• September 25, 2017

Lost Wax Casting

Lost-Wax Casting

Lost-wax casting is an ancient process used for making jewelry. Bronze (an alloy of copper and tin) and gold objects have been found that can be dated to the fourth millennium B.C. The earliest recorded reference to lost wax casting is found in a Babylonian cuneiform tablet dated 1789 B.C., produced during the reign of King Hammurabi: the tablet is essentially a receipt for wax being issued to a metal worker, for making a bronze key to a temple.

Lost-wax casting involves making a model in wax, which in early times would have been hand-packed in clay. The clay and wax would then be placed in a fire or primitive oven, melting out the wax (hence “lost-wax”), and leaving an impression of the original object baked into the clay (resulting in a mold). The clay mold would then be ready for the introduction of molten metal, making an exact copy of the model.

Modern jewelry production still makes use of lost-wax casting, although technology has changed how the wax models are produced. Wax models of rings, earrings and pendants are no longer crudely packed by hand in moistened clay, but instead a high-grade liquid plaster of Paris is used, known as investment. The wax models are thus invested, and the entire process is now referred to as investment casting. Wax models are attached to wax sprues (essentially solid wax dowels), and the assembly is known as a tree sprue. The tree sprue is then attached to a round rubber base, which is then fitted with a steel sleeve (called a flask); the flask forms a watertight seal with the base. The models are now enclosed on the sides by the steel flask, and the next step is to mix up a quantity of investment. The mixture is poured into the flask, which in turn is vacuumed, ensuring that there are minimal air bubbles.

After approximately an hour, the rubber base is removed from the steel flask. The flask is now ready to be transferred to a high-temperature oven, known as a kiln. Flasks are initially heated to (and held at) slightly above the boiling point of water (212 degrees Fahrenheit or 100 degrees Centigrade), and the abundant moisture in the investment becomes transformed into steam, which in turn facilitates melting the wax models in the flasks. The flasks eventually become red hot, reaching a temperature of over 1,300 degrees Fahrenheit. The kiln remains at this temperature for a couple of hours, during which time any remnants of wax (which is a carbon compound) are completely oxidized into carbon dioxide gas, which dissipates from the flasks.

The burnout cycle is now completed, and the flasks are ready to be charged with molten gold, silver or brass (known as casting). The two principal forms of casting are centrifugal (or spin) casting, and vacuum casting. Centrifugal casting involves heating a measured quantity of metal in ceramic crucible, placing the flask into a holder adjacent to the crucible of molten metal, and then allowing the flask & crucible to spin at very high speeds. During this process, which occurs in the blink of an eye, the liquid metal exits through a hole in the crucible, and is literally injected into the flask, which of course has voids conforming exactly to the original wax models. The liquid metal fills those voids and rapidly hardens. The casting machine continues spinning for several minutes, as the spring mechanism slowly unwinds.

Vacuum casting achieves the same results, but involves placing the flask into a large steel sleeve that is connected to a vacuum pump. Vacuum casting requires the use of special perforated flasks, which in turn allow the vacuum pump to exert negative pressure on the investment ─ ensuring that the molten metal will enter into the tiniest crevices of the voids.

The flask must cool down for approximately 10-15 minutes, after which time it is plunged into a bucket of water. With plenty of steam and hissing, the hardened investment in the flask almost instantly “breaks down” into a white slurry and washes away, leaving the metal casting behind in the flask. The casting consists of a perfect impression of the tree sprue and any rings, earrings or pendants attached to the sprue. The raw (unfinished) castings are removed from the tree, and are now ready for finishing operations: castings are first “cleaned up” (filed and sanded), and are then transformed into beautiful pieces of jewelry, set with precious stones and given a brilliant final polish.

Twenty-first century jewelry manufacturers no longer have to carve wax models solely by hand, or even use CNC (computer numerical control) milling machines on pieces of wax. State-of-the-art CAD (computer assisted design) programs now send design instructions to 3D printers that have extremely high resolutions, as fine as 25 microns (= 25/1,000 of a millimeter). By comparison, the diameter of a human hair ranges from approximately 20 microns to 100 microns. 3D printers can produce “wax” models (actually made from high-tech liquid resins) with remarkably fine details. The end result of this technology is truly dazzling pieces of jewelry.