The working time of zinc phosphate cement is shortened by:
a- concentrating the acid
b - warming of glass slab***
c- incremental mixing of powder
d- all of the above.
------------------------------
HOW AND WHAT IS THE GLASS MADE?
IN ADDITION to the role he has played in everyday life, glass has had a momentous participation in the development of technology and our conception of nature. Thanks to him we know what microorganisms are like, through a microscope; how is the Universe, with the use of telescopes; what is the nature of the atom and the dynamism of a living cell. The variety of uses found only is limited by the ability and ingenuity of man. Its versatility is hardly replaceable, so that its study becomes more interesting.
Basically, the manufacturing principle of glass has remained unchanged since its inception, as the main raw materials and melting temperatures have not been modified. However, the techniques have been transformed to achieve a more accelerated production process, and researchers have developed different compounds to combine them with the raw material and thus vary the physical and chemical properties, so that it is possible to have a wide range of glasses for various applications.
The glass is made in a fusion reactor, where a mixture that almost always consists of siliceous sand (clays) and powdered or granulated dry metal oxides is heated. In the melting process (passage from solid to liquid) a viscous liquid is formed and the dough becomes transparent and homogeneous at temperatures greater than 1 000 ° C. When removed from the reactor, the glass acquires a rigidity that allows it to be shaped and manipulated. Controlling the cooling temperature prevents devitrification or crystallization.
In ancient times the fusion was done in homemade sand molds, as seen in figure 8, but for the industrialization of this process it was necessary to build large furnaces (figure 9), where in addition to the raw materials pieces could be added of old waste glass.
During the primitive times of the glass industry, the only raw materials that were used in its manufacture were clays. Today different mixtures are used to obtain different types. For example, glass blocks are manufactured in molds with a mixture of silica sand, lime and soda, and dolomite, aluminum clay and products for refining are added. Many materials currently play an important role, but clays remain fundamental.
Although the word may be known to us, we may not know that clay is the product of the geological aging of the Earth's surface, and that since this degeneration is continuous and occurs everywhere, it is a very abundant earthy material in the nature. In fact, for the grower, the miner or the road builder is a hindrance. In our country we have numerous clays. The deposits in the Republic are distributed in the territory corresponding to 10 of the states that form the political confederation of the country. The locations where they have been found are illustrated on the corresponding map (figure 10). This shows that in Chihuahua, Zacatecas, Aguascalientes, Jalisco, Guanajuato, Querétaro, Hidalgo, Tlaxcala, Puebla and Guerrero are the important areas. Often these territories are related to mineralization regions, such as those corresponding to the states of Guanajuato, Hidalgo and Querétaro.
The primary igneous rocks that gave rise to the clays were, among others, granites, pegmatites and feldspars. The aging of these primary rocks was caused by the mechanical action of water, wind, glaciers and earth movements, combined with the chemical action of water and carbon dioxide at high temperatures. Today the same natural forces continue to produce clay, thus forming more quantity than man can use.
Clay is a deceptively simple material. It does not have the stubborn hardness of the stone, nor the temperamental fiber of the wood, nor the solidity of the metal, but it has a fragility and an inconstancy that seem to demand special care. It is soft, tame, plastic, malleable, without grain or direction. Classifying it is a difficult task and leads to different results, depending on the characteristic of the material taken as a reference. We can order it from a geological, mineralogical point of view or according to its use.
------------------------------
HOW AND WHAT IS THE GLASS MADE?
IN ADDITION to the role he has played in everyday life, glass has had a momentous participation in the development of technology and our conception of nature. Thanks to him we know what microorganisms are like, through a microscope; how is the Universe, with the use of telescopes; what is the nature of the atom and the dynamism of a living cell. The variety of uses found only is limited by the ability and ingenuity of man. Its versatility is hardly replaceable, so that its study becomes more interesting.
Basically, the manufacturing principle of glass has remained unchanged since its inception, as the main raw materials and melting temperatures have not been modified. However, the techniques have been transformed to achieve a more accelerated production process, and researchers have developed different compounds to combine them with the raw material and thus vary the physical and chemical properties, so that it is possible to have a wide range of glasses for various applications.
The glass is made in a fusion reactor, where a mixture that almost always consists of siliceous sand (clays) and powdered or granulated dry metal oxides is heated. In the melting process (passage from solid to liquid) a viscous liquid is formed and the dough becomes transparent and homogeneous at temperatures greater than 1 000 ° C. When removed from the reactor, the glass acquires a rigidity that allows it to be shaped and manipulated. Controlling the cooling temperature prevents devitrification or crystallization.
In ancient times the fusion was done in homemade sand molds, as seen in figure 8, but for the industrialization of this process it was necessary to build large furnaces (figure 9), where in addition to the raw materials pieces could be added of old waste glass.
