(1) cutting spherical gems
Arc gemstone cutting can be classified according to waist shape and cross-sectional shape.
1. Classification according to waist shape
There are round, oval, olive, heart-shaped, rectangular, square, octagonal, cushion, pituitary and random shapes (see Figure 5- 1-25).
Fig. 5- 1-25 waist shape of globoidal gem
2. Classification according to section shape
There are single convex cutting, double convex cutting, lentil cutting, hollow convex cutting and concave cutting (see Figure 5- 1-26).
1) The characteristic of single convex cutting is that one end is convex and the other end is flat. Suitable for all kinds of gem ring.
2) The feature of biconvex cutting is that both ends are convex, but the upper convex surface is higher than the lower convex surface, which is suitable for gem ring with special optical effects, such as cat's eye gemstone, starlight gemstone and moonstone.
3) The characteristics of lentil cutting are that both ends are convex, and the radians of the upper and lower convex surfaces are the same. This kind of cutting is often used in opals.
4) Hollow convex cutting is characterized by digging a concave surface from the bottom on the basis of single convex cutting. Suitable for gemstones with dark color and poor transparency. This can improve color and increase transparency. Jade is often cut in this way.
5) Concave cutting is to dig a concave surface from the top down on the basis of single convex cutting. In this way, more expensive gems can be embedded in the concave surface. For example, setting a diamond on black chalcedony.
Fig. 5- 1-26 Cross-sectional shape of globoidal gem
(2) Design and positioning of spherical gemstones
The cambered style is usually suitable for the following situations:
1) When there are too many impurities in transparent gemstones, faceted gemstones, such as rubies, sapphires and emeralds, cannot give full play to their advantages.
2) All opaque and translucent gems, such as jadeite, agate, turquoise and malachite.
3) Gems with special optical effects, such as cat's eye gems, star rubies and sapphires, opals, Labradors, etc.
1. Design of globoidal gems
The design step of gem is the process of gem examination and design scheme selection. After a piece of rough stone is brought, it should be inspected first, and the color, transparency and special optical effects of the rough stone should be analyzed as much as possible. The process is as follows:
(1) original stone naming
Gemsmiths know very well that it is better to process known gems than unknown ones.
(2) Understand the external characteristics of gems.
Including the shape, weight, color of raw materials and whether there are big cracks and inclusions on the surface.
(3) Original stone analysis
By knowing what is known, we can infer the unknown. For example, whether the color is uniform, how deep the cracks are, how many cracks there are, whether there are special optical effects and so on.
(4) Select the design scheme
For small rough stones, the processing design can directly refer to the style design. For a large piece of rough stone, choose the design scheme. For example, the same rough stone has different design schemes, and which scheme to choose depends on its value.
As for what kind of cambered surface the original stone is ground into, it depends on the situation. Generally, gems with cat's eye effect are cut with high convex surface, because it can better reflect the cat's eye effect and make the cat's eye thinner and brighter. Opal stone mostly adopts lentil type, which is related to the shape of Opal original stone. Starlight red and sapphire are often cut into biconvex shapes, because this can not only effectively show the starlight effect, but also take care of it; Hollow convex type is often used for crimson almandine because it can reduce the color concentration.
2. Positioning of spherical gemstones
For the original stone with special optical effect, the following positioning is required.
(1) Cat's eye effect
Any rough stone with parallel fibrous inclusions can be ground into a jewel with cat's eye effect, and the cat's eye effect can only be displayed in a convex shape. Generally, it is cut with a single convex surface, and the waist shape is round and oval. Its positioning is relatively simple. According to the cat's eye effect principle, the bottom of the incision is parallel to the fiber content. If the outline of the gemstone is oval, the fiber direction is perpendicular to the long axis direction of the gemstone. In order to make the cat's eye effect appear finer and more vivid, the thickness of the convex surface can be appropriately increased to increase the curvature of the convex surface, so that the reflected light can be concentrated in a narrow band to form a clear, bright and vivid cat's eye light band (see Figure 5- 1-27(a)).
