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CHAPTER XIX.

EXPRESSION.

EXPRESSION is the process of forcibly separating liquids from solids. It is generally effected by the use of a press, although for many pharmaceutical operations, upon the small scale, the use of straining-cloths with hand-pressure suffices. Pressing-cloths are generally employed, but they are troublesome, and of late years strong efforts have been made to construct presses which could be used without them. At least six mechanical principles are recognized in the operation of expression as now practised: namely, by the use of-1. The spiral twist press. 2. The screw press. 3. The roller press. 4. The wedge press. 5. The lever press. 6. The hydraulic press.

FIG. 303. FIG. 304.

1. The Spiral Twist Press.-The principle of this press is best and most practically illustrated in the usual process of expressing a substance contained in a cloth with the hands. The mixture to be expressed is placed in a bag or a cloth held in one hand by the four corners, which are gathered together, and the lower portion, or bag, is rotated with the other hand, so that, beginning at the top, the point of smallest diameter, the strainer is spirally twisted, the pressure forcing the liquid between the meshes of the cloth. Figs. 303 and 304 show Gigot's press, in which this principle is used upon a larger scale. Upon the left hand is shown a sectional view of the press as it appears when filled with material. The perforated cone, d, forms the bottom of a conical strong cloth tube; the upper end is connected with a funnel, l, which is so constructed that none of the material that has passed down into the bag can leak back. The lower portion of the cloth is secured to a ring, which may be fastened, so that it will not rotate when the press is used. If the upper end of the cloth is twisted, the space occupied by the material is contracted, and the liquid oozes through the meshes of the cloth. The dry residue is discharged by untwisting the cloth and unhinging the bottom, which opens downwards. 2. The Screw Press is the most useful of all forms for pharmaceutical work where very great power is not desired. The screw is always used in combination with one or more levers, and this form of press is employed in great variety to accomplish special purposes. There are two forms of screw presses: 1. Single-screw presses. 2. Double-screw presses. Each of these forms may be subdivided into

Gigot's press.

those in which the position of the screw or screws is vertical, and those in which the position of the screw or screws is horizontal.

1. Single-Screw Presses.-In this form of press the single screw is generally used in a vertical position, and operated with a lever or a combination of levers. It is the simplest kind of press, and if well made will admirably answer the general purposes of the pharmacist. The screw should have a square-faced thread, and be well made. The plunger should be disconnected from the end of the screw, or else move freely around it. The parts of the press which come in direct contact with the material to be pressed should be coated with tin or porcelain, so that liquids containing acid or tannin shall not be affected injuriously. Fig. 305 shows a press of this description, made by H. Troemner, of Philadelphia. A perforated tin case accompanies the press. This may be used when pressing bulky drugs, like arnica flowers, and press-cloths may be abandoned. The best material for press-cloths

FIG. 305.

FIG. 306.

[graphic][merged small][merged small][merged small]

is that which is especially made for the purpose. It is twilled and elastic in one direction, and, if proper care be taken when enveloping the material and introducing it into the press, the same cloth may be used many. times, as the pressure causes the meshes to open without breaking the threads. The manufacturers of linseed oil use press-cloth largely. For small operations, in the absence of press-cloth, which is very expensive, new Russia crash may be used. The press-cloth should be moistened, if possible, with some of the same liquid expressed at a previous operation. Water answers very well if the liquid is aqueous. The substance to be pressed is laid upon the cloth, and one corner of the cloth laid over it. The opposite corner is then placed upon the first; next one of the remaining corners is laid in the same way upon the first two, followed by its opposite. The corners should be folded over so that a square, somewhat flat package is produced, in size somewhat smaller than the press-plate, especial care being observed to suit the quantity of material to the capacity of the press. If too much is taken, the press-cloth will be too small to permit of folding it over sufficiently; the corners

of the package will therefore not withstand the pressure, portions of the material itself will ooze out, and the whole operation must be repeated. The principal objection to the single-screw press is, that unless the material in the press is nearly homogeneous, so that the press-cake is equally pressed upon at all points, unequal action results, the pressblock is pressed against one side of the case, causing violent friction and and resistance, and the thread of the screw binds upon one side, full pressure thus being defeated; whilst the main objection to the vertical screw is, that the press-block and plate must be in a horizontal position, so that the liquid pressed out adheres to the cloth, and cannot be collected readily without tilting the press. Fig. 306 is an illustration of a German single-screw press which is well adapted for pressing the residues from macerated tinctures. The large lip of the containing vessel is a practical convenience, whilst the lever, L, in combination with the catch, C, gives unusual power and ease in working, for a small press. In the horizontal screw press, the jaws being vertical, there can be no obstruction to the dropping of the expressed liquid, which may be caught in a vessel placed immediately under it. Oberdoffer, of Hamburg, Germany, makes a very powerful horizontal screw and compound lever press. Fig. 307 illustrates a novel horizontal screw press, made by the Enterprise Manufacturing Company, of Philadelphia, which is operated without a press-cloth. It consists essentially of a tapering cylinder, with a hopper on the upper side at its large end, and a strong screw fitting closely to the inner surface of the case, the thread of which diminishes in size as the screw becomes smaller. Along the under side of the cylinder is formed a channel adapted to receive a perforated brass plate. This latter has a transverse concavity corresponding to that of the inner surface

FIG. 307.

