Part 3

In the projection of this part of the Powder Works, I conceived the design of making the central portion present the appearance of a grand monumental structure. For this purpose the chimney was placed centrally, and its exterior dimensions considerably enlarged; in fact, it is composed of two distinct parts, the chimney and outside obelisk; the former being enclosed at its base by a square tower, nineteen by thirty-five feet in height, whose battlements arose to view above the front walls. From the top of this tower the enveloping obelisk commenced, and ascended one hundred and fifteen feet, making the complete structure one hundred and fifty feet from the ground to the coping. The interior chimney flue is five feet square from bottom to top. The corner stone, or rather the box, containing the usual documents, was, by a fancy of the architect, placed in one of the corners of the top coping of the obelisk.

The saltpetre refinery occupied the right central portion of the front, being sixty-five feet long, fifty-five feet broad and thirty feet high, open from the floor to the ventilated roof. At the east end were four of the large evaporating iron pans, placed side by side, and elevated three feet above the floor by the brick work which surrounded them; five similar pans were in a corresponding position at the west end, and the large copper drying pans occupied forty feet along the north side at the same height. Each evaporating pan had a separate furnace, and the heated air from the whole passed beneath, and in contact with the bottoms of the drying pans on its way to the great chimney; the furnaces opened into side rooms communicating with the outside open space in the rear of the building. Thus the refining room was entirely free from ashes, dust and smoke.

The centre space of the floor, about thirty-six feet square, was sunk four feet to allow water from the canal to pass around the bottoms of two of the large evaporating pans, which were placed therein near the centre of this area, and nine feet apart; these were used for a special purpose.

The best quality of gunpowder can only be made from the purest saltpetre; the impurities of the crude material are mainly deliquescent salts, which rapidly deteriorate the strength of the powder by the moisture absorbed. To refine more or less the rough saltpetre of commerce is then a necessity even in producing an inferior article.

To carry the refining process to the extent of nearly absolute purity, required several successive crystallizations and washings, involving a large amount of manual labor in the manipulation, and consuming much time. This was particularly the case in the very large amount of saltpetre, eight to ten thousand pounds per day, used by the Works, the refining of which would demand extended buildings and apparatus, as well as requiring a large number of operatives. Hence, it became desirable to devise methods by which hand labor could be superseded by motive power and machinery; in this I was entirely successful. Thus, in the operations of filling the various boiling pans with water or mother-liquor; the transference of the boiling solution of saltpetre to the draining trough, and thence to the crystallizing machines; the cooling down of the solutions, and their constant agitation to break up the forming crystals into fine particles, and transferring of these to an adjoining tank; the washing of the crystallized mass, and the subsequent removal of the mother-liquor and wash-waters, were all accomplished by machinery, with the assistance of two or three workmen only.

The saving of time and labor was thus manifest, and the rapidity with which these operations were performed, permitted a double and triple process in a single day; thus allowing a degree of purity in the product of refined saltpetre not attained in any other refinery. Its purity was such generally, that there was not the one-hundred-thousandth part of chlorides left in the salt.

Of the machinery used, the most important was a bronze revolving wheel with buckets attached to the periphery, which worked into an iron pan or kettle, whose section was an arc of a circle; the buckets grazed the surface of the bottom and sides of this kettle, the bottom of the latter being immersed in a current of cold water. The hot filtered solution of the crude saltpetre was received into this kettle, and thus kept into a state of rapid agitation, the effect being to produce a wet mass of minute crystals, which, as fast as formed, were taken up by the sharp edged buckets, and lifted sufficiently high to pour into a receiving vat; this permitted the liquid part to flow back into the kettle. By this means in a short time the entire mass of fine deposited crystals from the rapidly cooled liquid, were removed to the vat. When the operation was completed the remaining liquid in the kettle was by the revolutions of the bronze wheel, discharged into one of the eight capacious cisterns below the floor; there were two of these machines employed.

The facility for work which this apparatus, with the other mechanical appliances afforded, enabled the refinery to carry the purification of the saltpetre beyond that of the most celebrated powder factories.

