Cabochon Cutting - A Collection of Historical Articles on the Methods and Equipment Used for Working Gemstones

By Read Books Ltd.

This book contains classic material dating back to the 1900s and before. The content has been carefully selected for its interest and relevance to a modern audience. Carefully selecting the best articles from our collection we have compiled a series of historical and informative publications on the subjects of gemology and crystallography. The titles in this range include "Gemstone Manufacturing" "The Optical Properties of Gemstones and Crystals" "The Thirty-Two Classes of Crystal Symmetry" and many more. Each publication has been professionally curated and includes all details on the original source material. This particular instalment, "Cabochon Cutting" contains information on the methods and equipment of the gem cutter. Intended to illustrate the main aspects of cabochon cutting it is a comprehensive guide for anyone wishing to obtain a general knowledge of the subject and to understand the field in its historical context. We are republishing these classic works in affordable, high quality, modern editions, using the original text and artwork.

• Language: English
• Publisher: Read Books Ltd.
• Release date: May 6, 2015
• ISBN: 9781473394537

Book preview

CABOCHON CUTTING -- PART I

CHAPTER ONE

Types of Equipment

The type of equipment most suitable for your purpose is largely a matter of determining how far you wish to carry the art of gem cutting. It is possible to finish gem stones with virtually no equipment whatever, working by hand, as is done in the Orient. Hand labor is tedious and unsatisfactory and not suited to the Occidental way of working. Foot power operated equipment was widely used at one time, and in the days of medieval gem cutting it was customary to hold the work in the left hand and rotate a lever to operate the lap with the other hand.

Obviously, inferior equipment or machinery that requires constant changing of wheels, buffs and other accessories will not add to the pleasure of gem cutting. Whatever equipment you may elect to purchase or construct, it is well to keep in mind the ease with which same may be operated. It is possible to conduct all operations on a single spindle, disregarding all speed recommendations, and still do satisfactory work or passable work. At a small additional cost it is generally possible to purchase or construct proper equipment and it is here advised that this be done. Investment in lapidary equipment is not necessarily a luxury; a well-equipped shop can turn out work having a distinct commercial value. Many home lapidarists manage to produce a sufficient surplus of finished gems and specimens to more than pay for the original investment and subsequent operating costs.

If you construct your own equipment a number of cast-off parts of other machinery can often be utilized. This will of course depend upon your mechanical ability to so arrange these parts into workable form. Certain parts and accessories must of course be purchased, but the cost of assembling can be eliminated. Purchased equipment is quite likely to give the greatest satisfaction; it has been tried and tested by the manufacturer, and will enable you to immediately start operations.

CABOCHON GEM CUTTING

Semi-precious gem minerals, like turquoise, agate, jasper, opal and numerous others, are usually cut with flat or rounded smooth surfaces, termed cabochon styles. In shape these may be rectangular, oval, square, or similar outlines. In short, there are only two principal worked surfaces, while in facet cut styles there may be 57 or more individual flat surfaces on the gem. All of these must first be cut, then polished.

Four Operations

Cabochon cutting may be divided into four separate operations, and most lapidary shops have a special tool for each operation—(1) Sawing (2) Grinding (3) Sanding (4) Polishing. The rough gem material as found or mined, often requires reduction to proper shape, hence sawing is usually the first operation. The grinding wheels reduce the stone to proper shape, while the sanding operation removes the deep scratches left by the grinding wheels. The final polishing operation gives the surface of the cabochon a high glossy finish.

Different speeds are required for the various operations in order to get the best results. It is possible to finish stones with little regard for speeds of the moving tool, but difficulties will be encountered, hence it is advisable to give due regard to the recommended speeds. Silicon carbide wheels will give best efficiency when run at indicated speeds, otherwise the work will proceed slowly or the wheel will tend to develop a glaze.

Likewise if a diamond or mud saw is operated at higher or lower rates than the indicated speeds trouble will develop. High speed operation of the felt polishing buff will cause undue heating of the gem and the abrasive grit will be thrown from the polishing buff.

