Crucibles

In the scheme which we outlined in the opening paragraph of this section, of having a place for everything, it was stated that there should be a separate table for ignitions. The size of this table will, of course, depend on the number of ignitions which are to be made at one time. Ten or twelve can easily be carried out at the same time on a table with a top 3 x 4 feet. There is nothing particularly different about the ignition table from any other table in the laboratory. It should, of course, have a concrete, stone, slate or lead top and can be made similar to the hood table described here. Nothing is gained by having a very wide table. If in the middle of the room, a table three feet wide is of ample width, while two feet will do for one set against the wall. The gas tubes should be frequent and there should be some mechanical means for supplying air to the blast lamps. Double gas jets every foot will allow an ignition every six inches. The gas pipes should be run around in front just below the top, as in the hood table.

Crucible Supports

Tripods may be used for holding the crucibles; or ring supports, sold by all dealers, can be used. These latter are to be preferred, as the height of the crucible above the flame can be regulated. They cost, however, about four or five times as much as the tripods. In getting supports, those with triangular bases take up a little less room than the other kind. If a concrete top is to be used, the cheapest and best way of supporting the crucibles is by iron rods projecting from the concrete top, from six to seven inches apart and ten to twelve inches from the edge of the table. Each rod should project fourteen inches above the table top. To do this, cut the rods of the proper height and insert in an upright position in the rough wood top of the table before the concrete is poured on. This latter will of course run around them and hold them very firmly in position. The crucibles are then held at any height desired by means of ring clamps such as are used with supports.

The best triangle to use with the crucible is one of platinum. Its first cost is of course high, but they will stand, if carefully used, any amount of wear. The platinum triangle for a 15 CC. crucible weighs about 8 grams and one for a 30 CC. crucible will weigh about 12 grams. Special clamp supports, for platinum triangles are made. These hold the triangle firmly in place and prevent the wire from bending down. They also require much less platinum wire than the ordinary form, three to five grams being sufficient for ordinary crucibles. Nothing is gained by buying platinum wire and bending triangles from this. The best of the pipe stem triangles are those made of iron covered with flanged pipe stems.

Blast Lamp

Every laboratory should have a blast lamp, since its use saves much time and simplifies many operations of the laboratory, and indeed certain determinations (such as silica) cannot be performed correctly without one. They can be obtained in many forms. Bunsen’s is one of the oldest and best of these, and it is illustrated in all catalogues of chemical apparatus. It is so fixed that the direction of the flame can be altered by turning a thumbscrew. Another good blast lamp is Weisnegg’s, which is mounted on ball joints. It can be turned about a greater range than Bunsen’s, but is also more expensive. The upright blasts cost about as much as the adjustable ones and are not so useful. It is generally better to play the flame at an angle upon the bottom of the crucible than directly upon it, for in the former method the products of combustion are swept away from the crucible, while in the latter they are carried up and around it, entirely surrounding the crucible and cutting off the supply of oxygen from the latter, possibly causing a reduction, etc., of the precipitate being ignited. For this reason the adjustable blast lamps are better than the upright ones. Crucibles to be ignited over the blast lamps should be placed with their bottoms projecting through a round hole in a piece of platinum foil, which in turn rests upon a piece of asbestos board with a slightly larger hole cut into it. While this method of supporting a crucible is not essential and indeed is not the one generally used, it will probably give better results in all cases where a precipitate is to be ignited to a constant weight or when reduction by or absorption of the products of combustion by the precipitate is likely to occur.

Air Blast

For supplying air to blast lamps, every chemist is familiar with the foot bellows. No manufacturing concern, however, can afford to use their chemist’s time for pumping a foot blower, so it is more economical to arrange to have some mechanical means of doing this work. If high pressure air is used around the plant, it may be run into the laboratory for this purpose. In blast furnaces, the air from the blowing engine; in machine shops, air for the supply of pneumatic tools, etc., is usually at hand. If the pressure is high, reducing valves should be put in between the laboratory and the compressor or blowing engine.

The writer has in his laboratory a blower which gives excellent service and is working under a water pressure of about 60 lbs. While there is nothing new about it, still it may be worth describing here.