Adjoining this part of the Works was the Sulphur Refinery, where this material was prepared from the crude stock, and made ready for the incorporating process. About one hundred and thirty tons of very impure sulphur had been received from Louisiana, for the use of the Powder Works; it had been purchased before the war by the planters for use in the making of sugar, and was bought up by the Confederate officers. The best quality of gunpowder has its sulphur chemically pure, which could be demonstrated by showing no trace of acid when powdered and boiled in water, and should entirely evaporate on a piece of glass when heated, leaving no stain. This can only be accomplished practically by distillation. The crude article was melted and poured into upright, thick wooden boxes five feet high and ten inches square at the bottom, tapering upwards; when cold the earthy matters would be found in the lower portion by subsidence, leaving about three feet apparently pure. This was broken off and placed into two kettles of suitable form and dimensions, having furnaces; the tops of these kettles were connected by a bent iron pipe to an enlarged portion, which was surrounded with water. On the application of heat the sulphur vaporized, and passing over through the pipe was condensed in the cooled portion, whence it trickled in a thick stream into a receiving vessel below; the first portions being rejected, the remainder was of a beautiful citron yellow when cold, and entirely pure.

Unlike the refined saltpetre, the purified sulphur had to be pulverized and bolted like flour before being used. The former was done by two iron wheels of twelve inches face and five feet diameter, weighing six hundred pounds each, revolving on a bed circle of iron like the incorporating rollers; the later was accomplished by bolters, but when these were worn out and could not be replaced, for want of the silk cloth, which was not to be found in the South, necessity compelled me to devise a different, and as it proved, a superior method.

The pulverized sulphur was placed in barrels or cylinders, with hollow axles, which were made to revolve slowly by machinery; there were ledges on the interior which caused the sulphur to be lifted and poured over as the cylinders revolved; a light current of air was blown through each, entering the hollow axle at one end, and passing out through the axle at the other end, which led into an adjoining room; there the impalpable sulphur dust was deposited, much finer than by the usual bolting process.

Adjoining this Refinery was the department in which charcoal was made and pulverized. Charcoal for gunpowder has to be made of a porous fine-grained wood, having very little ashes when burned; willow is generally preferred, and was used at first in the Powder Works, but the exigencies of the war taking away those who would ordinarily have supplied it, rendered it impracticable to procure a sufficient quantity. Recourse was had to the cotton wood, which was abundant; on trial its charcoal was found fully equal to that of the willow for the purpose, and was, thereafter always used.

Charcoal for gunpowder must be made by what is termed the distilling process; that is, the wood must be heated in iron retorts to the proper degree, to have it of the best quality and free from sand or grit. For this purpose cast iron cylinders, or retorts, six feet long and four feet in diameter were used, placed over furnaces, each having one end solid and the other with a movable cover; into these were run the slip cylinders, which contained the kiln dried cotton wood, split up into sticks about one and a half inches in diameter, and entirely filling it.

The slip cylinders were charged with the wood in an outside apartment, their covers put on, then readily moved by cranes to the retorts, into which they were pushed; the covers of which were then luted with clay and closely applied. The bottoms of the retorts being perforated, permitted the escape of the vapors and gases into the furnaces beneath, where inflaming, they supplied mainly the heat required in the operation. In about two hours the slip cylinders were withdrawn from the retorts and moved by the cranes over, and lowered into the cast iron coolers beneath the floor; these had water from the canal circulating around them; the covers being then put on to exclude the air, the mass of charcoal was rapidly cooled. As soon as a slip cylinder was removed from a retort a freshly charged one would take its place, and thus the process was continued. The slip cylinders were taken out of the coolers in succession by the cranes, and swung over a long and broad table upon which their contents were dropped; here the sticks of charcoal were separately examined and the imperfect rejected. The charcoal was then placed in pulverizing barrels with bronze balls, which revolving by machinery, soon reduced it more or less to a fine powder; it was then bolted, and with the sulphur and saltpetre taken to the weighing house. Here the three materials were arranged into sixty pounds charges, by mingling forty-five pounds of saltpetre, nine pounds of charcoal and six pounds of sulphur, which was then moistened and ready for incorporation.