Figure 5- 1-27 Positioning of Gemstones with Special Optical Effects
(2) Starlight effect
This is the reflection caused by the distribution of microscopic needle-like and tubular inclusions in two or three directions. Generally, inclusions arranged in two directions can form four starlight, such as starlight pyroxene; Inclusions arranged in three directions can form six starlight, such as starlight sapphire. The positioning method is to make the bottom surface parallel to the arrangement plane of inclusions in all directions, that is, perpendicular to the Z axis (see Figure 5- 1-27(b)).
(3) Moonstone moonlight effect, Labrador halo effect and placer gold effect.
The moonlight effect of moonstone is the phenomenon of light reflection and diffuse reflection due to the layered structure of polycrystalline or feldspar with different refractive indexes. Labrador halo effect is interference caused by light reflection from the double crystal plane of polymer sheet. The placer gold effect is the reflected light produced by the layered distribution of fine flake inclusions. Although these three kinds of gems with special optical effects are slightly different in formation principle, they all belong to layered structure, so when positioning, the bottom surface of the cutter should be parallel to the layered structure.
(3) Processing technology of spherical gemstones
There is a wide range of materials for globoidal gems, and there is generally no specific requirement. The advantage of globoidal gem is that it can fully display the special optical effects (cat's eye effect and starlight effect) of gem, and can fully make up for the shortcomings of gem in fire color, transparency and defects.
There are many kinds of materials for curved gems, such as jadeite, ruby, sapphire, opal, golden green cat's eye, garnet, agate and so on.
The processing technology of globoidal gem is relatively simple, which is divided into eight processes: line drawing, blanking, ring forming, rod mounting, preforming, fine grinding, polishing and post-treatment.
1. Draw a line.
In order to accurately and conveniently cut materials and make full use of materials, it is necessary to draw lines on the plate. Line drawing is to draw the waist shape of a curved gem on a piece of material with a brush. The brush used for drawing lines is a pointed brass rod or aluminum rod with a length of 10 ~ 15 cm.
In order to accurately draw the waist shape according to certain specifications, templates are usually used. Formwork is usually made of hard plastic or metal plate. There are many round holes, oval holes and square holes of different sizes on the template (see Figure 5- 1-28). These holes are designed according to production requirements and are easy to assemble.
Figure 5 1-28 Template
The process of drawing lines includes the process of design. For example, on an agate raw material with beautiful patterns, if many tiny micro-cracks are found on it before drawing lines, then this raw material cannot be used. If inclusions, flaws or soft spots are found, they should be circled in time to avoid being drawn on the outline of the curved gem. In addition, according to the shape of the pattern, different waist-shaped globoidal gems can be designed.
When drawing lines, we should pay attention to whether the patterns on both sides of the raw materials are the same, that is, we should consider the increase or decrease or disappearance of the original patterns after the arc gemstone is polished.
The above is the process of large low-grade gems. If the raw materials or lumps of high-grade gems are small, we can start directly from the next process.
Step 2 make disappear
The method of blanking is sawing. That is, use a trimming saw to cut the line along the drawn outline, leaving a few millimeters of margin, usually 2 ~ 4 millimeters ... because some jade is brittle and prone to cracks when cutting. If the off-line is too close, the crack may extend into the contour line.
The equipment used for cutting is a trimming saw, and the specific operation steps are as follows:
1) Clean the stone table without sundries, otherwise it will easily topple when feeding, damaging the saw blade and stone. Turn on the cooling water to cool the saw blade, so as to prevent the raw material and the saw blade from overheating when rubbing.
2) Hold the raw materials, push them evenly into the saw blade, and start feeding slowly, so that the saw blade does not generate sparks.
3) After the raw materials enter a section, accelerate in a straight line, but don't suddenly exert force. If it is found that the feeding resistance is great and sparks burst out, it should be returned by the original way, and the reason should be analyzed before feeding.
4) When sawing raw materials, the speed should be slow to avoid cracking.
5) Repeat the above process to remove the redundant edges and corners to reach the standard as shown in Figure 5- 1-29.
Figure 5- 1-29 Blank shape during blanking
3. Ring shape
After the blanking process is completed, you get a blank that is roughly similar to the design requirements. To get a blank with better waist shape, it needs to go through a ring process. Ring is to circle the waist of the blank with a grinding wheel.