Its construction is shown in Fig. 22. Referring to this illustration, A is the aspirator whose detail construction is shown in Fig. 23. J, Fig. 23, is an ordinary half-inch T. K is a piece of half-inch pipe connected with the valve H. Into the tube, K, is screwed a small tap through the middle of which has been bored a 1-16 inch hole. L is another piece of half-inch pipe leading to the drum, B. The third opening of the T can be connected with the line for aspirating as shown. The drum B, Fig. 22, consists of a three-inch pipe, 22 inches long and capped at both ends. C, G, and E are all made of half-inch pipe. G leads to the waste. The air for the blast lamps is drawn off at D. This apparatus will easily give sufficient air for two blast lamps. It can be purchased, if desired, in a little neater form, from several dealers in chemists’ supplies. A small drum may be placed after the blower to catch any water splashed into the air line. One may be made by capping a piece of 3-inch pipe, 12 or 14 inches long, at both ends. It should stand upright and the air should enter and leave at the upper end, and there should be a small tap at the bottom for drawing off any water that may collect in the pipe. The water blower in use by the writer, however, seems to give air free from water and no trap would be needed with it.

In place of the water blower described, a small automatic air pump such as is used for forcing beer from the cellar into the bar, and by physicians and barbers, may be used. It should be of the piston variety, double acting and automatic. A large receiver or drum should follow this to equalize the pressure. A small Root blower or a Crowell blower run from a shaft or by a motor may be used, but either is somewhat noisy if placed in the laboratory and like all machines needs repair and attention.

Dr. Porter W. Shimer described in the Chemical Engineer for April, 1905, an apparatus for producing either blast or Suction. It consists of a No. 16, “Goulds Air-Pressure or Vacuum Pump” (manufactured by The Goulds Manufacturing Co., Seneca Falls, New York). It is a hand pump, operated by a wooden lever about five feet in length, and, by proper arrangement of valves, it can be used either to compress air or to create vacuum. The diameter of cylinder is 6 inches and the stroke is 10 inches. The displacement of free air per stroke is 280 cubic inches. The inlet and outlet are 1¼-inch pipes.

The pump is connected with two boilers such as are used for hot water in connection with kitchen ranges. These boilers need not be new, for rejected ones may be made, with a little repairing, as good as new ones for this purpose. The cylinder for blast in Dr. Shimer’s laboratory is 6 feet high and 18 inches in diameter. The one for suction is 5 feet high and 14 inches in diameter. The manner of making the connections and the position of the valves are shown in Fig. 24.

By a proper arrangement of the valves the pump may be set for vacuum and the air may be exhausted from the other cylinder for purposes of filtration. By this means any degree of vacuum desired for hastening filtration may be obtained. This cylinder and the piping should be coated inside with acid-proof asphaltum paint to protect it from the corrosive action of acid fumes drawn in during filtration of strong acid solutions. This may also be guarded against by sucking the air through two bottles partially filled with water containing a little caustic soda. The entrance tube to each bottle should pass below the surface of the liquid.

Both blast and suction cylinders may be connected with piping containing as many valved outlets as may be desired.

Ignitions in a Muffle Furnace

Small muffle furnaces, such as are used in assaying gold and silver ores, are often used in large laboratories. When many ignitions are to be made they undoubtedly have some points in their favor. In most small laboratories, however, they are uneconomical, as the gas or gasoline required to heat them up is considerably more than that which would be required to ignite a dozen or so precipitates over Bunsen burners and blast lamps. If heated by coal they are a nuisance and a source of dirt. When fire assays are also made the muffle is of course convenient for ignitions. They are described in Chapter XIII.

Platinum Crucibles

These should be made of the best hammered ware free from alloy. They usually weigh as much, lids included, in grams as they hold in cubic centimeters—that is a 15 c. c. crucible with lid should weigh about 15 grams. For ordinary ignitions a 15 c. c. crucible will be large enough, and, indeed, where 9 cm. filters are used and small precipitates are collected, even 10 c. c. crucibles will answer satisfactorily. For fusions, a 30 c. c. crucible will be found sufficient. For ignitions it is a good plan to have all of the crucibles weigh approximately between 15 and 16 grams, as by this means, considerable time is saved in weighing a lot of them, one after the other, because the large weights on the balance pan do not have to be changed.

