods. To the ends of these rods, four 3 in. iron wheels are fastened on which the doors run. The tiles are secured in the frame in such a manner that there is a clearance of 0.25 in. above the furnace top to freely move the doors. Each furnace has 8 grate bars of 1 in. square wrought iron resting at the ends on two similar bars, placed on the brickwork. The bars are 14 in. long except the two center ones which are 18 in. long, and may be withdrawn through the opening G for dumping the fire.

There are four muffles H, in the furnace, 15x9x6 in. outside measurements. The lower muffle and perhaps the next upper one may be used for scorification and cupellation. The other two muffles will not heat to a high enough temperature for anything except annealing and roasting. The muffles rest at the back and sides on the ends of bricks cut to a level as shown and projecting from the furnace body. At the front they lie flush on 1.5 in. angle irons A.

The furnace is built with the front entirely open; the grate bars are the same as described for the crucible furnace. When the muffles have been placed in position the space around the front of the muffles is filled with a mixture of fire clay and silicate of soda to a depth of 3 in. A strong solution of silicate of soda or “ water glass” is mixed with 3 times its weight of water until homogeneous. This solution is then mixed with fire clay to a stiff paste; usually 1 part of solution is required for 7 parts of fire clay. The mixture usually contracts on heating and shrinks away at the edges. These cracks then have to be filled again.

The top of the muffle furnace is covered with tile laid in 1.5 in. angle irons A’. The flue V from the muffle furnace into the stack is 12 x 3 in. in size. The draft of the furnaces is controlled by placing sheet iron plates in front of the ash pit doors.

Gasoline Fired Furnaces

Furnaces of this type are in common use, and for small offices, where the pressure of work is not great, they afford a convenient and cheap method of operation. Gasoline, on account of ease of transportation and great calorific power, is also employed in out-of-the-way districts for extensive daily work. Where coal is reasonably cheap, not above $6.50 per ton, gasoline at 30 cents per gal. cannot compete with it in large offices or schools, where the assay furnaces are operated continuously for the greater part of the day.

Fig. 13 shows a gasolene furnace apparatus. The furnace, divided into crucible and muffle compartments, is made of fire-clay tiling, bound with sheet iron. It is heated by a brass and copper burner, provided with a generating device. The burners

are made in varying sizes to suit different furnaces. The gasolene is stored in a steel tank, of 5 or 10 gal. capacity, provided with an air pump to furnish pressure. A pressure gauge is attached to the tank. Generally, 0.25- to 0.375-in. piping joins the tank and the burner. The burner and piping are connected by a special universal joint, so that the burner can be swung into and out of position. The burner (if the Cary) should fit tightly against the fire-clay ring or boss in the opening of the furnace, so that all the air for the combustion of the gasolene is drawn in through the burner tube. To insure tight joints, glue or soap, or shellac, not white or red lead, must be used in the screw connections. The gasolene is fed to the burner under a pressure of 10 to 20 lb., though for special purposes higher pressures are used.

Fig. 14 shows a detailed view of the Cary burner. The upper valve controls the main gasolene supply, and the lower one controls the generator. The burner is heated by the generator, so that the gasolene issuing from the main needle-valve is vaporized, and in its passage to the furnace draws in air through the burner tube, the mixture igniting and burning at the mouth of the burner in the hot furnace. Burners are listed by the diameter of their tubes. Five sizes are made, from 1.25 to 2.25 in., each size varying by 0.25 in.

Fig. 15 shows the tank and pump apparatus. It is best to place this at a considerable distance from the furnace, in order to avoid accidental explosions. Fig. 16 shows a crucible furnace, and Fig. 17 a large gasolene muffle-furnace. The writer has attained a temperature of 1350° C. in small gasolene furnaces, such as Fig. 16 represents, and 1250° C. in large furnaces, as represented by Fig. 17. By a special construction of furnace, with graphite muffle and heavy insulation against radiation, with good draft, the writer has attained (for metallurgical experimentation) temperatures of 1500° to 1530° C., after three hours,

with a 2-in. gasolene burner as shown in Fig. 14, with gasolene at a pressure of 55 lb. and a consumption of 1.53 gal. per hour. A 2-in. Cary burner, under 10 lb. pressure, will consume from 0.65 to 0.75 gal. per hour. A No. 31 Cary combination furnace, holding at a charge in the crucible compartment six 20-gram crucibles and having a muffle 7×10.5×4.5 in. in size, has a capacity of 10 fusions per hour, including cupellation. With gasolene at 30 cents per gallon, the cost of fuel per assay is 2.25 cents.