CHAPTER TWO

Sawing

Sawing into proper shape and size is generally the first procedure in lapidary technic. Where the rough gem material is presented in small fragments or water worn pebbles, the work can then start directly on the silicon carbide grinding wheels. It is also possible to break a large mass and thus obtain suitable size fragments for cabochon cutting, but this method will prove wasteful. If the material is worth cutting, it is also worth conserving.

There are several types of saws which may be utilized to section hard gem materials, these include the diamond charged metal disk generally refered to as the diamond saw; the mud saw; metal bonded saws charged with diamond grit; and various types of silicon carbide cut-off saws.

The hand charged metal disk diamond saw is an efficient tool and is the most widely used in the lapidary industry. The hand charged metal diamond saw has some obvious disadvantages, but with recent improvements in manufacture, this tool is the best available. When used properly and when properly charged the metal disk diamond saw will give efficient and long service.

Diamond charged metal bonded saws are operated at speeds considerably higher than those of the metal diamond saw. The metal bonded saw is heavily charged with diamond grit and is costly, but where large quantities of small size material is to be sectioned, this tool finds its usefulness. The small size saws are primarily intended for free hand sawing, while the metal diamond hand charged disk is generally operated with a clamp and saw carriage holding the work.

Mud Sawing

The tool used as a mud saw is a disk of Armco or auto fender iron. The disk may be of any size, and the gauge of the metal will be dependent upon the diameter of the blade. A 20 guage thickness will serve for a large blade, while 22 gauge will suffice for blades of 8 to 12 inches in diameter. Some workers prefer the bronze or copper blades, but these appear to have no special advantages other than higher cost. It is suggested that saw disks be purchased from supply firms, so the disk will be true at the periphery as well as at the arbor hole. Disks sold by supply firms are generally first made true on the lathe. In ordering blank disks or charged blades be sure and specify arbor hole size.

A 10 or 12 inch diameter mud saw should be operated at around 300 r.p.m. Smaller blades faster in proportion and larger blades slower, so as to obtain the correct peripheral speed. The mud saw is not charged with abrasive in the manner of the diamond saw. The former tool depends upon the abrasive grit which adheres to the blade for its cutting ability, as the blade passes through a thick mixture of silicon carbide and water. Silicon carbide grit number 120 is generally indicated. The abrasive mixture is kept in a metal container below the saw, with the edge of the blade passing through the mixture. Arrangements can also be made to feed the mixture to the work from above. Silicon carbide grit has a high specific gravity and thus tends to settle out of the water. To give the mud mixture greater viscosity, fine clay or flour may be added to the mix, to suspend the grit and enable the mixture to adhere to the saw blade. Fresh grit should be added from time to time.

Small amounts of the new hard abrasive Norbide (grit No. 120) added to the silicon carbide mixture will increase the speed of cutting. Most mud saw operators are now adding Norbide to the silicon carbide grits. The saw blade should be shielded to prevent splashing.

In using the mud saw the work should not be held by hand (except small fragments), but should be held in some type of mechanical arm or carriage, as shown in the various illustrations. There are several types of mechanical holders, including the swinging arms and the saw carriages. The newly developed sawing carriages are superior to the swinging arms and similar methods. See Fig. 1 and Fig. 2. For one thing the saw carriage holds the work more rigidly and does not permit side movement. Often in diamond sawing the flange on one side of the saw may be quickly lost, due work proceeds without further special attention. As the saw cuts deeper slightly additional pressure can be applied. A system of weights and pulleys or spring tension can be used. While it is customary to run both the mud and diamond saw in the vertical position, we know of at least one prominent amateur who has done an enormous amount of work during the past ten years on the mud saw operated in the horizontal position. Mr. William Pitts of Sunnyvale, California, uses this method and finds it effective. About ten pounds tension pressure is applied when sawing a four-to the work not lining up true with the arbor and blade. The saw will then tend to travel off at an angle, or, in the case of the mud saw, binding will result when sawing thick specimens. Moreover, the sawing carriages enable the operator to clamp a much larger mass into proper position.

