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It's now in the dictionary. You all would not have guessed some of these. Some imitative words are more surprising than others. How to use a word that literally drives some people nuts. The awkward case of 'his or her'. Urine consists of over 3, components, many of which are valuable, including various minerals, water , urea , and more. First, the urine is collected and distilled, separating the water from the other compounds. The remaining dark-coloured solution is then filtered, which leaves sediment consisting of minerals. This filtered sludge is then fired at a high temperature to 9 to 10 hours with 20 minutes of soaking.
The mineral residue crystallises and forms an opaque glass coating on the ceramic surface. It is typical of coatings used in the home-laundry industry.
The amount of zirconium oxide added is a compromise between the need for alkaline durability, served by adding more zirconium oxide, and the need for workability, which zirconium oxide hinders. For an application such as strong alkali resistance in a chemical reactor, the amount of zirconium oxide would be increased at the expense of workability. Enamels can also be formulated for acid resistance. Silica is highly acid resistant, so acid-resistant enamels tend to be high in SiO,. That is why the laundry enamel has some titania in its formula. Enamels can also be formulated for thermal resistance in service.
This is normally accomplished by raising the refractoriness, or firing temperature, of the enamel. Coatings C5 and C6 in Table are examples of the formulations used when the outstanding thermal and corrosion resistance required of hot water tank systems are needed. The composition of these enamels bears a similarity to that of the home laundry enamels because the action of water on these enamels is similar to that of alkali attack. It is instructive to compare these formulations with formulations B6 through B8 in Table It can be seen that the two important differences are the addition of adherence oxides to facilitate bonding to a metal substrate and the use of greatly increased concentrations of boric oxide and reduced concentrations of alumina and silica to permit the very much lower firing temperature.
E24 A substantially different type of ground coat formulation is given in D7 in Table This coating has been developed to provide a way to oxidize and hence remove food soils from the surfaces of ovens at normal operating temperatures.
They range from opaque whites through pastels and medium-strength colors to strong, dark colors. A wide selection of glosses is also available, ranging from the high-gloss sanitary ware finishes to the full-matte architectural enamels. Porcelain enamel cover coats are classified as opaque, semiopaque, and clear. Opaque enamels are used for white and pastel cover coats, semi-opaque enamels are used for most of the mediumstrength colors, and clear enamels are necessary to produce bright, strong colors. Although zirconia and antimony oxides were used for opacification in the past, current opaque enamels are opacified with titanium dioxide.
Some fully opaque titania-opacified porcelain enamels are given in section A of Table These materials have been formulated to show excellent acid resistance and in most cases fairly good alkaline resistance.
The five examples shown here are all suitable for application directly to decarburized steel. However, not all titania-opacified porcelain enamels are suitable for application direct-on, because they do not produce a satisfactory surface. As with the other porcelain enamels, these systems are basically alkali borosilicates. Content mole ratio 0.
Finally, small additions are often made of materials that affect the color of the enamel by altering the crystallization properties. Two examples of semi-opaque cover coat enamels are also given in Table These materials do not differ in essential constituents from the fully opaque enamels. Instead, the concentration of titanium dioxide is reduced to the point where the system is compatible with the use of pigments for the production of mediumstrength colors.
Clear cover coat porcelain enamels are used in conjunction with appropriate pigments for the production of strong and mediumstrength colors. Some typical examples are given in section A of Table There is a rather wide variation in formulation between these products compared with the opaque and semi-opaque enamels. Although some titanium dioxide is present in order to improve acid resistance, the concentrations are low enough that substantial crystallization does not occur.
Therefore, the inclusion of pigment in the mill formulation will permit the development of strong colors. Particularly in the architectural industry there is need for cover coat enamels with lower gloss but with no reduction in weatherability and durability. There are several deficiencies in this technique, however, and recently coatings have been developed specifically for matte finishes. Table includes examples of these coatings. Example B19 illustrates a fully opaque matte coating, whereas example B20 is typical of a semiopaque coating suitable for use with darker colors.
Many of these minerals contain more than one of the constituent oxides. Hence, the choice of raw materials is limited by the presence of all the oxides in a given material. Table shows most of the raw materials commonly used in the formulation of ceramic coatings. As will be discussed in Chapter 7, most ceramic coatings are applied as an aqueous slip. For this reason, materials that are highly soluble in water are not included in Table Such materials can only be used after fritting, a process to be discussed in Section 5.
While no water-soluble materials are included in Table , a few of these materials are toxic, and are more or less soluble in mild acids. Great care is required when handling such materials to prevent the ingestion or breathing of dusts.
One should always read the Material Safety Data Sheets supplied with materials to determine appropriate handling procedures. A number of factors must be considered in selecting a raw material for use in a ceramic coatingT1 These factors include: The chemical composition of the material, including the impurities impurities to to be be expected expected The uniformity of the composition over extended periods of time The particle size distribution of the material The availability of the material and the location s of its source s5 77 G L A Z E SA N D G L A S SC O A T I N G S Its behavior in transit and in storage Its behavior in water suspension Its behavior in processing Its effect on the environment cost Chemical composition involves not only the desired elements, but also the impurities.
Among impurities, ferric oxide hematite , titania rutile , iron chromite, and zircon are particularly undesirable. Also undesirable is any element that is not called for in the formulation. On the other hand, some impurities may be desirable. As a general rule, less pure materials, especially those containing volatiles that decompose below SOOT, are more reactive than materials of high purity.
Consistency of composition over time is very important. Most mined materials can vary slowly over time. To combat this problem, some producers, especially in the clay mining business, blend material from various parts of their mines, in order to produce a uniform product over time. O n the other hand, the increased surface-energy-tobulk-energy ratio of a fine particle will lead to agglomeration. Agglomerates melt very slowly and must be avoided.
Thus, a balance is required. The location of a raw material source affects shipping costs and delivery times. For some of the less expensive raw materials, shipping cost can be an appreciable part of the delivered cost. On the other hand, some unique materials are worth the added cost and inconvenience. Behavior in transit and storage can seriously affect the ability to use a material.
Hygroscopic materials, for example, must be protected from water. Excessive humidity can cause cementing reactions to take place. R5 Fine particles can cause dust clouds, with consequent loss of material. As will be discussed in detail in Chapter 7, the water solubility of a material affects its behavior in an aqueous suspension. Even materials considered to be relatively insoluble, such as feldspars, can leach alkalis when left for a long time in aqueous suspension. T1 The behavior of a material during processing can have a significant effect on the results obtained.
This will be discussed in detail in Section 5. In particular, it is important to realize that reactivity is generally inversely related to purity. High-purity materials are often relatively inert and difficult to use.