Synthetic glass is made up of sodium oxide, calcium oxide and silicon dioxide (sand) mixture, which is heated in the furnace, where it is converted into molten state. Calcium oxide provides strength to the glass and make sure that is it not soluble in water. The glass obtained from the above mixture has a light green color and to remove the green color manganese oxide is added to obtain a transparent glass. Generally there are four types of glass. Soda lime glass (lowest quality), Lead glass, Borosilicate glasses, High silica content glasses (highest quality).
Soda lime glass are easy to work at low temperature compare to other type of glass and also cost less than other glass. Generally soda lime glass are used, where cost is a major factor, like light bulbs, bottles, fiberglass, building blocks and windowpanes
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silicon dioxide (silica)
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72 %
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sodium oxide (soda)
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15 %
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calcium oxide (lime)
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9 %
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magnesium oxide (magnesia)
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3 %
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aluminum oxide (alumina)
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1 %
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Lead glass has a good workability, high refractive index and high electrical resistance, it is also used as a shield from x-ray and ultraviolet ray. It is also used in light bulbs, crystal tableware, neon sign tubing and some optical components
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silicon dioxide
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68 %
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lead oxide
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15 %
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sodium oxide
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10 %
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potassium oxide (potash)
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6 %
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calcium oxide
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1 %
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Borosilicate glasses have high chemical stability, high heat shock resistance, low coefficients of expansion
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silicon dioxide
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80 %
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boron oxide
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14 %
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sodium oxide
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4 %
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aluminum oxide
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2 %
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High silica content glasses resist extreme thermal stress and have a very high chemical resistance.
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silicon dioxide
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96 %
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boron oxide
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3 %
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other oxides
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1 %
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The job of the glass blower requires talent, experience, and precision, dexterity of a juggler and sensibility of an artist. Massive strength is required to create and handle large size items. No machine can substitute the masterly skill of human hand. Used glass (waste) is brought from the local market and washed in a cylindrical shaped glass cleaner using fresh water and loaded on the conveyor belt. With the help of the conveyor belt, glass is taken to a crusher, where the glass is converted into fine glass powder. Using a furnace, glass powder is melted.
Making of decorative glass is done with the help of glass blowing technique, there is no much change in this technique from the time it has been invented to the modern day. Glass blower knows as gaffers, use a four feet long hollow metal pipe, this pipe is dipped in hot molten mixture of glass (gather) and small amount of molten glass is rolled at the end of pipe. Gaffer then place the hot glass onto a metal plate, for the initial shape, this process is known as marvering and then blows in air through the pipe to create a bubble, this process is known as parison. Gaffer then insecrt parison into the furnace to reheat it, he then blows the air into the parison to create the final required shape. With the help of wooden paddles, which have hole and a pack of wet newspaper is used to give fine shape. Using shears, soften excess glass is removed. The hot glassware item is dipped into the mixture of molten colored glass mixture. Throughout the process, gaffer keeps on rotating the hot glass, otherwise there are chances of sagging. To get different texture and specific shape, mold are used.
The color of the glass can be changed with the addition of metals, metal oxides or other compounds to the molten mixture. These coloring compound either fully dissolve or suspended in the molten glass mixture. Generally, there is no change in the physical property of the glass unless high concentrated coloring agent is added. The type of color which does not alter while blowing the glass is used for this process.
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Compound Added to the molten glass
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Color of the Glass Produced
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cobalt oxide
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blue
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magnesium oxide
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violet
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gold or selenium
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red
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uranium, iron, or silver oxides
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yellow
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ceric oxide
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brown
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iridium oxide
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black
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copper or chromium oxides
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green
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calcium fluoride or stannic oxide
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white or opal
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Once the products is completely formed at the end of the pipe, the neck of the product which is attached to the rod is heated (if it has cooled down) and small amount of water is applied on it, which form micro- cracks on the neck’s surface, craftsman carefully taps the glassware on to a solid surface to break and separate the glassware from the rod. Then it undergoes Annealing, it is the process where, the article is reheated at 580oC and held at this temperature for five minutes. During this process, any fine and microscopic cracked would be removed with the help of heat, this process also helps to avoid the internal stress of the glass.
Next is the fire polishing, after the glassware is separated from the metal blow pipe, the uneven, shape and pointed glass still remains on the end of the pipe. The pipe is directed to the flame torch, as the flame heats the glass, it soften and start to melt. Gaffer continuously rotate the rod at a constant speed due to which the hot and molten glass take the shape of a golf ball. If the gaffer wants to continue the blowing process, he dips the hot molten glass into gather and repeats the blowing process otherwise the hot glass is heated at higher temperature, which falls of the metal pipe. Some of the important point to remember while shaping are:-
- Pulling decreases both the diameter and the wall thickness.
- Blowing increases the diameter and decreases the wall thickness.
- Heating decreases the diameter and increases the wall thickness.
- Pushing an enlarged tube increases both the diameter and the wall thickness.
- Pushing a constricted tube decreases the diameter and increases the wall thickness.
To remove excess glass from the formed glassware, it is introduced to the flame. The neck or the area which is supposed to be removed is heated at higher temperature and the excess part is separated from the glass wear. Glass wear then undergo sanding process, the edge of glass wear is placed against the sanding belt, continues flow of the water is required while the sanding process, to avoided a formation of cracks. These glass wear items undergo inspection by the expert glass craftsman, he carefully inspect the glass for any cracks, dent, uneven surface or form. If any error is found, the glass wear is crushed and loaded on the conveyor belt. Rest of the selected items are sent for packaging and dispatching unit.