Reflecting over the processes for making gunpowder, it suggested itself that the chemical reactions would necessarily have the most favorable conditions, when there should be the most intimate approximation of the component molecules. That, as the charcoal by its combustion with the oxygen of the saltpetre, supplied the expanded gases which produced the explosive force, it was of the first consideration that there should be the most perfect mixture practicable between these two ingredients. Under the microscope a fine particle of charcoal was seen to be a mass of carbon penetrated by numerous pores, hence it became necessary to completely fill these minute pores with the saltpetre to have the best condition. This might be accomplished by the usual processes, as the charge is kept moistened when stamped or rolled, but as it will not answer to have the mass wet during the incorporating operation, only moist or damp, the completion of the process was necessarily delayed. If this mass of material could be made into a semi-liquid condition by the action of steam, the hot solution of saltpetre would speedily penetrate the minute pores of the charcoal, and thus the desired end would be rapidly attained.

Accordingly, the following process was devised: The moistened sixty pounds charges, roughly mixed and moistened with water, were introduced into horizontal cylinders of sheet copper thirty inches long by eighteen inches in diameter. These cylinders revolved slowly on a common axis, consisting of a heavy brass tube three inches in diameter, perforated with holes. High pressure steam was introduced through the tube raising the temperature to the boiling point while the water produced by condensation, added to that originally used to moisten the materials, reduced them to a semi-liquid slush, which was run out of the cylinders after about eight minutes rotation. On cooling, this mud became a damp solid cake, the saltpetre which in the state of boiling hot saturated solution had entered the minutest pores of the charcoal, now crystallizing. The cake as produced was transferred to the incorporating mills, and under the five ton rollers was in an hour brought to the condition of finished mill cake, ready to be cooled and granulated, while without the steaming process, four hours incorporation in the mills had previously been necessary to produce powder of the same first-class character. The capacity of the work of the mills was thus practically quadrupled, the thorough saturation of the charcoal with saltpetre being accomplished by the steaming, while it remained for the rollers merely to complete the incorporation of the whole mass and give the required density to the mill cake.

The Incorporating Mills, twelve in number, extended along the canal beyond the Refinery building and further back from its bank, having the Laboratory between the two; they were two hundred and ninety-six feet long. This separation was for safety, as they worked explosive material. The walls were massive, being four to ten feet thick, the horizontal section of each being that of a huge mortar of seventeen feet wide by twenty-four feet long; the height of the walls was twenty-eight feet; they faced alternately in opposite directions, so that an explosion of one would not be communicated to those adjoining.

The fronts were constructed of light wood and glass, and the roofs of sheet zinc, so that but slight resistance would be offered, upwards and outwards, to the explosive force. A wing wall, nearly as high as the main walls, and three feet thick, extended outwards from the centre of the exterior back wall of each mill twenty feet, to guard still further against the effects of an explosion. Behind these the powder-makers stood, for safety, while starting or stopping the motion of the ponderous rollers. This was done by means of a long lever, which threw in or out of gear the friction arrangement, which worked each set beneath the floor, in the thick archway which extended from end to end beneath the mills. It has already been stated that this archway contained the great iron shaft which imparted motion to all the mills, and which derived its own from the large steam engine, which was located above, in the centre apartments separating the mills into two divisions.

In addition to the above precautions to prevent the explosion of a mill from extending to the others, above each set of rollers was balanced a vessel containing about thirty gallons of water. This was connected by means of a small iron shaft with a similar vessel to each mill of the division. Thus, on an explosion in one mill, its bed-plate was instantly drenched with water, and this caused the same to take place at the same moment with all the others.

These precautions were rendered the more necessary by the carelessness of the powder-makers, who might not remove the broke up powder cake from the mill enclosure before placing a new charge under the rollers, thus having one hundred and twenty pounds of material to take fire at the same time--as once happened--producing a powerful explosion. There occurred only three explosions at these mills--all before the steaming process was adopted--and in the first only was any one injured. In that one no material harm was done, as the two powder-makers--exposed by their own carelessness--were at work again in a few days. This explosion completely destroyed the slight roof, as well as the wood and glass front, but did scarcely any other damage to the mill, and had no action on the other mills further than drenching their beds with water. The other two explosions were insignificant.

