J4unufffcture oj Lead Pencils. W-7
Results in the preceding Table, showing the progressive and increased rptia of clejieclions fron; Marcii 6& 1857, to June %3rd, 1838.
The greater degree of weakness here exhibited in the hot blast iron than the cold renders our comparative experiments in this Table defective; several bars were tried in succession, but they separatelygave way, some on laying on the load., 448 Ibs., and others after supporting it for a few seconds.
In expcrlment lst, Table XIV.,, it will be noticed that a bar from the cold blast iron, after being charged with the full load,448 Ibs., continued tosup port it for a period of37 days; this was not, however, accomplished without signs of weakness, as will be seen from the progressive increase which took place in the deflections from the 6th to the 17th of March; and alsofrom observed discrepancies some time previous to its rupture. In making these statements it must be observed, that the bar in experiment 1st was thinner than any of the others, and had borne for thirty-seven days a weight greater than had broken bars of the same size in previous experiments upon the Coed-ralon iron, when the weights were laid on without loss of time.
Alrstracl qf comparative itxrense and ratio of deJections on the whole Barsfrom Afurch Gth, 1837, lo June 23-4 1838.
Cold Mast Iron, Hot Blast Iron, Ratio ef increase of de- increase ofdeffec- deAeotions flection in inches. tion in inches.
Increase of defiection, Table XI. ,033 .043 *. Increase of deflection, Table XII. ,046 .077 *. Increase of deflection, Table XIII. ,140 .088 .I Increase of deflection, Table XIV. ,047 . . . .
.Mean, .066 .069 1000 : 1045
The mean increase of deflections on the whole bars is therefore ,066 for the cold blast, and .069 for the hot blast, being in the ratioof 1000 to 1045.
The interest which experiments of this kind may be expected to excite, and the nature as well as the value of the material on which they are here made, will, it is hoped, prove an inducetnent for extended investigation on this subject.
[TO BE CONTINUED.]
Manufacture of Lead Pen&. The best black-lead pencils of this country are formed of slender &&al-
lelopipeds, cut out by a saw from sound pieces of plumbago, which have been previously calcined in close vessels at a bright red beat. These pnrallelopipeds are generally enclosed in cases made ofcedar wood, though of late years they are also used alone, in peculiar pencil cases, under the name of ever-pointed pencils, provided with an iron wire and screw, to pro-
lTOL. XXIV.---ru'O, 6. -DECEMBER, 183% 34
306 Progress of Pmclicnl Q Theoreticnl Mechanics & Ci~enhlry. trude a minute portion of the plumbago beyond the tubular metallic case, in proportion as it ix wanted.
In the year 1795, M. ContG, a French gentleman, well acquainted with the mechanical arts, invented an ingenious process for making artificial black-lead pencils of superior quality, by which he and his successor and son-in-law, M. Humblot, have realized large fortunes.
Pure clay or clay containing the smallest proportion of calcareous or sili- ceous matter, is the substance which he employelI to give aggregation and solidity, not only to plumbago dust, but to all sorts of coloured powders. That earth has the property of diminishing in bulk, and increasing in hard- ness, in exact proportion to the degree of heat it is exposed to, and hence may be made to give 1 every degree of solidity to crayons. The clay is pre- pared by diffusirq it in la&c tubs through clear river water, and letting the thin mixture settle for two minutes. The supernatent,Inilky liquor is drawn OK by a syphon front near the surface, so that only the finest particles of clay are transferred into the second tub, upon a lower level. The sediment which falls very stowly in this tub, is extremely soft and plastic. The clear water being run OK, the deposit is placed upon a linen filter, and allowed to dry. It is now ready for use.
The plumbago must be reduced to a fine powder in an iron mortar, then put into a crucible, and calcinetl at a heat approaching to whiteness. The action of the fire gives it a brilliancy and soltness whicII it would not othcr- wise possess,and prevents it from bcingalKectet1 by the clay, which it isapt to be in its natural state. The less clay there is mixed with the plumbago, and the less the mixture is calcined, the softer are the pencils made of it; the more clay used the harder are the pencils. Some of the best pencils made by M. Cont6, were formed of two parts of plumbago and three parts of clay; others of equal parts. Tl;is composition admits of indefinite variations, both as to the shade and hardness; advantages not possessed by the native mineral. \Vhile the traces may be made as black as those of pure plumbago, they have not that glistening aspect which often impairs the beauty of black-lead draw- ings. The same lustrc may, however, be obtained by increasing the pro- portion of powdered plumbago relatively to the clay.