Enterprise press.

of the cylinder, and the perforations allow the escape of the expressed fluids into the channel or gutter beneath, from which it escapes by a proper outlet. The substance to be expressed is placed in the hopper, and, the crank attached to the screw being turned in the proper direction, the thread of the screw compresses the substance into a smaller and smaller space, until finally it is discharged at the farther end of the cylinder in a comparatively dry state. A large screw, passing through a removable end of the conical cylinder, regulates the size of the outlet, and serves to increase or diminish the amount of pressure to which the mass is subjected by the screw. The brass plate in the bottom of the machine can be replaced by others having perforations of different sizes. Clogging of the holes by bits of wood, stems, or seeds is prevented by the shear-like action of the edge of the screwthread, which shaves off the protruding fragment, while the rest is forced through the perforation. To increase the power of the press upon slippery substances, the inside of the casing has a few longitudinal

or spiral grooves at a greater or less angle to the direction of the screwthread. Arrangements are provided for detaching the casing from the

FIG. 308.

Enterprise press.

screw, and for attaching the whole to the edge of a table or bench. Fig. 308 shows the press as taken apart.

2. Double-Screw Presses are preferred by many. These are always of the horizontal screw form, and in Fig. 309 is shown one made as proposed by Chas. T. George, of Harrisburg,

[graphic]

Pa.

A strong and substantial framework, made of ash wood, forms the base. Two pieces of timber rest upon this frame, one firmly attached by an

[merged small][graphic][merged small]

iron rod and keepers to one end of the frame or table, and the other free or movable, both blocks being bored at the same distance from the end with smooth holes to receive the two iron screws.

Upon the movable block, and opposite the holes, a female nut of bellmetal is firmly fixed. Upon the inner face of the two timber blocks, iron castings, saddle-shaped and hollow, are inserted, flush with the face of the block, each capable of holding six pints of boiling water, and each casting having a hole on top to receive hot water or steam : a brass pet-cock is fixed at the bottom to discharge the chilled water.

The two iron screws pass through the holes of both blocks of timber, the head of each screw having an iron cog-wheel attached to it; into the cogs of each wheel a pinion-wheel is firmly fixed, which in turn is fastened to a short shaft having at its end an iron fly-wheel.

By turning this wheel both screws are evenly and rapidly rotated without danger of bending or breaking, and consequently one block is drawn surely and with great power against the other, thus expressing whatever drug may be placed in a sack of strong linen towelling between the two blocks of timber.

It is apparent that the double-screw press is very powerful, and, the pressure being equalized, good results are obtained.

3. The Roller Press is used upon the large scale for pressing oily seeds, fatty substances, etc. Its principle is thoroughly shown in the well-known clothes-wringer, which, although made for the laundry, serves as a very efficient press for many substances. Care must be taken to apply the force gradually to the bag containing the berries or other material to be pressed, and not to use it upon substances which will soften or dissolve the rubber rollers.

4. The Wedge Press.-This form of press is powerful, economical, and inexpensive. It is objectionable principally on account of the noise necessarily made in driving the wedges. Fig. 310, taken from Knapp's Technology, illustrates the wedge press. The filled cloths are laid between strong plates, h and g, and placed in a square space cut in a solid block of oak wood or in a cast-iron case, b, and the plates are forced nearer and nearer to each other by driving in the wooden wedges which occupy the remaining space. One

of these wedges, a, serves to facilitate the disconnection of the apparatus; the strokes which drive in the wedge i tending, from the reverse position of a, to drive the latter out; c, e, and d are intermediate pieces to prevent the wedges from coming into immediate contact. The pressing-plates are each pro

FIG. 310.

[graphic][merged small]

vided with three side ribs: the immovable ones, g, g, press against the sides of the case, and the movable ones, h, h, against the intermediate wedges, c, e, and are pierced with numerous holes to allow the liquid to flow out more easily. On filling the press, the wedge a must be suspended (by a string) at a distance from the bottom, so that the apparatus may be easily taken to pieces. The liquid trickles from the pressing-plates through the pierced horizontal plates, n, n, upon which these rest, into the pipe o. Both a and i are driven by separate stampers, which are raised by a toothed wheel, or mallets may be used.

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