Fig. 18 shows the Case burner for gasolene or similar distillate. When in the proper position it is inverted, i.e., the preheating system is at the top instead of at the bottom as in the Cary burner. The generator or boss in which the gasolene is vaporized is cast in one piece with the mixing chamber which is in the form of a truncated cone and very much shorter than in other gasolene burners. The burner is smaller and more compact than the ordinary burner of the same capacity. The fact that it is inverted permits the gas formed from the gasolene

in the generator to pass freely upward to the valves. The valves are of special design. Ordinarily the needle valve is used in burners of this type, that is, a pointed hard steel needle works in the circular valve orifice, making an annular opening for the escape of the gas. This annular opening varies in dimensions according to the position of the needle, and may be closed completely by screwing the needle up as far as it will go. With use, the tendency of the needle is to enlarge the valve orifice and cause increased consumption of gasolene. The valve of the Case burner is closed by the valve seat meeting a shoulder on the valve stem, both planed surfaces. The opening for the flow of gas is annular as before, but the end of the blunt valve pin does not close the valve. The burner is made of phosphor bronze, and operates best under a pressure of from 40 to 50 lb.

Fig. 19 shows a gas burner, for assay furnaces. The air supply is controlled by the butterfly valve A. The gas issues from the circular opening D and mixes with the air from the annular

opening C, for combustion. The gas flow is regulated by the cock G. The burner is provided with the pilot tube B to ignite the gas in starting the burner.

Fig. 20 shows a Case gasolene, oil, or gas fired muffle furnace of new design. It is provided with a heating chamber the features of which are first—a set of fire-clay blocks so constructed as to form channels or flues under the muffle to direct the flame and hot products of combustion, insuring a uniform heat distribution and acting as a firm support for the muffle; and second—sets of vertical ribbed channels or flues in the side walls to guide the hot gases and accomplish an even distribution of the heat. The channels between the ribs are wider near the front than at the rear of the furnace in order to lessen friction to the gases in this part, thus causing the heating of the front of the muffle uniformly with the rest of it.

The fire clay blocks or “muffle heaters” may be readily replaced by new ones when necessary.

Gas Furnaces

Where municipal illuminating gas or other gaseous fuel is available, gas-fired furnaces are convenient and cheap of operation. The Reichhelm furnace (American Gas Furnace Company) is frequently used. The furnaces require air at low pressure, which is mixed with gas in proper proportion before it enters the furnace through the several burners. The proportion of gas to air is controlled by valves. Fig. 21 shows the furnace. Gas furnaces permit of close control of heat and are desirable for accurate temperature work.


Convenient tools are necessary for the handling of crucibles, scorifiers and cupels. The features essential in these tools are that they be light, grasp the crucible, etc., firmly, with no danger of tipping, and take up little room in the furnace. As an illustration of a tool deficient in these qualities and therefore undesirable, Fig. 22 is given. This shows a pair of crucible tongs designed to grasp the body of the crucible. It cannot be handled in a muffle full of crucibles, owing to the space it takes up in opening. Fig. 23 shows a pair of crucible tongs to grasp the sides of the crucible, and operating in little space.

Fig. 24 shows two types of cupel tongs. Fig. 25 shows a good form of scorifier tongs, and Fig. 26 another form.

For large offices where much work must be quickly accomplished, special forms of tools may be used. Figs. 27 and 28 show a multiple tongs for scorifiers. This apparatus will handle 25 scorifiers, practically a muffleful at one time. It is composed of quintuple tongs, corresponding to the five longitudinal rows of scorifiers in the muffle. The lower part of each pair of the tongs consists of a fork on which the scorifiers rest, and one of

whose prongs is rectilinearly extended through two bearings in a frame and held in position by collars. This extension is free to revolve on the bearings, and it is the axis of rotation of the tongs. To each of them is attached, at a right angle, a lever extending upward at 45°, and all the levers are connected by slotted joints to a cross-rod. Therefore if, by means of a crank fastened to the end of one of the extended prongs, one of the forks is turned and the scorifiers tilted to the desired angle, the others rotate to the same extent. The center of gravity of the scorifiers lies to one side of the rotation point, and they would, therefore, on being lifted, tilt in that direction; this, however, is prevented by the cross-bar resting against a post at that end of the frame toward which the inclination tends. The scorifiers are clutched by the upper prongs of the tongs, which is fastened to a spring on a post of the fork below, and which is free to move in a vertical plane, the pivotal point lying over the spring and post. By bringing

pressure on the extended ends of these clutch bars behind the pivot, their other end will rise above the scorifiers, and thus release them, or permit the placing of them onto the tongs. The pressure exerted on the rear ends of the clutches is accomplished by means of a cross-bar fastened to a spring bar, which is itself fastened to the handle of the instrument. An ordinary mold with 20 holes, arranged to receive the contents of the scorifiers, goes with the tongs.