The diameter of grinding wheel is generally above 250mm, and the abrasive used is 80 # ~ 100 # silicon carbide. The specific operation steps are as follows:
1) Turn on the wheel mill and cooling water.
2) The bottom of the blank is upward, hold it firmly by hand, and smooth the edges and corners of the blank. The shape of the circle should be smooth and naturally curved, equidistant from the drawn outline, with a margin of about 0.5 ~ 1 mm, and the waist surface of the circle should be perpendicular to the bottom surface.
3) In order to prevent the edge of the bottom surface from collapsing due to sharpness, a small inclined plane can be ground on the edge of the bottom surface after the circle is completed, and the included angle between the small inclined plane and the bottom surface is 45. Suitable. At this time, one side of the small inclined plane can coincide with the drawn outline, but don't grind off the outline (see Figure 5- 1-30).
Figure 5- 1-30 Blank Side View
Stand on a pole
Before forming the cambered surface, the annular blank should be placed on the rod. Sticking refers to gluing the blank and stick together with glue. For experienced jewelers, blanks can be prefabricated directly in their hands, but for novices, they need to be placed on poles. Because this can not only avoid hurting hands, but also fix gems.
Stick for bonding blanks, made of brass or iron, can also be hardwood. The length is generally around 100 ~ 150 mm, and one end of the stick is round. As the stick end, its diameter depends on the size of the blank, which is generally 6.3 ~12.7 mm. The glue is generally black wax glue or red wax glue.
The specific operation steps are as follows:
1) Wash the gem blank and paste it.
2) Break the glue, put it in a beaker and heat it with an alcohol lamp until it melts.
3) Immerse the bonded end of the stick in the molten glue to a depth of about 10mm, and then take it out for later use.
4) Clamp the blank with tweezers and heat it on an alcohol lamp. Note that the blank should be heated evenly at this time to avoid cracking. For gems that are afraid of heat, they can be heated by asbestos mesh.
5) Stick the bottom surface of the blank with adhesive. When the glue is not completely hardened, straighten the blank by hand to make the bottom of the blank fit vertically. And then naturally cool.
preforming
Preforming is grinding the shape with a grinding wheel. Through preforming, the blank basically obtains the arc shape in the design. The tool used for preforming is a wheel mill with two silicon carbide grinding wheels: one is a coarse grinding wheel with a diameter of about 300 ~ 400 mm and an abrasive particle size of 80 # or100 #; The other is a fine grinding wheel with a diameter of about 200~300mm and a particle size of 180 # or 220 #. No matter what kind of grinding wheel, its purpose is to grind the allowance of the blank. The fine grinding wheel can further grind off some scratches and microcracks, thus improving the smoothness of the blank surface.
The specific operation steps are as follows:
1) Start the wheel mill with coarse grinding wheel and cooling water.
2) Grind an inclined plane on the top of the blank with a hand-held stick, and the edge of the inclined plane should not exceed the edge of the bottom surface and the small inclined plane (the small inclined plane of the ring mill), and it is best to leave a margin. When grinding the inclined plane, the force should be even, and the sticky rod should be rotated continuously to make the inclined plane as round as possible. The included angle between the girdle and the vertical plane is about 10 ~ 15.
3) After grinding the first round belt, grind the second round belt to the top, grind the third and fourth round belts to the top in turn, and so on, so that the top surface of the blank approaches to a smoother surface from a stepped surface.
4) Use a fine grinding wheel to clean the blank to prevent coarse particles from being brought to the fine grinding wheel. Turn on the cooling water again.
5) After the rough grinding wheel is preliminarily preformed, there are still obvious facets and scratches, and replacing it with a fine grinding wheel can further improve the surface smoothness. The operation steps are the same as before (see Figure 5- 1-3 1).
Figure 5- 1-3 1 preform process
6. Fine grinding
The function of fine grinding is to improve the smoothness of blank surface again. Essentially, it is the same as the preformed fine grinding wheel, but the equipment used is different, so its polishing accuracy is different.
There are many tools used in fine grinding, including grinding wheel mill, belt mill and disc refiner. They * * * have the same feature that when they are pressed by the gem blank, they will automatically produce a yield surface to wrap the blank. It is this yield surface that increases the contact area between the milling cutter and the gem, and the grinding force of this yield surface is much softer than that of the grinding wheel, which greatly improves the smoothness of the gem blank.
