Finishing

Metal Finishing

• Cleaning
  Cleaning of metal casting refers to all operations and activities that are performed to remove sand, scale and / or excess metal from the casting. In jewelry making, cleaning is a continuous process used at various stages. Casting comes for cleaning either directly from separated mold or after improvement by welding, soldering or other procedures. Burned-on sand and scale are removed to improve the surface appearance of the casting whereas excess metal, in the form of fins, wires, parting line fins and gates, is also removed. Cleaning of metal casting can be done by many ways:
  
  
• • pickling - pickling is a process in which chemicals are used to remove oxide and carbon deposits from metal to obtain a clean surface. This process will also remove all of the fire stain and oxidation caused by the soldering. In this process, a metal piece is submerged in a heated acid solution, usually acetic, citric or nitric acid. The acid solution is then removed by rinsing the metal piece in cool water as well as in limewater to neutralize any remaining acid. But due to the environmental reasons, shot blasting has largely replaced pickling.
  • Sand Blasting - Sandblasting is a general term used for the process of cleaning, smoothing or etching a hard surface by forcing very fine bits of solid material across that surface at high speeds. Sandblasting method gives batter finish than the finish achieved by using sandpaper. In Sandblasting method, sand is the most commonly used material but sometimes other relatively uniform particles like synthetic beads, bits of coconut shell etc are also used. Sandblasting as a cleaning and smoothing method is widely used where small bits of abrasive launched at the metal or other surfaces at a high velocity so that all dirt, imperfections are knocked loose and can then be easily washed off. Whereas sandblasting as an etching method is also very popular and usually applied onto crystal or glass. In etching process, abrasive is blasted at the crystal or glass lightly to turn it semi-opaque and this semi-opaque effect is used to produce words or images on that crystal or glass.
  • Steam Cleaning - Steam cleaning refers to cleaning of metal casting through steam and it is suitable for removing oily or greasy residue.
  • Soap Water Cleaning - Soap water cleaning refers to cleaning of metal casting by using mild soap detergent in water.
  • Supercritical Fluid Cleaning - Supercritical fluid cleaning refers to cleaning of metal casting with small openings or complex design. This cleaning process is used mainly on liquid contaminants like petroleum, silicone, lubricants, waxes etc.
  • Ultrasonic Cleaning - It refers to the cleaning of a metal piece by using ultrasonic cleaners. Ultrasonic cleaner is a machine which uses a fluid that is vibrated at 20,000 cycles per second. When the vibration speed rises above the ultrasonic frequency level, bubbles explode and generate strong power, cleaning the surfaces and cavities of hard-to-clean metal pieces. This cleaning does not scratch the surface or harm metal pieces.
  
  
• Polishing
  Metal polishing is the process of rubbing a metal piece to make it smooth and shiny as well as reduce the appearance of flaws.
  
  
• • Tumbling - Tumbling is the process used to get shine and brightness in a metal piece. This process primarily uses steel shots and does not remove any metal in the form of fins or gates from the metal piece.
  • Bombing or Stripping - Both bombing and stripping techniques are used to brighten metal pieces primarily by way of cyanide and water baths. However, bombing removes metal and brightens the entire piece evenly but stripping tends to remove more metal from the high points such as prongs.
  • Buffing - Buffing refers to the use of grease compound in combination with wheels to produce very smooth polished surface. It is the final step in the professional finishing process.
    
    
• Soldering
  Soldering is a technique, used by jewelers, in making and repairing jewelry. By using this technique, two or more metal pieces are joined by applying a molten metal or metal alloy that has a lower melting point than the two metals being joined. This molten metal forms a bond when cooled and solidified. Soldering is also used in adding findings or links in a metal piece.
  
  
• Plating
  In terms of jewelry, plating is the technique to cover or coat a thin layer of a metal to another metal surface mainly for the decorative purposes.
  
