Thursday, 6 December 2012

Diamond Cutting

Diamonds
Since Venice was the gateway to the East, it is not surprising that the first reports of an active diamond-cutting industry originate in that city in about 1330. From there the art quickly spread to Paris, probably because France was very much the principal market for finished diamonds at that time. As early as 1360, an inventory of the jewels of Louis, duke of Anjou, refers to seven faceted diamonds in his collection, and records show that a famous cutter called Herman was working there in 1407. Cutting centers soon began to grow up in other parts of Europe, notably in Bruges in Flanders, in Nuremberg, in Antwerp, in Amsterdam and in Lisbon, all key cities linking trade routes with the market for diamonds. Over the years the importance of some centers grew while that of others declined and new ones emerged. 

The unquestioned Jewish domination of the diamond trade began very early. As an international trade of a very specialized nature requiring great skill, it was a natural choice for a people whose culture and experience bridged the gap between Europe and the East. But there was another reason. Diamond cutting was a new trade and not subject to the rules and regulations of the medieval craft guilds which governed so many activities and effectively excluded new entrants. The Jews therefore gravitated toward diamond cutting as it was one of the few trades from which they were not barred. Religious persecution in the sixteenth century led to a further concentration of the diamond trade in Amsterdam as refugees from the tyranny of the Inquisition in Spain and Portugal and from the Spanish raids on Antwerp fled to a country with a long reputation for religious tolerance. 

Diverse Shaped DiamondsThe first great name in European diamond cutting is Louis de Berquen. A native of Bruges, active in the fifteenth century, he is reputed to have been the first lapidary to use diamond powder in conjunction with a wheel in order to cut and polish diamonds with scientific precision. Some commentators believe that the story owes more to legend than fact and doubt that there ever was such a person as Louis de Berquen, but nevertheless a statue of him stands in the diamond district of Antwerp. In any case a Parisian jeweler, Robert de Berquen, writing in 1669, claimed to be a descendant and recounted a number of incidents in his ancestor’s life.

Tavernier reported that the Indians certainly had the facility for cutting diamonds, although their obsession with weight-saving seems to have made them reluctant to indulge in anything more than the most cursory smoothing out of rough edges and polishing pits. Incompetent cutting of the Great Mogul, which reduced its weight from nearly 800 carats to just 280 carats, very nearly cost the life of the traveling Venetian cutter Hortensio Borgio. As it was, the furious owner of the diamond, Aurangzeb, the son of Shah Jahan, fined him 100,000 rupees, his entire fortune.

Tavernier's description of the equipment used at the Indian mines points out the differences between it and that which he must have had in his Paris workshop:

There are several Diamond Cutters at this Mine but none of them have above one Mill, which is of Steel. They never cut but one Stone at a time upon each Mill, to find out the grain of the Stone; which being found, they pour on Oil (not sparing for Powder of Diamonds, though it be very dear) to make the Stone slide the faster; and they lay on more weight than we do; . . and the Mill was like ours, only the great Wheel was turned by four Negroes . . . their Wheel does not go as fast as ours, because the Wooden-wheel that turns the Steel-wheel is not above three foot in Diameter. 

The famous Italian goldsmith, Benevento Cellini described his method of cutting and polishing diamonds as follows: 

Different Diamond CuttingsOne diamond is rubbed against another until by mutual abrasion both take a form which the skilled polisher wishes to achieve. With the powder which falls from the diamond, the last operation for the completion of the cut is made. For this purpose, the stones are fixed into small lead or tin cups and, with a special clamping device, held against a steel wheel which is provided with oil and diamond dust. This wheel must have the thickness of a finger and the size of the palm of the hand; it must consist of the finest well-hardened steel and be fixed to a mill-stone so that through the rotation of the latter, it also comes into rapid movement. At the same time four to six diamonds can be attached to the wheel. A weight placed on the clamping device can increase the friction of the stone against the moving wheel. In this way, the polishing is completed.

