Firing (ceramics)

“ The ideal firing is where there is no smoke but this cannot be reached until the kiln is hot enough to cause the smoke to burn.
Persons who have burned kilns for overglaze work will find the method of burning pottery very different. Instead of a fire brought as rapidly as possible to the finishing point, there must now be a slow soaking burn in which the heat shall have time to saturate the ware. ”

“ It consists of a single firing-chamber in which the unfired bricks are placed, and in the walls of which are contrived a number of fire-mouths where wood or coal is burned. In the older forms known as up-draught kilns, the products of combustion pass from the fire-mouth, through flues, into the bottom of the firing-chamber, and thence directly upwards and out at the top. The modern plan is to introduce the products of combustion near the top, or crown, of the kiln, and to draw them downwards through holes in the bottom which lead to flues connected with an independent chimney. ”

“ Twelve hours for the firing and twelve for cooling is a rough estimate for the firing of an ordinary kiln. It is important not to hurry the process of cooling, as a too rapidly cooling fire may crack the ware or affect the glaze injuriously.
The Greeks fired their pottery at a considerably lower temperature than potters do today. It seems to have been about 960° centigrade (corresponding to about cone 010) since any increase over this temperature causes a change in the color of the clay together with an additional contraction. ”

“ The blaze of the furnace and the spluttering, white-hot metal make it as light as day in the shed. The forger and his mates stagger under the weight of the ingot and porter-bar and incline their heads to escape the fierce heat. Their faces and necks are burnt red and purple — of the colour of blood-poisoning. Their shirt sleeves are hanging loose to protect their arms; they wear thin, round calico caps on their heads and leathern aprons about their waists. At the first blow or two the sparks shriek around, and especially if the ingot is of steel and happens to be well-heated. ”

“ Then, remembering the old vault-keeper’s remark, “It doesn’t seem to take the heat right. Gets all sort of crumbly when it’s been heated,” he dashed for the forge, seized a pair of tongs, and drew the piece of metal from the fire. It slipped from the tongs and fell upon the cement floor with a dull thud.
In an agony of fear lest the steel had been ruined he seized a hammer and cold chisel and, placing the edge of the chisel against the still white-hot surface, struck it sharply with the hammer. ”

“ The flash of the blazing metal half blinds him. He cannot stand the heat so well; it is all against turning out good work. Unless the bloom is kept exactly square under the stroke of the hammer it lops over on one side and obtains an ugly [226] shape, which it will be impossible to rectify; there is nothing more unsightly to the eye of the careful smith or hammerman than a shabby piece of forging. Very often, too, a portion of slag or sand from the bed of the furnace has adhered to the pile and, falling away, has left a hole in the metal. ”

“ Iron-filings, (for this Steel-dust is nothing more than pure iron reduced into small particles by filing or some other method,) when free from rust, and not mixed with any impurities, answer very well; but fire-work [36] makers generally prefer cast-iron reduced to powder, by beating thin plates of it on a cast-iron anvil with a heavy hammer, and sifting the broken particles through sieves of brass or iron wire, of different degrees of fineness, so as to separate the particles into grains of various sizes, according to the magnitude of the pieces. ”

“ The iron ore is very pure. Here were the broad-bladed spears of southern Uregga, and the equally broad knives of all sizes, from the small waist-knife, an inch and a half in length, to the heavy Roman swordlike cleaver. The bellows for the smelting-furnace are four in number, double-handled, and manned by four men, who, by a quick up-and-down motion, supply a powerful blast, the noise of which is heard nearly half a mile from the scent. The furnace consists of tamped clay, raised into a mound about four feet high. A hollow is then excavated in it, two feet in diameter and two feet deep. ”

“ This iron, or rather steel, is formed into various instruments by being repeatedly heated in a forge, the heat of which is urged by a pair of double bellows of a very simple construction, being made of two goats’ skins the tubes from which unite before they enter the forge, and supply a constant and very regular blast. The hammer, forceps, and anvil are all very simple, and the workmanship (particularly in the formation of knives and spears) is not destitute of merit. ”

“ As soon as the furnace-door on one side of this hammer was opened, a large lump of scrap-iron at a white heat was lifted and then conducted by a crane on to the anvil beneath. At the same moment from an opposite furnace a long iron bar, heated only at one extremity, was by a gentle blow of the hammer no sooner welded to the mass than the head smith, using it as a handle, turned and re-turned the lump on the anvil so as to enable the steam-hammer to weld its contents into proper form. Of course there has been selected for this extremely heavy work the strongest man that could be obtained. ”

“ When handling brass pieces having thin sections there is danger of melting the brass and causing it to flow away from under the flame, with the result that the work is ruined. If, in the judgment of the workman, this may happen with the particular job in hand, it is well to build up a mould of fire clay back of the thin parts or preferably back of the whole piece, so that the metal will have the necessary support. ”

“ The existence of a steel for the purpose of sharpening seems to imply not only the knowledge of the preparation of the metal and its subsequent hardening, but also of files or of other tools to produce the peculiar striated surface to which the sharpening property of a steel is due. Had such tools been known, it seems almost impossible that no trace of them should have come down to our times. Moreover, if used for sharpening bronze knives, a steel such as at present used would sooner become clogged and unfit for use than if employed for sharpening steel knives. ”

“ When savages learned to make bronze, their former weapons and tools of stone and bone were thrown away. The flint knife, which lost its brittle edge at the first cut into wood, was replaced by tough metal which could be sharpened anew every day, and would last for years. ”

