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bell founding and bell metal


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bell founding and bell metal - bell founding and bell metal
bell founding and bell metal - See also alloys, bronze founding.

The following is condensed from a paper on Clocks, Carillons, and Bells, read before the Society of Arts by Mr. A. A. Johnston.

Alloys

  1. For large bells. Copper 100 lb., tin 25 down to 26 lb.
    For small bells. Copper 12 lb., tin 4 lb.
  2. Thompson's. Copper 80 lb., tin 10.1 lb., zinc 5.6 lb., lead 4.3 lb.
  3. Clock bell metal. (1) Copper 75.19 lb., tin 48.81 lb. (2) German: copper 73, tin 24.3, zinc 2.7. (3) Swiss: copper 74.5, tin 25, lead 0.5.
  4. Parisian, for small ornamental clocks. Copper 72 parts, tin 26½ parts, iron 1½ parts.
  5. Locomotive bells (American). Copper 80 lb., tin 20 lb., zinc ½ lb., lead ½ lb.
  6. White table-bells. (1) Copper 2.06 parts, tin 97.31 parts, bismuth 0.63 parts. (2) Tin 7, antimony 1. (3) Tin 14, antimony 2. (4) Fine tone: copper 40, tin 60. (5) Tin 19, nickel 80, platinum 1.
  7. Gong metal. (1) Copper 78 parts, tin 22 parts. (2) Copper 78.5, tin 10.27, lead 0.52, silver 0.18. (3) Copper 10, tin 4, zinc 1.5, silver 0.5.
  8. For small bells, said not to tarnish, and be light in weight, and of good sound. Copper 6 lb., nickel 1 lb., melt and cool; add 1 lb. zinc and ½ oz. aluminium, melt and cool again; now finally melt, and add ½ oz. mercury and another 6 lb. of copper in a molten state.
  9. Japanese, and known as "Karakene." (1) Copper 20 parts, tin 8 parts, iron 1 part, zinc 3 parts. (2) Copper 20, tin 5, lead 2⅔, zinc 1. (3) Copper 20, tin 6, lead 4, iron 1, zinc 2. (4) Copper 20, tin 4, lead 4.
  10. Railway signal bells. Copper 60, zinc 36, iron 4.
  11. Sleigh bells. Copper 84, tin 16.
  12. Ship. Copper 82, tin 12, zinc 6.


Proportions

The usual proportions are, to have the upper thin part of the bell one-third the thickness of the thickest part (known as the "sound-bow"). The thickness of the sound-bow varies with the size of the bell, those of large size having this part as thin as one-fifteenth the diameter, while with small ones it has been as thick as one-tenth the diameter. These may be considered extremes, the generally accepted effective thickness of the sound bow being one-twelfth to one-thirteenth the diameter. If a peal of bells is to be undertaken, it is desirable - necessary, in fact - to adopt a rather wider range of thickness to prevent the treble being so small and weak as to be overpowered by the tenor, though care must be taken not to run into the opposite extreme and make the large bells too thin. In calculating the sizes of bells to produce particular notes, and assuming that eight bells are made of similar material, and their sections exactly similar figures, in the mathematical sense they will sound the eight notes of the diatonic scale, if all their dimensions are in these proportions: 60, 53⅓, 48, 45, 40, 36, 32, 30, which are merely convenient figures for representing the inverse proportions of the times of vibration belonging to the eight notes of the scale. So that if it is required to make a bell a fifth above a given one, it must be ⅔ of the size in every dimension, unless it is intended to vary the proportion of thickness to diameter, for the same rule then no longer holds, as a thinner bell will give the same note with a less diameter. The reason is, that according to the law of vibrating plates or springs the time of vibration of similar bells varies as (thickness)2/diameter. When the bells are also completely similar solids, the thickness itself varies as the diameter, and then the time of vibration may be said simply to vary inversely as the diameter.

Shape. - Next to the alloy of the bell, its diameter, shape and mould require close attention. A bell of a given weight must also be of a given diameter and thickness. For example, a ton bell should be 4 ft. diameter at the mouth, and 3½ in. thick at the sound-bow, whilst a 1-cwt. bell should be 18 in. diameter and 1¼ in. thick. There is no getting away from this rule if you wish your casting to turn out a success. The same principle applies to all bells of various sizes. The heavier the bell, the larger and thicker it becomes; the lighter, the smaller and thinner.

Weights

The weights of bells of similar figures vary as the cubes [ of their diameters, and may be nearly enough represented by the figures 216, 152, 110, 91, 64, 46, 33, 27. The exact tune of a set of bells as they come out of the moulds is a secondary consideration to their tone or quality of sound, because the notes can be altered a little either way by cutting; but the quality of the tone will remain the same for ever, except that [ it grows louder for the first two or three years that the bell is used, probably from the particles arranging themselves more completely in a crystalline order under the hammering.