Cellini wrote his description in 1568, but a diamond cutter of today would feel very much at home in the Italian's workroom. And indeed, if it was Louis de Berquen, a hundred years earlier, who constructed the first polishing wheel to be used with diamond dust and devised the first systematic arrangement of the facets, then he fully deserves his popular image as the father of the diamond-cutting industry. There is, of course, a greater measure of mechanization and a far higher degree of skill as might be expected after five hundred years of experimentation, but the art of the cutter still relies very much on the eye and the hand. 

Almost certainly the first regular cut was the table cut. It originated in India and involved taking an octahedron, flattening one point into a table and the opposite point into a smaller table, called the culet, and then bruiting or grinding the four ribs above the girdle to provide facets on the upper part, the crown, and the four below to form facets on the lower part, the pavilion. It was this cut, along with its many variations, which dominated the European scene until well into the seventeenth century. The stone's octahedral origins were clearly in evidence, with the angle of the crown and the pavilion to the girdle usually remaining unchanged at 55°.

Like the Indian cutters, the Europeans at this time were more concerned with maximization of weight and preservation of the original outline of the stone than with brilliance.

Another early cut that tended to follow the shape of the rough was the rose cut. Flat underneath, the upper and convex part was covered in facets: twenty-four for a Holland Rose; eighteen to twenty for a Half Holland; and six to eight for an Antwerp Rose. Again, there were many variations on the cut, largely depending on the shape of the rough. A Double Rose was a diamond faceted in a dome on both sides, the first use of which is attributed to Louis de Berquen, when he cut the Florentine for Charles the Bold. Rounded stones were suitable for a bead cut and pear-shaped ones would become briolette or pendeloques.

Depicted here are a variety of the diamond cuts used over the centuries. Because the rose cuts lack "fire," they are rarely used today. Only the emerald and baguette shapes are in current use, together with the three variations of the brilliant cut.

ENGRAVING THE DIAMOND

Because of the extreme hardness of the diamond, few European lapidaries took to engraving them as they did other precious stones. As a result, the best-known engraved diamonds in the world are Indian diamonds—the Akbar Shah and the Shah, both from the Peacock Throne; and the Mumtaz Mahal, or Taylor Heart, presented to Elizabeth Taylor by Richard Burton in the early 1970s. However, there are a number of examples of this obscure branch of the engraver's art in museums and private collections, all carried out by European lapidaries. The first recorded example is that of the arms of Charles V, engraved by Jacopo da Trezzo in Milan in 1556. His pupil Clement Birago later engraved on another diamond a portrait of the Spanish prince Don Carlos. 

According to the Privy Seal records, the sum of £267 was paid to one Francis Walwyn, on January 16, 1628, for "cutting, finishing and polishing a Diamond and engraving upon it the arms of Charles I with the initial letters of his Queen on each side". Francis Walwyn's expertise was obviously in great demand at the English court. The diamond signet ring used by Charles I when Prince of Wales, and engraved with the Prince of Wales's plume of feathers is also his work. 

Diamond CuttingFrance, too, has produced several engraved diamonds of great historical interest. One is a thin stone engraved with the head of Napoleon. It was on display at the Paris Exhibition of 1867, but its present whereabouts are un-known. Another is a ring that once belonged to Marie Antoinette, which has at its center an oblong diamond inscribed with "Marie."

As diamonds grew in numbers and in popularity, the cutters inevitably acquired greater knowledge and skill. They had begun to understand the ability of a diamond to "play" with light and were learning to take advantage of the fact. One of the earliest faceted cuts which marked a step toward the modern brilliant was the Mazarin cut named after Cardinal Mazarin of France in the mid-seventeenth century. It was a cushion-shaped cut with seventeen facets above the girdle and seventeen below, but it still lost a great deal of light through the bottom and the sides. 

The next step is generally attributed to a Venetian lapidary called Peruzzi who, in 1700, cut a total of fifty-eight facets on the crown and pavilion, the same number regarded as standard for a brilliant cut today. But if Peruzzi's cut retained much of the light lost by a Mazarin,it failed to demonstrate a diamond's refractive abilities at their best. It lacked "fire." Nevertheless the cut was a great advance, and with the popularity of diamonds boosted by the Brazilian finds, it became very much the standard for the next two centuries. A later variation called the triple cut or the old mine cut was the immediate precursor of what we now know as the modern brilliant. 