These incorporating mills consisted, each, of an iron circular flat bed of seven feet diameter, fixed in a mass of masonry built up above the brick archway, through the center of the floor, to a convenient height. On this bed two massive iron rollers, six feet in diameter and fifteen inches face, revolved. Each weighed five tons. They had a common axle of wrought iron, of five inches diameter, and a vertical shaft of cast iron passing through the centre of the bed, having a rectangular cross-head through which the axle worked. This shaft connected below with the machinery which gave it motion from the main shaft.

These rollers were not equi-distant from the centre of revolution, by which arrangement every part of the charge of materials on the bed was subjected to their action--which was crushing, grinding, mixing and compressing; grinding and mixing from the twisting motion which followed from so large a diameter revolving in so small a circle, and crushing and compressing from the weight of the rollers.

To keep the powder on the bed, a wooden curb, funnel-shaped, two feet high was placed around the circumference, fitting closely, extending outwards at an angle of forty-five degrees. In the centre of the bed was a short cylinder of metal, two feet in diameter and six inches high, through the top of which the vertical shaft passed. This prevented the powder working inwards. It also acted as a steam-chamber to keep the bed-plate warm; but this was not used for the purpose, since the steaming process rendered it unnecessary. A scraper, or plow, followed each roller, which continually broke up the powder-cake, mixed its fragments, and kept them in the path of the rollers.

At the commencement of the operation the charge of sixty pounds of steamed materials was uniformly distributed over the bed; the rollers were then set into motion, revolving about ten times each minute, which continued for an hour; the broken up powder, or mill cake, which was about five-eighths of an inch thick, was then removed from the bed, having a blackish grey color and taken to the cooling magazines. These were excavated in the clay and rock on the other side of the canal, about one hundred yards distant; were four in number and separated from each other; here the mill cake became cold and hard, and was ready for the next operation, that of granulation.

The permanent building in which this was done was about fifteen hundred feet distant from the Powder Mills, on the same side, further up the canal; this, as well as each of the other permanent structures, was made of brick, having thin walls and light roofs. Wood in the damp atmosphere of the canal speedily decayed.

A natural growth of trees and brush-wood intervened between the buildings along the canal, which were generally situated about one thousand feet apart; thus the explosion of any one of them would be harmless to the remainder. There was a temporary structure of wood used at first for granulation, about one hundred yards distant from the permanent building, on the opposite side of the canal; this, after a use of some months, exploded with about three tons of gunpowder.

The explosion was heavy, shaking the earth for some distance, and throwing up a convolving column of flame and white smoke five hundred feet in height. It was composed of a series of confused masses of smoke and heated air revolving in vertical planes with extraordinary velocity, through which the flames flashed outwards in all directions; this was followed by the thundering sound of the explosion, which vibrated the air for a mile around, and was heard within the limits of the city.

There were seven men within the structure, a sentinel outside, and a boy with a mule in a shed adjoining. The bodies of the seven men and the boy, with the debris, were carried up with the ascending column, and by its revolving action, reduced mainly to small fragments and dispersed; the sentinel was killed by the shock, but his body was not otherwise disturbed. A growth of small pines surrounded the place, which effectually intercepted the lateral flying fragments; in fact the force of the explosion did not extend outside a diameter of one hundred feet, but within that area the trees were destroyed and the space where the structure stood was ploughed up and nothing remained. At the time there was no work being done, as the workmen were awaiting the arrival of the boat with the mill cake. The careful foreman, Gibson, had been called away, and probably the accident happened from matches falling on the floor, as it had been found impossible to prevent their use by the workmen, for smoking, when off duty. This was the only explosion at the Works during the war, except the three at the Mills, already mentioned. It demonstrated the safety of the arrangements, since there was no damage to any portion of the Works except the destruction of the glass sashes, and a slight movement of the roof of the permanent granulating building, about one hundred yards distant. This was about to be occupied, having been completed.