There are three kinds of equipment for fine grinding.
(1) wheel grinder
The grinding wheel used for fine grinding is similar to the grinding wheel in appearance. Its base body is made of metal or wooden wheel, and its outer ring is bonded to the base body with rubber sheath. The thickness of rubber is about10 ~15mm, and it contains diamond powder. This rubber wheel is suitable for grinding rubies and sapphires with high hardness. The outer ring can also be replaced by leather, and the abrasive used is 240 # or 280 # silicon carbide.
(2) Belt mill
It is very effective to use belt mill for fine grinding. When the convex surface of the gem is pressed against the abrasive belt, the abrasive belt will produce a yield surface, which will wrap the gem and increase the grinding area of the gem.
(3) Disc refiner
Disc grinding is a common fine grinding method. Its matrix is mostly metal or hardwood, and its disk surface is slightly convex. Stick a piece of felt or soft foam with a thickness of 5 ~ 10 mm on it, and use tweed or canvas as the outermost layer, or a belt. The abrasive is 220 # ~ 280 # silicon carbide.
polish
Polishing process is the most effective process, and the polished gems can show brilliance. As the saying goes, "ten treasures and nine shots" shows the great role of polishing.
Tools commonly used for polishing are wood, plastic, asphalt and tin-lead alloy. Wooden pallet surface should be covered with felt, leather, woolen cloth, canvas and other materials. The structure of polishing disc is similar to thin disc, including convex disc, concave disc and flat disc. Generally, diamond powder, ruby powder, green powder, agate powder and tin oxide powder are used as polishing agents. The specific operation steps are as follows:
1) Cleaning the gem blank and polishing the disc.
2) Turn on the polishing machine and apply a little polishing agent to the polishing disc with a brush, and the amount must be small.
3) In the process of gem polishing, the force should be uniform, starting from the edge of the blank, and then gradually reaching the top to make its cambered surface smooth. Any polishing tool works best when it is near drying. The polishing of jadeite and ruby must be very hard.
4) During polishing, check the progress at any time, check whether there are any omissions, scratches and so on. When many pits are found on the surface of the gem, it may be that the original stone has been eroded. You should immediately polish the disc with leather or wood and polish it again according to the above steps.
Whether the gem blank has been polished to a very bright luster can be observed by slowly rotating under strong light. If you still see scratches, or the words on the bulb are reflected on the polished surface and can't be seen clearly, it means that the polishing degree is not enough. Different combinations of polishing discs and polishing agents should also be selected for polishing gems with different properties. Generally, for gems with lower hardness, floppy disks such as felt and leather can be used, and polishing agents can be chromium oxide, iron oxide and agate powder. For gems with higher hardness, asphalt or tin-lead disks should be used, and the best polishing agents are ruby powder and diamond powder, followed by cerium oxide and chromium oxide. Several special gemstones should adopt specific combinations. For example, plexiglass disk and cerium oxide for crystal have the best polishing effect, while woolen cloth and chromium oxide for Iceland spar have the best polishing effect, and so on. See table 5- 1-3 for polishing combinations of other gems.
8. Post-processing
After polishing, the gem blank usually needs post-treatment. The specific operation steps are as follows:
1) Debonding treatment: heat the gem, stick it with alcohol lamp, melt the glue, and unscrew the gem.
2) Cleaning treatment: put the gem and the stick with a small amount of glue into alcohol respectively. After the glue is dissolved, take them out and clean them with clear water.
Table 5- 1-3 Polishing discs and polishing agents for common gemstones
3) Bottom surface treatment: grind the bottom surface with 220 # fine sandpaper or flat sand table, with less grinding amount, so as not to grind off the small inclined surface of the bottom surface. If the transparency of the gem is good, the bottom surface needs polishing. If it is opaque, polishing is not required.
4) Cleaning and waxing. After cleaning and drying, the gemstone with the bottom surface treated should be waxed to fill the microcracks and increase the smoothness of the gemstone.
At this point, the processing of the globoidal gem is completed.