  

• • Techniques of Plating
    
    • Electroplating - Electroplating is the process where inexpensive metals are frequently electroplated or coated with a thin layer of more expensive metals like gold (gold plating), silver (silver plating), rhodium (rhodium plating), copper (electro coppering) etc. In this process, an electric current is flown to coat an electrically conductive metal object with a relatively thin layer of precious metal.
    • Electroless plating - Electroless plating is the chemical deposition of a metal coating onto a metal piece using chemical reactions rather than electricity.
    • Other Plating Techniques - Some other techniques are also used for plating like chemical plating, electrochemical plating, mechanical plating, spray plating, hot dip plating etc.
      
      
  • Specific Metal Plating
    
    • Gold Plating - Gold plating is a method of depositing a gold layer of .175 microns (approximately 7/1,000,000ths of an inch) with at least 10-karat gold onto the surface of another metal, most often silver or copper, by chemical or electrochemical mode.
    • Silver Plating - Silver plating is a method of depositing a silver layer on other metal surface mainly for decorative purposes on household and jewelry items.
    • Rhodium Plating - Rhodium is a noble metal, from the platinum family, with a whitish gray appearance. Rhodium, in raw natural state, comes in liquid state and not in solid state like platinum. In jewelry industry, rhodium is very popular plating metal which gives very smooth, shiny and expensive look, just like platinum, to a metal piece. This metal is applied through the electroplating process to base metals like gold, sterling silver or some other metal alloy. This rhodium plating gives silver tone finish, which is darker than a silver plated finish and darkens with time rather than tarnishing like silver plate. Rhodium plating is virtually identical to Platinum but on a fraction of the cost and often it is used on fashion rings and brooches.
    • Chrome plating - Chrome plating is a method of depositing a thin layer of chromium on other metal surface mainly for giving a bright and lustrous metallic surface, which is highly resistant to tarnish.
    • Other Metal Plating - Some other metals are also used for plating or coating a metal surface. For Example, zinc, tin etc.
      
      
• Texturing
  
  • Florentine Finish - A Florentine finish is a crosshatched decorative technique engraved into the surface of a precious metal. It is usually applied by a hand tool and consist a series of parallel lines engraved in two directions perpendicular to each other. This finish reduces the reflectivity of metal and the lines are often coarser and more deeply incised than the brushed or matte finish methods.
  • Hammered Finish -
    A hammered finish is a texture applied to the surface of a metal piece with a hammer to give it a dimpled look. A hammered texture gives impression of a series of small depressions in the metal. This finish is varied from light to deep hammering texture and usually increases the size of the metal piece.
  • High Polish Finish - A high polish finish is super shiny and smooth polish applied to a metal surface that gives a highly reflective and mirror-like finish.
  • Satin Finish - Satin finish is a texture of a metal surface that is in between matte finish and a brilliant one. It is a series of tiny parallel lines scratched onto a metal surface with a wire brush or polishing tool to produce texture. Satin finish is achieved by various means and methods like sandblasting, wire brushing or chemically altering a shiny metallic surface. This finish gives impression of a soft, pearl-like luster instead of a bright polish.
  • Matt or Brushed Finish - A matte finish, also known as a brushed finish, is a texture applied to metal surface, which gives a dull and non-reflective finish. Matte finish is achieved either by a chemical process or by using an abrasive material to scratch the top layers of the piece.
    

• Stone Cutting & Polishing
  Cut refers to the geometric proportions and finish of a gemstone. It is one of the most important factors in determining gemstone’s sparkle and brilliance. The stone should be symmetrical in all dimensions so that it will appear balanced, and so that its facets will reflect light evenly, which will provide good brilliance to stone. A person who cuts and polishes rough gemstone and converts it into a finished gemstone is known as cutter or lapidary. Usually all gemstones are cut and polished with grits of harder substances. Since diamond is the hardest substance, it is often used to polish most gems, including the diamond itself.
  A lapidary uses many techniques like sawing, drilling, tumbling, grinding, sanding, lapping and polishing to cut and provide the surface finishing to these gemstones. These techniques are explained as under:
  