Moulding

Bells of small size are generally moulded in sand from a metal or wooden pattern, and the sand mould is dried in a stove. Large bells are moulded in loam. The core is built in brick on an iron platform, which must have nugs in case the mould is made above ground. This brick core is covered with ¾ in or 1 in. thick of hair loam, and the last surface-washing is given by finely ground composition of clay and brickdust. This latter is mixed with an extract of horse-dung, to which is added a little salammoniac. Upon the core the "thickness" is laid in loam sand, but the thickness is again washed with fine clay to give it a smooth surface. Ornaments which have been previously moulded, either in wax, wood or metal, are now attached by means of wax, glue, or any other kind of cement. If the ornaments are of such a nature as to prevent the lifting of the cope without them, for the cope cannot be divided, the ornaments are fastened to the thickness by tallow, or a mixture of tallow and wax. A little heat given to the mould will melt the tallow, after which the ornaments adhere to the cope, from which they may be removed when the cope is lifted off the core. It is necessary to well polish the thickness, and, as it is not possible to use coal for parting, wood ashes should be lightly dusted over. Wood ashes are also used for the parting between the core and the thickness. A paint brush is used to lay the cope on at first, the liquid, thin and fine, being made up of clay, ground brick and horse-water. Upon this hair loam, and, finally, straw loam are laid.

The moulding of the crown of the bell is done over a wood pattern after removing the spindle. In the hole in the core left by the spindle the iron or steel loop for the hammer is set, projecting into the thickness so that it will be cast into the metal when it is run. When the cope is lifted off, the facing of the mould can be finished; and if there are small defects they can-be left, any excess of metal occurring at these places being chiselled off afterwards. Nothing much can be done towards polishing the facing of the mould, except to dust it uniformly with ashes, and when the mould is dry it is put together for casting. The core can be left open, or filled with sand; if open, there is no danger, as bell metal gives off very little gas. The chief security of the cope is the well-rammed sand of the pit, but it is to some extent secured by iron. The cast gate is on the top of the bell, either on the crown, or, if the crown is ornamental, at the side of it. Flow gates serve no good purpose, the metal must be clean before it is poured.

Casting

The mould, which has previously been built up on an iron plate, is lowered into a pit already dug in the ground, of a sufficient height to envelop the bell. This mould is called the "core," and has been built up of bricks and loam, and dried hard in an oven. The core being now placed below the level of the base of the furnace, the case, which has been built up and baked in a similar manner, is then placed over it like a glove, but leaving a vacant space between the two of the prescribed thickness of the bell. Any inscription to be cast on has already been impressed on the loam of the case, so as to form part of the metal when run. The alloy having previously been mixed in the proportions already referred to, and being now reduced to liquefaction, as the result of being in the furnace some hours, the furnace is tapped, the molten metal, finding an outlet, rushes down an inclined conduit into the crown of the mould, and in a very few minutes the vacuum is filled, and the whole - core, case, and bell - are allowed to remain in the ground for a day or two to cool. A 5-cwt. bell could be dug out the next day. A ton bell would remain too hot to touch for two or three days. The process of casting is but the consummation of days and weeks of preparation.

Tuning

The tuning of a peal of bells is a delicate business. So long as bells are used singly, it does not matter much what note they turn out; but where bells are required to act in concert with each other, then the question of tuning them to harmonise has to be taken into account. The bell is fixed mouth upwards, and held firmly in the grip of powerful vices. Having taken a plumb of the centre, we adjust our steel-cutter to the sound-bow of the bell, and proceed to pare off the metal at its thickest part. This is how a sharp bell is flattened in tone, and it can be done without detriment to the bell up to half or even three-quarters of a note. If you keep on turning it out, the bell would become thin and "panny" in sound.

To sharpen a flat bell is not so easy - indeed, we seldom attempt it, preferring rather to take the bell into stock, or even to recast, it than to waste time over what generally results in failure. There is a theory extant that a flat bell can be sharpened by reducing its diameter. I believe the note of a flat bell can be sharpened in this way, but we have also found that the application of this principle prejudicially affects the tone of the bell; hence we generally contrive to cast our bells sharp, so that they can be easily flattened, if necessary, to the required note. Sometimes they come out exactly right, and that is the best form of "tuning."

When a bell gets worn in one place through the tongue striking there for generations, it becomes weak in that particular spot, and runs the risk of cracking in consequence. What should then be done is to quarter-turn it round, so that a fresh substance is presented to the clapper as the bell swings.

Good Bell Metal

Good Bell Metal has a fine grained fracture of a greyish colour, differing to bronze. It is hard, rather brittle, and sonorous. Cooled from red heat suddenly it becomes soft, but when reheated and allowed to cool very slowly it regains its hardness. The more copper in the alloy the deeper the tone, while tin, iron and zinc make the tone sharper. It has been believed that the addition of silver improved the tone, but this is now known not to be the case. In making the alloy the copper is melted first, then when the mass has been thoroughly heated the tin is added, the two being well stirred to intimately mix them. It is considered by some that best results are obtained by adding the tin in two parts: a little more than half at first, then the remainder, stirring well between.

Gong Metal

Gong Metal has about the same composition as the bell alloy, as seen above, but the metal undergoes different treatment. After the plates are cast they are taken from the mould, and then heated in a furnace to a cherry-red heat. When in this state they are put between iron plates, plunged into water, and when cool are tough enough to be worked with a hammer. (And see alloys).

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