Modern Brilliant Cut Diamond
The credit for the invention of the modern brilliant cut undoubtedly goes to Marcel Tolkowsky who, in 1919 at the age of twenty-one, published Diamond Design. In this article he set down for the first time the precise angles and proportions of the brilliant-cut diamond which would give the best optical results for both reflection and refraction. As has already been noted in "What Is a Diamond?", the angles of the facets which give maximum reflection of light back out of a diamond are not those which provide maximum refraction. What Marcel Tolkowsky did was to calculate the best compromise angles to give the greatest refraction or "fire" with the least loss of reflectivity or "life".

The modern brilliant-cut diamond, which is the result of theoretical calculations of the precise angles needed to provide the best optical effect.

There are two obvious differences between a modern brilliant and earlier brilliant cuts. One is the depth of the stone: the modern brilliant is much shallower than the old mine cut, for example, and involves a major departure from the angles of the original octahedron. The other is the shape of the girdle, which is now completely round instead of simply having the corners of the octahedral shape removed.

There have been some slight modifications to Tolkowsky's angles and proportions since 1919, notably in the Eppler Fine cut of 1940 and the Scandinavian Standard (Scan DN) in 1970, but his work remains the basis for the modern brilliant cut. Variations from the ideal standard occur constantly for a number of reasons. They may concern the positioning of flaws or inclusions, or the stone's color and clarity may be such that obtaining maximum weight becomes the prime objective.

Fine Finishing  The brilliant cut or round cut diamond is deservedly popular as a result of its brilliance, its regular shape and standardization, and because most rough diamonds lend them-selves to the cut with an average weight recovery of the order of 50 percent. But it is not the only cut. Approximately 2 percent of rough diamonds, because of their shape or flaws, cannot be cut into a round brilliant without an unacceptable loss of weight. It is these stones that are cut into the shapes known as marquise, pendeloques (pear-shaped or tear drop), oval, emerald, triangle and square cuts. As a group these are all classed as fancy cuts. The pendeloque is far and away the most rare. These cuts are more expensive than the ordinary round brilliant cut, because their cost of manufacture is high; but higher weight recovery should more than compensate for this factor. The emerald cut is used for long rough diamonds and although weight recovery is above average at approximately 60 percent, the cut sells at up to 25 percent less than a round brilliant because its long parallel facets fail to show the same degree of brilliance.

For all practical purposes, the smallest stone used today is a 1/2-pointer, of which there are 200 to the carat (there being 100 points to the carat). On stones up to 2 points in weight, single cuts (with twenty facets) are usually made, although full cuts can be made as small as 3/4-pointers (150 to the carat). The cutting of these very small stones is the basis of a cottage industry in Belgium in the Ardennes, and in France in the Jura Mountains. The cutters come into the city to collect a stock of roughs and take them back for cutting on benches set up in their homes.

There are five processes* used in the creation of a polished gem from the rough:

1. Cleavingsplitting a stone along the cleavage plane.

*Because many of the words used in the diamond trade are taken from the Dutch, their adaptation to other languages often results in a variety of spellings; and, indeed, words used in one country may be totally unknown in another. For example, brillianteer is used in Britain, while in the USA and on the Continent brilliandeer is more common, and girdling may be preferred rather than bruting as the proper term for the rounding process. The word scaife is from the Dutch word for wheel, and is still used in traditional centers, but wheel is now commonly used.

2. Sawingdividing a crystal by using a diamond saw.

3. Bruiting -shaping a diamond by re-moving part of it by rubbing against another diamond.

4. Grinding or Blocking—making a flat surface by holding the crystal against a rotating wheel applied with diamond powder.

5. Brillianteering or Polishing—preparing the finished gem by a more refined application of grinding techniques.

Sawing
But before anything at all is done to the stone, it is studied carefully by the cutter. Using a magnifying glass (usually a 10x loupe), or even the new polariscope in potentially difficult cases, he will examine the diamond in order to determine the direction of the grain, and the location of any inclusions or imperfections. He can then decide exactly how to cut the stone to produce the maximum value in its finished state.