During the primitive times of the glass industry, the only raw materials that were used in its manufacture were clays. Today different mixtures are used to obtain different types. For example, glass blocks are manufactured in molds with a mixture of silica sand, lime and soda, and dolomite, aluminum clay and products for refining are added. Many materials currently play an important role, but clays remain fundamental.
Although the word may be known to us, we may not know that clay is the product of the geological aging of the Earth's surface, and that since this degeneration is continuous and occurs everywhere, it is a very abundant earthy material in the nature. In fact, for the grower, the miner or the road builder is a hindrance. In our country we have numerous clays. The deposits in the Republic are distributed in the territory corresponding to 10 of the states that form the political confederation of the country. The locations where they have been found are illustrated on the corresponding map (figure 10). This shows that in Chihuahua, Zacatecas, Aguascalientes, Jalisco, Guanajuato, Querétaro, Hidalgo, Tlaxcala, Puebla and Guerrero are the important areas. Often these territories are related to mineralization regions, such as those corresponding to the states of Guanajuato, Hidalgo and Querétaro.
The primary igneous rocks that gave rise to the clays were, among others, granites, pegmatites and feldspars. The aging of these primary rocks was caused by the mechanical action of water, wind, glaciers and earth movements, combined with the chemical action of water and carbon dioxide at high temperatures. Today the same natural forces continue to produce clay, thus forming more quantity than man can use.
Clay is a deceptively simple material. It does not have the stubborn hardness of the stone, nor the temperamental fiber of the wood, nor the solidity of the metal, but it has a fragility and an inconstancy that seem to demand special care. It is soft, tame, plastic, malleable, without grain or direction. Classifying it is a difficult task and leads to different results, depending on the characteristic of the material taken as a reference. We can order it from a geological, mineralogical point of view or according to its use.
A geological classification is the most convenient in the case of clay, as it can be a useful preliminary guide to the raw materials used in the glass industry (Figure 11). They can also be divided into two large groups: primary and secondary. The primary clays, also known as residual clays, are those that have formed in the place of their mother rocks and have not been transported by water, wind or glacier. As there is no movement, there is almost no opportunity for mixtures of other sources to alter their composition, so they tend to be relatively pure and free of non-clay materials. They are valued for their cleanliness, their whiteness, their softness, their low cost and their difficulty in finding them. The secondary clays are those that have been displaced from the place where they were formed. They are much more common, less pure, because they have material from different sources, and their composition varies widely. These data are particularly important for people who are going to use these materials, since working conditions are significantly altered. Clays that are essentially pure require minimal treatment, while the others have to be treated thoroughly before being used in industry.
You may be wondering why we give so much importance to clays, if this is a book about glass. What happens is that we want to teach you and convince you that clay is like earth, like sand, and that glass is obtained from it. It is hard to imagine, right? Think of the surprise that the primitive man took when he discovered it. He started to heat earth and it began to get hard until it became a glass. Sounds like magic. Today we know that as the clay treatment temperature increases beyond red hot, hardening occurs, followed by compaction and finally a transformation of clay into glass. During vitrification there is a considerable contraction, due to the decrease in particle size and a restructuring of the molecules within the vitreous matrix. But what are the clays that allow them to do all that? Clays are complex silicoaluminatos. A silicoaluminate is a compound made of silicon and aluminum, which is formed when silica modifies its surface by interaction with aluminate ions, exchanging Si (OH) -4 ions for Al (OH) -4 ions, as illustrated in the figure 12. They can be exchanged for each other because they are very similar to each other. Al (OH) -4 has a negative charge and four OH groups, the same as Si (OH) -4. In addition, silicon and aluminum are of a similar size. Over time these compounds react and form soluble salts with the alkaline (Na, Li, and K) and alkaline earth (Be, Mg and Ca) ions, thus changing the structure of the original silicoaluminates. Aluminum can be surrounded by 4 or 6 oxygen atoms, and can have a +3 or +4 charge. Imagine a silicate where one of the atoms of Si + 4 is replaced by an Al + 3 ion.
Since the global load has to be the same and the silicon has four while the aluminum has three, a K + 1 or a Li + 1 is attached and solves the problem. Figure 13 shows a drawing of the structure of clays with and without metals. In Figure 13 (a) we see that there are two different types of layers. At the bottom we find a layer of silicon, in the middle one of aluminum and then another of silicon, with their respective oxygen each, of course. It is clear that aluminum changed the shape of the clay. In figure 13 (b) the situation is similar, except that it indicates the position taken by the potassium atoms (K). If we continue looking for differences, we will see that in the clay that lacks metals (figure 13 (a)) water molecules (H2O) appear between layers of silicon. That is why it is said that all these minerals have the property of absorbing water, which also helps to make the structures wider because, as you can see, the one in Figure 13 (a) measures between 9.6 and 21.4 Å, depending on the amount of water that has been absorbed, while that of figure 13 (b) measures 10 Å. These changes in the structure of the clay are the basis of its capricious nature.
