In pottery, a potter's wheel is a machine used in the shaping of round ceramic ware. The wheel may be used during the process of trimming the excess body from dried ware, for applying incised decoration or rings of colour. Use of the potter's wheel became widespread throughout the Old World but was unknown in the Pre-Columbian New World, where pottery was handmade by methods that included coiling and beating. A potter's wheel may be referred to as a "potter's lathe". However, that term is better used for another kind of machine, used for a different shaping process, similar to that used for shaping of metal and wooden articles; the techniques of jiggering and jolleying can be seen as extensions of the potter's wheel: in jiggering, a shaped tool is brought down onto the plastic clay body, placed on top of the rotating plaster mould. The jigger tool shapes the mould the other; the term is specific to the shaping of flat ware, such as plates, whilst a similar technique, refers to the production of hollow ware, such as cups.
Much early ceramic ware was hand-built using a simple coiling technique in which clay was rolled into long threads that were pinched and smoothed together to form the body of a vessel. In the coiling method of construction, all the energy required to form the main part of a piece is supplied indirectly by the hands of the potter. Early ceramics built by coiling were placed on mats or large leaves to allow them to be worked more conveniently; the evidence of this lies in leaf impressions left in the clay of the base of the pot. This arrangement allowed the potter to rotate the vessel during construction, rather than walk around it to add coils of clay; the earliest forms of the potter's wheel were developed as an extension to this procedure. Tournettes, in use around 4500 BC in the Near East, were turned by hand or by foot while coiling a pot. Only a small range of vessels were fashioned on the tournette, suggesting that it was used by a limited number of potters; the introduction of the slow wheel increased the efficiency of hand-powered pottery production.
In the mid to late 3rd millennium BC the fast wheel was developed, which operated on the flywheel principle. It utilised energy stored in the rotating mass of the heavy stone wheel; this wheel was wound up and charged with energy by kicking, or pushing it around with a stick, providing a centrifugal force. The fast wheel enabled a new process of pottery-making to develop, called throwing, in which a lump of clay was placed centrally on the wheel and squeezed and shaped as the wheel turned; the process tends to leave rings on the inside of the pot and can be used to create thinner-walled pieces and a wider variety of shapes, including stemmed vessels, so wheel-thrown pottery can be distinguished from handmade. Potters could now produce many more pots per a first step towards industrialization. Many modern scholars suggest that the first potter's wheel was first developed by the ancient Sumerians in Mesopotamia. A stone potter's wheel found at the Sumerian city of Ur in modern-day Iraq has been dated to about 3129 BC, but fragments of wheel-thrown pottery of an earlier date have been recovered in the same area.
However, southeastern Europe and China have been claimed as possible places of origin. Furthermore, the wheel was in popular use by potters starting around 3500 BC in major cities of the Indus Valley civilization in South Asia, namely Harappa and Mohenjo-daro. Others consider Egypt as "being the place of origin of the potter's wheel, it was here that the turntable shaft was lengthened about 3000 BC and a flywheel added. The flywheel was kicked and was moved by pulling the edge with the left hand while forming the clay with the right; this led to the counterclockwise motion for the potter's wheel, universal." Hence the exact origin of the wheel is not wholly clear yet. In the Iron Age, the potter's wheel in common use had a turning platform about one metre above the floor, connected by a long axle to a heavy flywheel at ground level; this arrangement allowed the potter to keep the turning wheel rotating by kicking the flywheel with the foot, leaving both hands free for manipulating the vessel under construction.
However, from an ergonomic standpoint, sweeping the foot from side to side against the spinning hub is rather awkward. At some point, an alternative solution was invented that involved a crankshaft with a lever that converted up-and-down motion into rotary motion; the use of the motor-driven wheel has become common in modern times with craft potters and educational institutions, although human-powered ones are still in use and are preferred by some studio potters. There are many techniques in use for throwing ceramic containers, although this is a typical procedure: A round, lumpy clump of clay body is thrown at a wheelhead or a bat attached to it; the term "bat" refers to a secondary disc or square, made of wood – or more plastic – to which the lump of clay is attached instead of the wheel head, thereby permitting the finished piece to be more lifted from the wheel. The clump is made and forced to the center of the wheel by applying pressure with the hands; the best way to muscle it towards the center is to hold one hand with the palm facing away from you and the other at an approximate 90 degrees to the first hand.