The crucibles may be numbered with a small steel die, or, if there are only a few, this may be done with dots; thus, etc. A wooden mould and plug should be provided and the crucibles kept free from dents and distortion by shaping them up in this. If a mould can not be obtained, a recess casting of the crucible in plaster or cement may easily be made, when it is new, and the end of a broomstick rounded off for a plug to fit inside.

Porcelain Crucibles

Platinum crucibles can not be used for the free metals, the easily reduced metallic oxides and the salts of the heavy metals, such as lead, tin, bismuth, etc., and for igniting such precipitates porcelain crucibles must be resorted to. When platinum can be used, little except first cost is saved by employing porcelain crucibles, as the breakage, etc., of these latter is greater than the wear and tear and interest on the former. Porcelain crucibles may be obtained in sizes ranging from a few cubic centimeters’ capacity up to that of several hundred c. c. They are made of both Royal Berlin and Royal Meissen procelain, the former being the best and costing considerably more than the latter. The Royal Berlin porcelain crucibles are of a more squat form than those of the other makes.

The crucibles may be obtained with vitrified consecutive numbers or letters on the side, by which to distinguish them, at a slightly higher price than the unmarked; or they may be numbered with a lead pencil under the unglazed bottom, before igniting and weighing, the ash of the lead burning into the crucible and making a permanent mark.

Porcelain Gooch crucibles may also be purchased and these are handy for some precipitates. Those with the movable bottom may be used to advantage to collect the carbon for combustion in steel analysis. When precipitates have to be heated in an atmosphere of hydrogen sulphide, hydrogen, etc., Rose’s form of crucible may be used. This consists of a porcelain crucible with a perforated cover through the hole of which a porcelain tube passes. When this form of crucible is not at hand an ordinary crucible may be made to serve, being covered by inverting over it an ordinary clay pipe, the gas being led in through the stem of the latter.

Crucible Tongs

These are made in a number of patterns and of a number of metals. For ordinary ignitions, in a platinum crucible, over a burner, the so-called double bent tongs will be found most useful. They allow the removal of the lid from the crucible-and the tilting of the latter on the triangle. For removing crucibles from a muffle furnace, Julian’s crucible tongs may be used. Blair also has devised a special form of tongs, scissors shaped, in which the curved and bent part of the tongs is of platinum.

A good pair of tongs is one made of solid nickel or German silver and having platinum shoes on the ends. The shoes are to be, preferred to the tips riveted on. The tongs may be obtained single or double bent, the former being the most convenient. In transferring a crucible from the triangle to the desiccator with these tongs, the lid is slipped to one side, just far enough to allow the edge of the crucible to be gripped by the point of the tongs. Where a cheaper pair of tongs is desired, steel forged, nickel-plated tongs are the best.

Desiccators, in which to cool crucibles before weighing, are of a number of forms, Scheibler’s is perhaps the best. This may be obtained in a number of sizes, of which the 6-inch is probably most suited to laboratory purposes, as it can be carried back and forth from the ignition table to the balance room. For holding the crucibles in the desiccators, porcelain or aluminum plates can be purchased. The latter cool the crucible quicker as the metal carries off the heat faster. Large desiccators may be had and, when these are to be left in the balance room, are found useful. The porcelain plates which come with large desiccators are usually perforated with holes entirely too small for crucibles. If the dealer is furnished a rough drawing, showing size and number of holes desired, he will usually make an aluminum plate at small additional cost. Calcium chloride is used as the moisture absorber in all portable desiccators.

It is often more convenient to lay precipitates aside as they are filtered and ignite later on; an 8-inch bell glass of the low form resting on a glass plate will be found convenient for keeping these out of the dust. The paper may be marked, either before being placed in the funnel or when taken out, with a soft lead pencil. If an acid dish or basin containing a little strong sulphuric acid is placed under the bell jar, and the precipitates are placed in a watch glass resting on the acid dish, the acid will dry the precipitates and filter papers, provided they are left in long enough.