The materials having been carefully sifted, a little of the clay is to be mixed with the plumbago, and the mixture is to be triturated with water into a perfectly uniform paste. A portion of this paste may be tested by calci- nation. If, on cutting the intluratell mass, particles of plumbago appear, the whole must be further levigated. The remainder of the clay is now to be introduced, and the paste IS to be ground with a multer upon a porphyry slab, till ii: be quite homogeneous, and of tile consistence of thin dough. It is now to be made into a ball, put upon a support, and placed under a bell glass inverted in a basin of water, so as to be exposed merely to the moist air.
Small grooves are to be made in a smooth board, similar to the pencik parallelepipeds, but a little longer and wider, to allow for the contraction of volume. The wood must be boiletl in grease, to prevent the paste from sticking to it. The above described paste being pressed with a spatula into these grooves, another board, also boiled in grease, is to be laid over them vc~y closely, and secured by means of screw-clamps. us the at- mospheric air can get access only to the ends of the grooves, the ends of the pencil pieces become dry first, and by their contraction in volume get loose In the grooves, allowing the air to insinuate further and to dry the re- mainder of the paste in succession, When the whole piece is dried, it be-
Mcmfucture of Lead Pemk 3%9
comes loose,and might be turned out of the grooves. But before this is done,, the mould must be put into an oven moderately heated, in order to render the. pencil pieces still drier. The mould should now be taken out, and emptied upon a table covered with cloth. The greater part of the pieces will be en- tire, and only a few will have been broken, if the above precautions have been duly observed. They are all, however, perfectly straight, which ia a matter of the first importance.
Jn order to give solidity to these pencils, they must be set upright in a crucible till it is filled with them, and then surrounded with charcoal powder, fine sand, or sifted wood ashes, The crucible, after having a luted cover applied, is to be put into a furnace, and exposed to a degree of heat regulated. by the pyrometer OF Wedgewood; which degreesis proportional to the in- tended hardness of the pencils. When they have been thus baked, the crucible is to be removed from the fire, and allowed to cool with the pencils in it.
Should the pencils be intended for drawing architectural plans, or for very fine lines, they must be immersed in melted wax,or suet, nearly boiling hot, before they are put into the cedar cases. This immersion is best done by heating the pencils first upon 3 gridiron, and then plunging them into the melted wax or tallow. lhey acquire by this means a certain degree of soft- ness, are less apt to be abraded by use, and preserve their points much better.
When these pencils are intended to draw ornamental subjects with much shading, they should not be dipped as above.
Second process for rnnfcirl~q nrtiJcial pencils, somewhat diflerent from the preceding.- All the operations are the same, except that some lamp-black is introduced along with the plumbago powder and the clay. In calcining these pencils in the crucible, the contact of air must be carefhlly excluded, to prevent the lamp-black from being burned away on the surface. An in- definite variety of pencils, of every possible black tint, may thus be produc- ed, admirably adapted to draw from nature.
Another ingenious form of mould is the following: Models of the pencil-pieces must be made in iron, and stuck upright upon
an iron tray, having ed.ges raised as high as the intended length of the [Jell- cils. A metallic alloy 1s made of tin, lead, bismuth and antimony, which melts at a moderate heat. This is poured into the sheet-iron tray, and after it is cooled and concreted, it is inverted, and shaken off from the made1 bars, so as to form a mass of metal perforated throughout with tubular cavities, corresponding to the intended pencil-pieces. The paste is intro- duced by pressure into these cavities, and set aside to dry slowly. When nearly dry, the pieces get so much shrunk that they may be readily turned out of the moulds upon a cloth table. They are then to be completely de- siccated in the shade, afterwards in a stove-room, next in the oven, and lastly ignited in the crucible, with the precautions above described.
M. ContC recommends the hardest pencils of the architect to be made of lead melted with some antimony and a little quicksilver.
in their further researches upon [his subject, M. ContC and M. Humblot found that the different degrees of hardness of crayons could n