Fig. 1. Arrangement of diamond saw compartment. Sawmill type of carriage. Stone is clamped in wooden carriage jaws, with an equivalent thickness of wooden blocks on opposite side to keep clamping face level. A small wing nut at bottom enables movement of carriage sideways for additional cuts of any desired thickness. (Photo Vreeland Lapidary Manufacturing Company, Portland, Oregon.)

After the work has been clamped into proper position, and the saw has worn a groove in the work, tension is then applied to the arm or carriage and the inch specimen with a twelve-inch diameter disk.

A little experimenting will enable you to adjust the proper tension of the work against the blade, as well as to get the correct speed. High speeds tend to throw the abrasive from the blade and, after all, the cutting is directly dependent on the ability of the disk to carry the mixture to the point of cutting. Low speeds also present some disadvantages. Mud sawing is very much slower compared to the diamond saws, and as a rule once the operator learns the use of the diamond saw he will abandon the mud saw, except for sectioning very large specimens.

Diamond Sawing

The hand-charged metal (disk) diamond saw is the prime favorite with most lapidarists and is to be found in practically every lapidary establishment, both commercial and private. This tool is operated at the same speeds as the mud saw, but is a far more delicate cutting blade, giving long and excellent service if properly charged with diamond grit and operated correctly.

The hand-charged metal diamond saw must be operated with proper lubrication, by passing the saw through a mixture of equal parts kerosene and automobile lubricating oil. The lubricant is held in a shallow metal reservoir below the blade. Only the edge of the saw need pass through the oil. Like the mud saw, this tool should be shielded with sheet metal and a cloth cover to prevent splashing. Generally a twelve-inch disk is operated at about 350 to 400 r.p.m.

Armco or auto steel fender iron will make an excellent disk, but some workers prefer bronze, copper or other metal. Disks of this kind can be purchased ready cut at a nominal cost, and if properly operated they can be recharged an indefinite number of times. If the disk meets with an accident and is badly bent, discarding is indicated. A bent saw blade will tend to travel sideways, thus increasing friction and tending to tear out the diamond grit embedded in the periphery. If the disk does not have a proper flange or set on each side of the cutting edge, tendency to travel sideways will be noted.

The inexperienced lapidarist will no doubt have some difficulty in properly charging and using a diamond saw at the outset, but a little experience and observation will soon correct improper use. Many home lapidarists find it advantageous to purchase diamond saws ready charged from the manufacturer. The amount saved by charging disks is not great if time be considered. Moreover, the manufacturer purchasing diamond bort in large quantities naturally gets wholesale rates.

The following outline is the technic used by many lapidarists in charging the metal diamond saw: (1) Mount the disk on arbor and make certain the periphery runs true. A sharp fragment of agate held on a rigid rest will serve to trim the edge of the disk to run true. (2) If disk tends to wobble when running, a heavy hardwood flat stick can be used to slightly bend and massage the blade to largely eliminate the side wobble. Both these operations are carried out while the saw is in motion.

(3) The periphery of the disk is then notched at intervals of about one-sixteenth inch, making the notches about one-sixteenth of an inch deep. A suitable tool for this purpose is an old hacksaw blade ground to a dull chisel shape edge. The edge should be left slightly dull or about .003 inch thick. Use a light hammer to drive the blade into edge of saw. (4) Diamond bort is then mixed with a bulk of vaseline about the size of a pea. Two carats of crushed bort should be used to charge a twelve-inch blade. Other disks in proportion. A smaller amount of diamond can be used, but since the efficiency of the blade is wholly dependent upon the amount of abrasive worked and held in the periphery, it is obvious that skimping on diamond is not a saving. Grit can be purchased ready crushed from dealers. Number 50 to 60 grit is generally used.

(5) The diamond paste is then applied to the notches and pressed in between the notches with the fingers. The notches are closed with light blows from a hammer. The skill with which the worker can close the notches on the saw and thus grip the diamond grit firmly, will largely determine the efficiency of this tool. (6) The saw is then placed in motion and a special grooved steel roller is run on the edge of the saw to further close the notches. The steel roller should be held steady on a rigid rest. This operation also gives the saw its proper set. Without this the cutting blade would tend to bind and not cut