Metal Casting Technologies : MCT-DEC-2014
34 www.metals.rala.com.au METAL casting Technologies December 2014 35 production of bronze tools he first molds were open-molds made of sand, soon to be followed by stone, then clay. From such molds came the first cast tools–the “flat celts” or axes such as found in Egypt and elsewhere; they were fashioned on the lines of the flint tools. Antedating these Egyptian celts, the founders of Sumer had made an important step forward in the “foundry art” by producing an axe that had a hollow end into which could be forced a wooden handle. This meant that a “cored mold” had been used, and the appearance of the casting indicates a closed mold. While tools for useful work were being constructed, so also were bronze duplicates of the flint weapons of war against both man and beast. The first spear was a solid leaf-shaped triangular pointed affair about 5 inches long affixed to a wooden handle. This same spear could easily be modified to make a useful knife with a short handle. The growth of the knife led to the dagger and then to the leaf-shaped sword. We of today can hardly appreciate the superiority of bronze over flint. One hefty blow, and the flint axe might easily shatter, and the labor of the long winter nights was gone in a moment. The bronze axe, however, did not shatter, and even when its edge dulled it could be re-hammered back to its original sharpness without the use of heat. The razor of today is rightly looked upon as one of the triumphs of metallurgical science. One can imagine with how much relief the bronze razor was received. Bronze razors have been found in early Sumerian tombs, as well as Egyptian razors with four beveled cutting edges. Also, In Denmark have been found razors dating possibly about 1000BC, of a shape very much like the hollow ground steel razors of our own great-grandparents. Tweezers, formed essentially of a bronze ribbon bent double, and rather wider at the ends than at the middle, have been found in association with relics of the early Egyptian dynasties. We also find similar tweezers in Crete and later still in Europe and even in Britain, although there, they are of rare occurrence. A slightly different-shaped tweezer is found in India and Mesopotamia, dating back to 3000BC. The tweezer draws attention to another feature of bronze–namely, the knowledge of the spring effect, for only resilient bronze would be of any use for this purpose. The sickle is an interesting example of the perpetuation of the early Neolithic tool. It occurred to some man, as he handled the jawbone of an animal, that here was a shape which, given a cutting edge, would be most useful in mowing the recently developed crops. (Whether Samson, in using the jawbone of an ass to slay the Philistines, had a tool of such nature or whether he merely used it as a club, we are not told; in any case, it was effective.) To get the cutting edge sharp, flints were inserted in the cavities where the teeth had been, and-it was an advance on tearing off the stalks by hand. It may have been that the first improvement of the Bronze Age was to replace the flint inserts with hammered bronze pieces. When the bronze sickle appeared, it had the shape of a jawbone. Such sickles were also cast in open stone molds. The sickle on a longer and differently shaped handle became the scythe, and again bronze had helped the farmer of that day. As to defensive weapons such as the shield or armor, we find that the Stone Age provided nothing of this nature, doubtless owing to the difficulty of forming stone, its weight, and its poor shock resistance. There is one exception to this, however, namely, the small rectangular plates of slate perforated with holes at the corners, supposed to have been bound on one’s arm to protect it from the recoil of the bow-string. The modern archer uses a leather sleeve. Bows of course need arrows with their arrowheads. The Assyrians were a nation of archers and their arrowheads were of flint, as were those of the Persians and the Parthians. It was said of the Parthian bow that it was the most efficient in shooting the arrow, hence the classic phrase “Parthian shaft.” Bronze arrowheads were made but as this is generally meant the loss of valuable metal, the more readily replaceable flint heads continued in use. Returning to defensive weapons, we find that with the advent of bronze came the metal shield, circular or oblong, the result of hammering our bronze sheet to the required form. Later came bronze plate armor, greaves for leg protection, and helmets. Bronze armor persisted even after the advent of iron as it did not rust and was easier to keep in order. Very striking examples of the superior durability of bronze over iron or steel are those bronze scabbards that have been found, containing only remnants of the iron or steel swords that had been left therein. Such is the resistance of bronze to the ravages of corrosive attack. As the metalworking art progressed, the founder noticed that when his molten metal was especially pure he could see his face in it. He had had a similar experience when looking into a still pool or lake. This may have caused him to search for a means of producing a solid metal surface that would also give an accurate reflection. Thus there followed on the heels of improved weapons and tools the bronze mirror. Frequently there were concave and must have given a very distorted image; then came the plane surface of the best polish then obtainable. As long as the tin content of the bronze was in the malleable area, the polished surface enabled the persons to see themselves, it is true, but in a rather dim manner. The manufacture of an improved mirror brought up the tin content to over 30% with the attendant brittleness, so that even with the bronze mirror it was possible to invoke “seven years’ bad luck.” As time progressed, the available amount of bronze increased, so that it could be used for household utensils either made from cast metal or from hammered and riveted sheet. The need for larger quantities of molten metal at one time called for enlarged and improved melting units. Anything to give us first-hand information as in the size and shape of such furnaces seems hard to come by. The probability is that the development occurred along the lines of larger blast furnaces of refractory material with air pressure supplied by water wheels. Air furnaces of the natural draft type, such as reverberators, may also have been used where long flame fuel such as wood was used in preference to the comparatively flameless charcoal. Whatever the means used, the bronze castings of Assyria and Egypt bear evidence of the use of such melting units. All this time, man was using his brain to an ever increasing extent. He was getting away from the time when all his energy was required for the necessities to a period when leisure became feasible. The beauty of nature, sunrise, and sunset, moonlight on a lake, sunshine scintillating through the autumn foliage–these and many other manifestations had awakened within him the desire for beauty not only outside himself but beauty produced by himself, so that we have the development of art and its application to everyday things, as well as others produced essentially for art’s sake. Making bronze statues Now man wants to perpetuate himself for other generations to admire and so we have statues in marble and bronze. This naturally required considerable refinement of technique, especially by the molder who had become thoroughly familiar with the making and use of sand and clay cores, either with a pattern which left its own core as it was withdrawn from the sand, or with dried cores that were inserted in the mold whenever hollow portions fro the casting were indicated. It became apparent, then, that there must be no undercut portion. The artist needed a method of molding that had more latitude. He desired to reproduce a man or a woman, for example, clothed in the flowing garments of the time. Such T Back to the cAsT BRONzE – its origin, use and influence (Part 4) Prof. John H. D. Bautista, PEE, RMetE, MBA; Technical Consultant, Phil. Metalcasting Assn., Inc. TWEEzERs, fORMED EssENTiALLY Of A BRONzE RiBBON BENT DOUBLE, AND RATHER WiDER AT THE ENDs THAN AT THE MiDDLE, HAVE BEEN fOUND iN AssOciATiON WiTH RELics Of THE EARLY EGYPTiAN DYNAsTiEs. As THE METALWORKiNG ART PROGREssED, THE fOUNDER NOTicED THAT WHEN His MOLTEN METAL WAs EsPEciALLY PURE HE cOULD sEE His fAcE iN iT.