This makes it easier to push the clay away towards the center of the wheel. The thrower finds the center of the clay by moving a thumb across the lump until no more friction is felt; the thumb is pressed into the centre of
Diamond cutting is the practice of changing a diamond from a rough stone into a faceted gem. Cutting diamond requires specialized knowledge, tools and techniques because of its extreme difficulty; the first guild of diamond cutters and polishers was formed in 1375 in Nuremberg and led to the development of various types of "cut". This has two meanings in relation to diamonds; the first is the shape: square, so on. The second relates to the specific quality of cut within the shape, the quality and price will vary based on the cut quality. Since diamonds are one of the hardest materials, special diamond-coated surfaces are used to grind the diamond down; the first major development in diamond cutting came with the "Point Cut" during the half of the 14th century: the Point Cut follows the natural shape of an octahedral rough diamond crystal, eliminating some waste in the cutting process. Diamond cutting, as well as overall processing, is concentrated in a few cities around the world; the main diamond trading centers are Antwerp, Tel Aviv, Dubai from where roughs are sent to the main processing centers of India and China.
Diamonds are cut and polished in Surat and the Chinese cities of Guangzhou and Shenzhen. India in recent years has held between 19–31% of the world market in polished diamonds and China has held 17% of the world market share in a recent year. Another important diamond center is New York City; the diamond cutting process includes these steps. Diamond manufacturers analyze diamond rough from an economic perspective, with two objectives steering decisions made about how a faceted diamond will be cut; the first objective is that of maximum return on investment for the piece of diamond rough. The second is how the finished diamond can be sold. Scanning devices are used to get a 3-dimensional computer model of the rough stone. Inclusions are photographed and placed on the 3D model, used to find an optimal way to cut the stone; the process of maximizing the value of finished diamonds, from a rough diamond into a polished gemstone, is both an art and a science. The choice of cut is influenced by many factors.
Market factors include the exponential increase in value of diamonds as weight increases, referred to as weight retention, the popularity of certain shapes amongst consumers. Physical factors include the original shape of the rough stone, location of the inclusions and flaws to be eliminated; the weight retention analysis studies the diamond rough to find the best combination of finished stones as it relates to per carat value. For instance, a 2.20 carat octahedron may produce either two half carat diamonds whose combined value may be higher than that of a 0.80 carat diamond + 0.30 carat diamond that could be cut from the same rough diamond. The round brilliant cut and square brilliant cuts are preferred when the crystal is an octahedron, as two stones may be cut from one such crystal. Oddly shaped crystals, such as macles are more to be cut in a fancy cut—that is, a cut other than the round brilliant—which the particular crystal shape lends itself to. With modern techniques, the cutting and polishing of a diamond crystal always results in a dramatic loss of weight, about 50%.
Sometimes the cutters compromise and accept lesser proportions and symmetry in order to avoid inclusions or to preserve the weight. Since the per-carat price of a diamond shifts around key milestones, many one-carat diamonds are the result of compromising Cut quality for Carat weight. In colored diamonds, cutting can influence the color grade of the diamond, thereby raising its value. Certain cut shapes are used to intensify the color of the diamond; the radiant cut is an example of this type of cut. Natural green color diamonds most have a surface coloration caused by natural irradiation, which does not extend through the stone. For this reason green diamonds are cut with significant portions of the original rough diamond's surface left on the finished gem, it is these naturals. The other consideration of diamond planning is how a diamond will sell; this consideration is unique to the type of manufacturer. While a certain cutting plan may yield a better value, a different plan may yield diamonds that will sell sooner, providing an earlier return on the investment.
Cleaving is the separation of a piece of diamond rough into separate pieces, to be finished as separate gems. Sawing is the use of a diamond laser to cut the diamond rough into separate pieces. Bruting is the art of cutting a diamond round. In the modern era diamonds are rounded using either a laser. Industrial diamonds can be used for bruting a diamond round. Modern computer software measures the roundness of each diamond and "Ideal Cut" diamonds have to round within a 10th of a millimeter to qualify as an excellent cut diamond. Diamond polishing is the final polishing of the diamond. In a diamond factory one would find a diamond "Crossworker" who first places the main facets on a diamond; this is done to ensure maximum weight and best angles for the specific shape of diamond. After initial crossworking is complete, the diamond is finalized by smoothing the main facets by the crossworker, known as polishing the diamond. After the main facets have been polished by the crossworker, the final facets are polished onto the diamond by a "Brillianteer."
The facets added are the stars and bottom halves known as upper and lower girdle facets. The final stage