In silica, the fundamental structural unit is a tetrahedron of SiO4, that is, a silicon atom always surrounded by four oxygen atoms (Figure 14). The forces that hold these atoms together comprise ionic and covalent bonds, which causes the bond strength to be very large. If we think of silica tetrahedra together, surrounding each other, we would have a combination of silica tetrahedra (with their respective oxygen) randomly oriented. In a crystal like the one in Figure 15 (a) the atoms follow a strict orientation pattern that is repeated n times, always in the same way. In a glass, the Si-O-Si bonds do not have a specific orientation; The separation distance between the atoms of Si and O is not homogeneous, the tetrahedral units are not repeated regularly and the compound is disordered. The latter is known as amorphous silica, while the ordinate is known as crystalline silica, and both are used in the manufacture of glass. Quartz is an example of crystalline silica widely used in this manufacture.
Glasses are made with clays, and as there is a great variety, the glass we obtain will depend on the clay we choose, which is why we need to know very well the raw materials. This is known by the glassmakers, and that is why they have learned that kaolinite is the simplest group of clay minerals, its basic structure is made up of oxygen atoms arranged in such a way that they give rise to alternating layers of tetrahedral holes, which are they occupy atoms of silicon and aluminum, and octahedral holes, occupied by atoms of aluminum, magnesium, iron and zinc.
There are also impurities that occupy interstitial sites, or put another way, they have ions that are poorly accommodated. The effect of impurities depends on their nature, the proportion they are in, the size and shape of the clay grains, and the reaction conditions, including the temperature reached, the duration of heating and the effects of some other substances present. When these impurities are composed of iron, for example, the color of the clay changes, and efflorescences of colors appear on the surface of the dry material and black or gray spots. The refractory properties are also modified. Ferric oxide is highly refractory when it is in an oxidizing atmosphere; in a reducer acts as a flux. The difference between the two situations is that in the first one the iron loses electrons, while in the second one it gains them. This disparity can radically change the properties of the raw material needed to make a glass. Impurities can help us manufacture, the important thing is to know how to choose and handle.
Since the primitive man discovered glass, its manufacture has changed little, and has depended heavily on the infrastructure available for the fusion of raw materials. Formerly used crucibles with a capacity of a few tons (nowadays they are still used to make special glasses). In the large modern factories the so-called tank furnace is used, which consists of a large closed tank, made with the best refractory materials. The fuel (gas or oil) burns inside the tank, produces huge flames that pass over the surface of molten glass and floating raw materials not yet melted. The most common tank furnaces are continuous, which means that the raw materials that are introduced by the melting end leave with the same speed on the opposite side in the form of molten glass, and then pass to the machines that shape it. There are very large continuous ovens, with a total capacity of 450 tons and a daily production of 250 tons of glass. The high temperatures with which these furnaces work (around 1,500ºC) require regenerative heating systems to recover part of the heat.
When the glass leaves the melting tank it cools and hardens quickly. In the few seconds that it remains at a temperature between yellow red and orange red it works in many ways to give it different aspects. It can be pressed, blown, stretched and rolled. The cold glass can be reheated and worked repeatedly with the same ease by applying the same method. It is important to keep the hot and soft glass from staying outdoors for too long, because it can crystallize.
In large-scale production, immediately after a glass article has been shaped, it is transported to a continuous annealing furnace, in which it is reheated to the appropriate temperature. This avoids tensions within the vitreous material. Subsequently it undergoes a slow and controlled cooling. After leaving the baking oven, each item is inspected, packed and, if necessary, undergoes finishing operations. A diagram of the glass manufacturing process is shown in Figure 18. The raw material is put in the fusion tank. Once melted, it is shaped and then annealed. It can be seen that the annealing temperature is relatively low compared to that of melting, and that the broken waste glass can be reused as many times as desired.
It is important to emphasize that the manufacturing process is practically the same for all types, and what changes from one copy to another is the material. All of them have silicon atoms in greater or lesser proportion, which is one of the elements of the periodic table that most closely resembles carbon. This is interesting if we think that carbon is the fundamental basis of life on our planet. If they are so similar, why is there no life on Earth based on the chemistry of silicon ?, and why can't we use carbon to make glass? The reason lies in the great ease that silicon has to form compounds with oxygen, thereby avoiding long chains that would be equivalent to those of carbon, and that are important in the chemistry of life. It is precisely this affinity with oxygen that makes it useful and indispensable in the formation of glass.
Labels
Prostho fixed