“ As the heat gradually rises, the metal separates. If the bar were now placed on the anvil and struck a hard blow with a hammer, it would fly to pieces. Therefore, judgment must be used in striking the first blow on any welding heat—it should be light. The succeeding blows should be made gradually harder. A hard blow at the start might make the metal fly to pieces, or make the upper piece slip away from the under piece. ”

“ Injuries.—One of the most common injuries to steel comes from carelessness in the heating for forging. It is one of the important operations, for unless the metal is uniformly heated, violent strains are liable to occur, and, when hardened, the steel will show these strains by cracking. These defects are known as fire cracks.
The smith should always have plenty of fuel surrounding the metal while it is in the fire so that the cold-air blast will not come in direct contact with the metal. The air should be heated by passing through a bed of hot coals before it strikes the steel. ”

“ A hollow made in the ground is the furnace in which they place the nozzle of their portable bellows, and they hammer the iron on a small anvil fixed in the earth.
Aided by these merely rudimentary means they execute pieces of metal-work, the dimensions of which are really surprising. They make nails and tacks, and even worm screws, repair locks, clean clocks, make [Pg 213] knives, mend skimmers, and restore umbrella-frames. They make bronze rings out of republican décimes, and sell these popular trinkets to the village beauties. ”

“ A blast furnace differs from a forge not in principle so much as in audacity, method and degree. The forge pricks nature and extracts iron one molten drop at a time. The blast furnace cuts a gash in her side and extracts iron in a blazing stream. ”

“ In the day, especially toward evening when the outer light was failing, when the forge fires burned brightly, and the white hot metal on the dies glowed at each stroke of the massive hammers, when the whang-whang-whang of steel on steel raised a mighty clamor, then it was a place to conjure about. ”

“ The opening for the fused iron was within the smelting-house, and I could run the ore on to the sandy floor in channels made for that purpose, and thus procure my pig iron or Bessemer steel as the case might be. In this blast-furnace I left several channels to [155] be connected in some way with a blast of cold air, for without this blast I could not of course expect to smelt the ore. To improve the draught, and to have Nature help me all possible, I built the chimney or cone of the blast furnace at least twenty feet high, of bricks, tapering the same in a cone form from the base to the apex. ”

“ Sometimes, however, it is almost a matter of life or death to be able to produce fire. The back of a pocket knife, or an old file with a fragment of flint, quartz, or pyrites struck smartly together over the remains of a burnt piece of calico, will in deft hands produce a spark which can be fanned to a glow, and so ignite other material, till a fire is produced. ”

“ To finish wrought iron, all of the scale and dirt should be scraped off with an old file while the piece is hot. When the iron is cooled, linseed or machine oil is rubbed on. If the work is held over the smoke of the fire and then oiled, it will take on a darker color. Never paint iron work. This destroys the texture of the metal. Do not file work bright. It should be dark—filing is not forging. ”

“ White cast-iron; this is brittle, and indicates always some derangement in the [689] working of the furnace; it flows imperfectly, and darts out in casting, abundance of brilliant white scintillations; it fixes very quickly; and on cooling, exhibits on its surface irregular asperities, which make it extremely rough. It is easily broken, and presents a lamellar and radiated fracture; and is so hard that tempered steel cannot act upon it. It is cast only into weights, bullets or bombs, but never into pieces of machinery. When exposed to the refinery processes, it affords a bad bar iron. ”

“ Two years ago No. 1 broke out and the molten metal caught and burned to death one of our men and injured several others. ‘Hanging’ does not occur when the furnace is working right, but failure of the charge to come down evenly is very serious sometimes. A blast furnace is like a coquette ”

“ When the furnace was charged, the doors were closed until the metal was sufficiently fused, when the workman opened an aperture and worked or stirred about the metal with iron bars, when an ebullition took place, during the continuance of which a bluish flame was emitted, the carbon of the cast-iron was burned off, the metal separated from the slag, and the iron, becoming reduced to nature, was then collected into lumps or loops of sizes suited to their intended uses, when they were drawn out of the doors of the furnace. ”

“ Where the objects had sockets cast over clay cores, those cores had to be removed, probably by means of pointed tools, such as that already described under Fig. 220. Where they were solid they seem in most cases to have undergone a considerable amount of hammering, which both rendered the metal more compact, and to a certain extent removed the asperities resulting from the joints in the mould. With edged tools and weapons, whether socketed or not, the edges especially were drawn down by means of the hammer. ”

“ It is filled up to a considerable height with clay to protect it from the hot fire inside, and the pressure of air is applied through those pipes, one or both of them, as may be necessary. When the fire is extinguished or removed, the debris or slag that comes from the metal, and the ashes and cinders from the material used for fuel, can be taken out through the trap at the bottom. ”

“ Jewelers' tools, especially for silversmith's work, probably have to stand the greatest punishment of any all-steel tools and to make a spoon die so hard that it will not sink under a blow from an 1,800-lb. hammer with a 4-ft. drop, and still not crack, demands careful treatment. ”

“ Then you can remove the iron crook from the mouth of the plug and let all the bronze go till the furnace be emptied. To this end it is necessary to have a man standing at each of the furnace mouths, [122] who, with the scraping iron [244] which is used in the craft, drives the bronze towards the outlet, until all is cleared out. Such of the flowing metal as is still left after your mould is filled you dam by means of throwing on to it with a shovel some of the earth you erstwhile dug out of the trench. That is how you complete your mould. ”

“ In an incredibly short time the piece of steel assumes a shape somewhat resembling a knife-blade, and being now too cold to yield longer to the hammer, it is again thrust into the fire. One by one the remaining pieces are similarly treated, and that done, they are all drawn out again, to be jammed by a blow from the striker into a mould which completely shapes the whole blade. ”