How Scientific Watch Jewels Are made

By William H. Samelius

From The Watch Word, November 1935

Jewel bearings for watches are mostly made of six materials: garnet, natural sapphire, natural ruby, scientific sapphire and scientific ruby and sometimes diamond. Garnet is not the equal of the latter five stones because it is considerably softer and more likely to wear and become chipped. Between the natural ruby and sapphire and the scientific jewels there is very little difference and it is with these four materials, especially the scientific stones, that the finest quality watch jewels are manufactured.

Natural sapphires are found in large quantities in the United States, Australia and Ceylon, while the ruby is mined in Burma. There are nearly 4,000 men and women employed in the art of jewel making. These stones found with many variations of hardness and uniformity are suitable for watch jewels. However, in recent years, the scientifically manufactured stones have become so perfect that they are now generally considered the best materials for watch jewels. Chemically they are composed of exactly the same elements as the natural stone, pure aluminum oxide (ALO.). This compound is mixed with a small quantity of chromium oxide to give the ruby color.

Pure ammonia alum is calcined in a muffle furnace at a temperature of over 18000 F., and the water of crystalization, sulphuric acid and ammonia, are drawn off, finally leaving powdered aluminum oxide in its pure state. This powder is fused in a small circular furnace by means of a hydrogen-oxygen flame with a temperature of over 38000 F. The aluminum powder is automatically sprinkled from a container into the center of the flame where it fuses almost instantly; thus, drop by drop, a pearshaped boule, or bullet of stone is built up. When the process is completed, the boule must be left in the furnace about an hour to cool very slowly in order that the crystal may be free from any internal stress. From three to five hours are required to make one boule, which weighs from 125 to 400 carats.

Each boule is really a single crystal because the raw material is so perfectly free from impurities and the process of building up is so gradual under such an intense heat. The purity and consistency are perfect except for the extreme top and bottom of the boule, both of which are cut off and thrown out in sawing-up blanks for watch jewels.

William H. Samelius
dean, Elgin Watchmakers' College

The average size of the boule is about one-half inch in diameter by one and one-half inches long.

Tiny airholes, slight impurities and cracks sometimes found in the natural jewels, do not occur in the scientific jewels. They have the same coefficent of hardness and friction, and in fact, average harder than the natural stone.

They will also take a high polish equally as well. There is very little grain in the scientific stone, and what grain there is has no effect on the finished jewel because the boule is always sawed into slabs crosswise to the vertical axis.

To saw the boule, it is first split in halves along the axis. These halves are then cemented to wooden blocks and sawed into suitably sized strips by copper, tin or bronze discs charged with diamond powder and cooled and lubricated with soda-water. These saws revolve about 1,000 r.p.m. Usually one saw is used at a time, since care must be taken not to overheat the material. The burr left on each lap, as the saw breaks through, is then removed by passing them over a copper lap charged with coarse diamond powder. Again these slabs are cemented on wooden blocks and sawed into strips.

These strips are then broken up into small squares with a knife, much the same as a candy-maker breaks up a string of taffy. These squares are then roughly graded according to size and cemented to a steel block to be ground on both sides to a thickness within a few thousandths of the finished jewel. Then the blanks are cemented together in lots of about 300 and ground into round discs in a centerless grinder, first roughly with a carborundum lap and then more closely with a copper lap charged with diamond powder, this first grinding comes within about .005 of an inch of the finished jewel size. Now the blank is ready for two of the most important operations in jewel manufacturing, mainly the drilling and the opening of the hole. The blank is held in a chuck, this chuck being stationary. Opposing it is a spindle and a chuck holding the drill which runs at 30,000 r.p.m., or 500 r.p. second. The drill is a small steel wire charged with diamond dust and it takes about 30 seconds to pierce this disc, one operator handles a machine with a battery of about twelve to fifteen spindles. After the holes are pierced, these small discs are then strung on a fine stretched wire and the discs caused to pass between two revolving laps facing each other, one being made of copper charged with diamond dust which does the cutting and the opposing lap is of rubber which drives the disc against the diamond-charged copper lap, giving them a revolving motion and by this means they are ground concentric to their required size. After these discs are drilled and ground to their proper diameter, they are cemented to a revoling plate and ground to the proper thickness by means of a diamond-charged copper lap lubricated with soda water. About .0002 inch is left on each side for polishing.

They are once more strung on a wire and cemented into a hollow holder which revolves at a high speed. The wire is withdrawn and a soft steel wire charged with diamond dust is then passed through the entire line of jewels and the holes are opened up to size and polished with a second wire charged with polishing powder. They are then cemented to a small chuck and fed across a copper lap at a proper angle in order to secure the beveled edge. This is a quick operation, one operator can handle about 10,000 jewels per day. They are then cemented to an adapter with the flat side down, revolving at an extremely high speed and a composition grinding-stick charged with diamond dust, also revolving at a high speed in the opposite direction is brought into place, grinding out the oil well or cup. This composition stick leaves a finished or polished surface.

After this operation they are once more put in a chuck and the under-side of the hole is then chamfered with a pencil-pointed copper lap. This is to prevent the holes chipping around the edge.

They are now pressed into round linoleum discs about four inches in diameter, several hundred at one time, and are faced on the upper and lower side under heavy pressure by means of a pearwood lap which is charged with a fine grade of powder and oil. This operation produces a flat gloss characteristic of a highgrade jewel. For the convex train jewels, the convex part is ground in a swing rest tool, two jewels are ground at once. There are two spindles, each spindle revolving at a high rate of speed located above and below the center of the grinding and polishing lap, which also revolves at a high speed, and by means of a swing rest, this lap is caused to move in a circular motion, thereby forming the convex side of the jewel. They are then cemented once more on a steel plate and the final finish of the convex surface is obtained by means of a revolving bristle brush charged with diamond powder, the brush and plates holding the jewels lying on the same plane. After this final finish, they are boiled in concentrated nitric acid to remove all cement and oil which may be on them, rinsed in hot water and dried in alcohol.

They are then inspected for imperfections and gauged for hole-size, diameter and thickness; the hole-size being held to .0001 tolerance, plus only.

For balance-hole jewels, the jewels are cemented on a spindle and small wires charged with diamond dust are passed through the hole to cut down the edge, thereby giving it the olive shape, characteristic of a first-class jewel. This operation must be done by hand and is the most expensive part in making jewels. The amount taken off is so small that it can hardly be seen with an ordinary eye-loupe and a microscope is used for final inspection.

Of all the features of a finished jewel, the most carefully inspected is the hole. Only the most experienced eye can detect defects of shape, polish or slightly rough corners in the hole.

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