Fig. 29 shows a device to charge 30 cupels at one time. It comprises a top sliding plate with openings corresponding exactly to the position of the cupels. The openings in the lower plate correspond with those of the upper one; the plate, however, rests on two adjacent sides extended downward at right angles to the plate and to each other, thus forming two closed sides of the instrument; one at the front and the other at the

right-hand side. The height of these sides is such that when resting on the bottom of the muffle the bottom plate will be some distance above the cupels, and by a slight pull forward and a push to the left with the handle of the instrument the set of cupels will be perfectly alined in both directions and the apertures in the lower plate will exactly cover the tops of the cupels. The lead buttons are placed in the apertures of the upper plate

and rest on the lower plate before introducing the instrument into the furnace, and when it is placed over the cupels, which have been properly alined in the muffle, the upper plate is pushed

forward to a stop-point, bringing the apertures of the two plates into register, thus causing the lead buttons to drop down into the cupels. The handle of the upper plate runs through guides

fixed to the handle of the lower plate; both handles are connected with a spring, which acts as a brake when the upper plate is pushed forward to drop the buttons, and also serves to bring it

back into its original position, in which the buttons cannot drop through the apertures in the lower plate.


Fig. 30 shows machined cast-iron molds to receive the molten fusions. The sharp cone-shaped mold is preferable to the shallow hemispherical type, as the lead buttons are then sharp and well defined and separate easily from the slag. The mold is best made with a screw-handle, so as to be easily repaired in case of breakage. The inner surface of the molds should be machined smooth, to permit the ready separation of slag and lead button from the mold. For scorification fusions, smaller molds are often used.

For the transfer of cupels to the parting room, iron cupel trays, as illustrated in Fig. 31, are used. The handle is removable, and one handle serves for a number of trays. For the annealing of gold beads, or cornets, fire-clay trays as shown in Fig. 32 are employed. Fire-clay, however, is very easily broken, and more satisfactory trays are made of sheet iron and heavy asbestos board.


Fire-clay crucibles are largely used in the United States, and fire-clay ware for assay purposes is made to a large extent in some of the western States. Following is the analysis of a Colorado crucible clay:

Loss on ignition…………………………………………10.14 per cent.
Alumina……………………………………………………15.09 per cent.
Silica…………………………………………………………71.81 per cent.
Ferric oxide…………………………………………………1.75 per cent.
Lime…………………………………………………………..0.14 per cent.
Magnesia……………………………………………………0.05 per cent.
Alkalies……………………………………………………….1.02 per cent.

The crucibles are rated by gram capacity, that is, by the number of grams of ore with the proper amount of fluxes necessary for fusion which the crucible will hold. The chief sizes are 5, 10, 12, 15, 20, 30 and 40 grams; of these the 20- and 30-gram sizes are mostly used, the 20-gram crucible for the 0.5 assay ton, and the 30-gram for the 1 assay ton fusions. Fig. 33 shows the various shapes employed. Imported Hessian triangular crucibles and sand crucibles are also used, but in small quantities.

Imported Battersea clay crucibles give good satisfaction and are used by some of the large assay offices in preference to domestic fire clay goods, for the reason that their quality is generally uniform and that they last for a larger number of fusions than the poorer grade of domestic goods which are sometimes sold. The highest grade of domestic material is, however, in most cases to be preferred as being fully as long lived and cheaper.

The special mixture of clays and their treatment for crucible manufacture is generally a trade secret, jealously guarded and little information concerning the subject is available.

Scorifiers are made of the same clays as the crucibles and are designated in size by their outside diameters; 1.5-, 2-, 2.5- and 3.5-in. sizes are made. These will hold a volume of 15 c.c.,

25 c.c., 37 c.c. and 100 c.c., respectively. The 2.5-in. scorifier is the one commonly used. Fig. 34 shows the ordinary type of scorifiers. Roasting dishes are shallow fire-clay dishes similar to scorifiers, but not so thick. They are rated by their diameters; the common sizes being 3, 4, 5 and 6 in. Fig. 35 shows the ordinary buck board and muller, and Fig. 36 buck board brushes. For the description of other minor tools and apparatus, as screens, pliers, and crushing and grinding machinery, necessary to the assay laboratory, the reader is referred to the voluminous and well-illustrated catalogues of the assay supply houses. Balances, weights, sampling tools, cupels, parting devices, etc., are discussed in their respective chapters.