MORTAR. A strong hollow instrument, usually made of marble, Wedgewood ware, or metal, in which hard or brittle substances are pulverized by percussion or grinding with another instrument called a pestle. Mortars usually partake of the shape of an inverted bell, but their form, capacity, and solidity, as well as the material of which they are made, vary with the object for which they are mainly designed. Thus, they may be purchased ready made, from an inch to eighteen inches, internal diameter, varying in weight, from an ounce or two, to several hundredweight. Large mortars are usually fixed upon a block of wood, of such a height, that the mortar may be level with the middle of the operator. When the pestle is large and heavy, it is sometimes suspended by a cord or chain, attached to a moveable pole placed horizontally above the mortar; this pole considerably relieves the operator, owing to its elasticity assisting the raising of the pestle.
In the annexed diagram is represented a plan for economizing the labour of pounding and sifting, which has been recommended by a person of practical experience in those operations, a and b are two large mortars, containing the material to be reduced; c and d are the pestles, with their rods; e and f are two levers suspended at their fulcrums to a simple frame to the ceiling, connected at one end by joints to the ends of the pestles; and by the other end, in a similar manner, to descending rods attached to treadles, which are operated upon alternately by the man in the centre stepping from one treadle to the other. In this manner a force of 150 lbs. is applied to one end of each lever in succession, and would consequently raise a similar weight at the other, if the fulcrums were in the centre. As it is however, desirable that the man should not have to step up high in lifting his weight from one treadle to the other, and as a pestle of fifty pounds weight is very considerable, those ends of the levers which are attached to the treadles are shortened, so as to make the force about 120 lbs.
This loss of power, in the first instance, is however fully compensated for, by the pestles being raised higher in the same space of time, or with greater velocity, and the increased momentum with which they (alternately) strike the z springs ij, in the ceiling, is returned by the action of the latter upon the substances in the mortars. When one of the pestles has struck, the man steps on that treadle which operates upon it, on to the other: the long arm of the lever of the former then descends by its superior weight, and being jointed near the extremity, it passes by the pin on the rod, (which should have an antifriction roller upon it,) by the joint opening, as shown in dotted lines, and afterwards closing, it locks itself under the pin. In its re-ascent it then takes up the rod and pestle, and allows them to drop when it has passed beyond the sphere of its action, as shown on the opposite side, where the lever is exhibited as being just beyond the point of contact, and the pestle is about to return to the mortar with all its accumulated force.
Underneath each treadle, a strong steel spring is fixed, to prevent those shocks which the man might experience by the treadles striking against the floor, after the levers have passed the rollers on the pestle rods; and the reaction of these springs is attended with the further advantage of assisting the man on to the other treadle.
It is apparent that by this method of pounding, a surplus of power, amounting to about 70 lbs. is devoted to the giving an accelerated force to the pestles. If we then take away a small portion of this surplus power for the purpose of sifting, it may so well be spared, as to make a scarcely perceptible difference in the impelling force to the pestles. There are several obvious modes of causing sieves to vibrate by this apparatus. Accordingly there may be placed a large semicircular sieve on a floor, with cords attached to each extremity or corner, which being made fast to the ends of the lever, cause it to rock, as they are alternately raised or depressed. In the drawing, the sieve is shown as moving upon a central bearing or pivot; this is, however, only another mode of producing the effect. The sieve is composed of two parts; viz. k, which contains the material to be sifted, and I, the receptacle for the resulting product or powder. The situation of this sieve between the two mortars, for receiving their contents alternately, will be found convenient.
It should be placed at a suitable distance behind or before the man at work; a rod should therefore be fixed to the end of each lever, at right angles with them, but in an horizontal position, which it would always maintain; and a long range of sieves, separate or connected, may be moved by the same means, according to the length of the horizontal rod. In the foregoing drawing, many of the subordinate parts, which every engineer knows how to supply, are omitted to avoid complexity.