Glaze formulas

Most of the oxides found in glaze formulas are found in naturally occurring clay. These clays, such as feldspars, contain small amounts of potassium, sodium, aluminum, and silicon oxides. When glazed pieces are fired, the clay oxides and glaze oxides become one. 

Students will design a glaze formula, predict the physical properties of the glaze, and test their predictions. 

By decreasing the boric acid content where it is included in the glaze formula, and substituting an equivalent amount of sUica, a glaze is made harder, more briUiant and more durable. The introduction of boric acid into the glaze as a substitute for sdica, however, decreases the coefficient of expansion. Thus, the relative amounts of boric acid and sdica may be proportioned to achieve the best possible fit between body and glaze. 

An analytical chemistry team analyzed a green mineral salt found in the glaze of an ancient pot. When they heated the mineral it gave off a colorless gas that turned limewater milky white. When they dissolved the mineral in sulfuric acid the same colorless gas was released and a blue solution formed. Suggest a possible formula for the compound and justify your conclusion. 

The composition of glazes is represented on the basis of three groups of oxides and according to the Seger formula. The most used oxides, divided into three groups, are listed in table 11.1.1. 

The composition of the glaze is expressed as a mole proportion. The Seger formula applies when the mass of the alkaline substances is 1 mole. Some examples ... 

The table on the following page illustrates the conversion of a glaze composition in grammes to the Seger formula in the last row of the table. 

Yellow lead(II) oxide, known as litharge, is widely used to glaze ceramic ware. Lead(IV) oxide does not exist in nature, but a substance with the formula PbOj.9 can be produced in the laboratory by oxidation of lead(II) compounds in basic solution. The nonstoichiometric nature of this compound is caused by defects in the crystal structure. The crystal has some vacancies in positions where there should be oxide ions. These imperfections in the crystal (called lattice defects) make lead(IV) oxide an electrical conductor, since the oxide ions jump from hole to hole. This makes possible the use of lead(IV) oxide as an electrode (the cathode) in the lead storage battery. 

Raw glazes are applied in particular to porcelain ware. As an example, we present a formula for the glaze listed second in the table ... 

Write the chemical formulas that correspond to the following names (a) aluminum chloride (used in cosmetics), (h) chromium(III) oxide (a pigment for coloring pottery glazes), (c) calcium nitrate (provides a red-orange color in fireworks), and (d) ammonium sulfide (used to make synthetic flavors). 

The ionic compound sometimes called uranium yellow is used to produce colored glazes for ceramics. It is 7.252% sodium, 75.084% uranium, and 17.664% oxygen. Wliat is the empirical formula for this compound ... 

Household window glass will eventually form crystals of devitrite, an orthorhombic crystal with a formula like NaiCasSisOie, these crystals usually form at the surface first. Crystalline glazes, which can make a pot look so spectacular, actually rely on spherulites consisting of many willemite (Zn2Si04) crystals, with each acicular crystal growing in the [001] direction, to achieve the effect. (More on this topic in is presented in Section 21.11.)... 

Brokes. Term used in the English ball-clay mines for clay that will not cut into balls such clay is generally of low plasticity and poor fired colour. Brongniart s Formula. A formula relating the weight (W, oz) of solid material in 1 pint of slip (or slop glaze), the weight (P, oz) of 1 pint of the slip. 

The formula was established for slop glazes by A. Brongniart (Traiti des Arts Ciramiques, Vol.l, p.249,1854). Brookfield Viscometer. An electrically operated rotating cylinder viscometer in which the drag is recorded directly on a dial it has been used in the testing of vitreous-enamel slips (/. Amer. Ceram. Soc., 31,18,1948). 

The author noted consistently over many years that everyone was interested and paid attention when formulae were presented, but eyes quickly glazed over when design philosophy, coastal management issues, environmental impacts, etc. were discussed. 

The experience of a physician family in Illinois illustrates the typical toxicity symptoms and some common source of lead. Five of the six members of the family experienced fatigue, poor appetite, pains in the stomach, and vomiting. (One exception was the infant who was on a formula). They became increasingly irritable. The blood test disclosed severe anemia. An exhaustive search identified an earthenware pitcher as the source of lead. The pitcher was glazed with lead-containing compounds and it was insufficiently fired. It was used for orange juice container, and the acidic juice leached out the lead. 

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