  
• • Sawing - Gemstone sawing is a technique of gemstone cutting in which a thin circular blade usually composed of steel, copper or a phosphor bronze alloy impregnated along the outer edge with diamond grit and is rotating at several thousand surface feet per minute literally scratching its way through a gemstone. A liquid material such as oil or water is used to wash away cutting debris and keeping the stone and the saw blade cool to avoid overheating.
  • Drilling - Drilling is a technique used to make a hole in a gemstone either to create a bead or to remove inclusions. To drill in a gemstone, gem-cutters often use a small rotating rod or tube with a diamond tip or slurry of silicon carbide. Ultrasonic drilling is also very effective but costly and thus it is reserved for high-volume commercial drilling.
  • Tumbling - In this process, large quantities of rough shaped stones are often tumbled slowly in a rotating barrel or in a vibratory machine with sharp abrasives (usually silicon carbide) and water for days or even weeks. After this process, these stones are finished in very attractive shapes. In vibratory machines, it is much easier to examine the progress of stones inside while machines are on whereas standard tumblers must be halted in order to check progress of the stones. In addition to polishing gemstones, tumbling is often used to polish large quantities of metal jewelry.
  • Grinding - Gemstone grinding is very important process in which silicon carbide wheels or diamond-impregnated wheels are used to convert a rough gemstone into a desired shaped gemstone. This is called perform. In this technique also a liquid material such as oil or water is used to wash away cutting debris and keeping the stone and the blades cool to avoid overheating.
  • Sanding - Sanding is a process that is very much similar to grinding process but uses sharper tools. The purpose of sanding is to remove deep scratches left behind from sawing and grinding processes. This process removes material less rapidly, which ultimately allows more delicate control over final shaping of the stone prior to polishing. It polishes to create smooth curves and avoids flat surfaces.
  • Lapping - Lapping is the process, which is very similar to grinding and sanding. The only difference is that this process is performed with low pressure on one side of a vibrating or rotating flat disk known as a lap, and it is used specially to create flat surfaces on a stone (as in faceting). Laps are often made of cast iron, steel, or a copper-bronze alloy, but other materials can also be used.
  • Polishing - Polishing is a technique used to provide a mirror like finish and shine to the surface of a gemstone to reflect light. This process is started once a gemstone is sawed and given the desired shape and sanded to remove rough marks. Polishing is achieved either by rubbing a gemstone with powder or grit, or against another gemstone.
    
    
• Stone Treatments / Enhancements
  Natural flawless gemstones are very rarely available. Gemstones are mined in a very rough state and they require stone finishing by numerous ways to enhance the appearance and durability of stones. Some of these treatments or enhancements are permanent in nature whereas others are temporary. Gemstone enhancement has become very common and accepted practice and these days, vast majority of the gemstones are treated in some way before setting into a jewelry piece. According to the Federal Trade Commission's Guides for the Jewelry Industry, jewelers must disclose to the consumer all gemstone treatments that are non-permanent or require special care. On the contrary, jewelers are not required to disclose permanent treatments that don't require special care. It's important to remember that most gemstone enhancements greatly improve the appearance and hence the value of a stone.
  
  
  • Heating - Heating is one of the oldest and most common treatment methods used to enhance the natural beauty of gemstones. It is a permanent process that can dramatically improve color and/or clarity in a number of stones, including sapphire, ruby, diamond, aquamarine, amethyst, tanzanite, topaz, tourmaline.
    
  • Irradiation - Irradiation is a permanent process in which gemstones are exposed to radiation usually to improve color. Irradiation is commonly used on yellow beryl, smoky quartz, freshwater pearls, diamonds, blue topaz and pink to red tourmaline.
  • Diffusion - Diffusion is a process in which surface color of gemstones is enhanced through the use of chemicals in conjunction with high temperatures. This treatment is usually used for rubies and sapphires.
  • Fracture-Filling - Fracture-filling is an enhancement process in which cavities or fractures are filled in a gemstone with an artificial substance like glass, plastic etc. This process is often used on diamonds to make flaws less visible.
  • Laser Drilling - Laser drilling is the permanent process of drilling holes into a stone with a laser to remove inclusions. This treatment is most commonly used for diamonds.
  • Oiling - Oiling is one of the oldest treatment methods used to enhance the color, smoothness and hide surface cracks of a gemstone. This treatment is often used on emeralds.
  • Dying / Bleaching - Dying or Bleaching is a process in which stones are dyed or bleached to improve their color or appearance. This treatment is often used on porous materials like jade, pearls, amber, coral, ivory etc.