 The factors the cutter has to take into account are many and varied. For example, cleaving or sawing may not be necessary; very many diamonds are placed on the wheel exactly as they are. It is up to the cutter to decide whether he has to take away a large enough piece to warrant either cleaving or sawing. He may also be tempted to compromise on the ideal proportions by cutting the stone to gain weight. This he would do by reducing the height of the crown and adding to the width. of the table. Cutting in this fashion can be worth as much as 10 percent in weight, and is commonly done to a diamond that would weigh just under a carat if cut to ideal proportions. However, as in everything to do with diamond cutting, the cutter's judgment is a fine one. He cannot deviate too far from the ideal proportions in his effort to gain weight, because the value of the additional weight may be more than offset by the loss in value attributed to a bad "make." (The closeness of the finished brilliant to the ideal proportions is known as its "make.")

Having decided what shape the stone is to be, the cutter will mark it with India ink to show where the first division is to be made, either by cleaving or by sawing.

Cleaving: This is the method used for dividing large irregular stones without crystal faces. It is done with the grain. The diamond is placed in the little cup, called the dop, at the end of an eight-inch wooden stick, and set there with a cleaver cement made out of a mixture of shellac, rosin and brick dust. The cleaver then makes a small notch, or kerf, in the stone using another diamond to do so and sets the stick holding the diamond to be cleaved upright in tapered hole in the workbench. He then places a knife blade in the kerf, and strikes it sharply with either a wooden mallet or an iron rod. The stone should then split along the cleavage plane.

The art of the cleaver has always been regarded as the greatest talent in the diamond cutting industry. In the early part of this century, the cleavers of Amsterdam would arrive at work in the morning, often in their own carriage, wearing high silk hats. They were the aristocrats of the diamond world.

 Because of the existence of stress points in a diamond, there is always some risk involved in the cleaving process. The invention of the mechanical saw has meant that sawing, rather than cleaving, is now done almost exclusively. For large stones, however, there may be no alternative to cleaving; and on such occasions, the cleaver once again takes his place in the sun.

Sawing: Clean regular octahedral are normally divided by sawing. The diamond cutter is said to "saw grain"—that is, he cuts in a non cleaving direction. The diamond to be sawn is set in a holder at the end of a mechanical arm which allows the stone to rest by its own weight against the blade of the saw. The saw itself is a wafer-thin disc of phosphor bronze, the edge of which is coated with a paste of olive oil and diamond dust. The disc revolves at between 4,500 and 6,500 rpm, but it still takes forty minutes to saw through an average .25-carat octahedron and between two hours and a whole day to divide a stone of 1 carat.

Grinding or Blocking
Bruiting or Girdling: The next step is to shape the sawn or cleaved stone into a circular outline. This is done by setting the stone to be rounded in a holder on a revolving shaft and holding another diamond against it. The result of this very skilled operation should be a diamond with a perfectly rounded girdle set exactly parallel to the table. The diamond used as a tool is usually itself a rough and will be bruited in its turn.

Grinding or Blocking: The partly finished stone is now ready for faceting. The blocker mounts the diamond in a dop on the end of a mechanical arm called a tang and places it against the scaife, a flat, revolving cast-iron wheel dressed with olive oil and diamond dust. First, he grinds the table of the diamond and then a crown or bezel facet between the girdle and the table, followed by a second crown facet exactly opposite the first and another facet on either side between the first two. The stone is then turned over and four pavilion facets are ground below the girdle. These eight facets are followed by four more on the crown and four more on the pavilion. The next step is to grind the culet facet, at the base of the diamond where the pavilion facets meet. It should be exactly parallel to the table. The blocker then polishes the facets he has cut by swinging the tang across the polishing ring on the scaife.

Brillianteering
Brillianteering: The brillianteerer completes the whole operation by adding the remaining forty facets, twenty-four above the girdle and sixteen below it. First of all he cuts eight star facets where the crown facets meet the table, and then sixteen girdle facets where they meet the girdle. Finally he cuts two long facets called halves into each of the pavilion facets. All the facts are then polished.

The cutting process is a long and exacting one and the cutters stop and check the accuracy of their work continuously. An experienced cutter can develop exceptional skill; and working by eye alone, he can achieve the required angles to within a few minutes of a degree by aligning edges and reflections of edges.

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