Glaze

Glaze (glaze, enamel), a glassy mass covering a ceramic product with an impermeable layer for liquids. It is obtained by firing in special furnaces clay products, so-called 'biscuit', due to the melting of the glaze mixture of various composition covering its surface. The mixture for glaze is sometimes 'fritted', i.e., melted before use; the resulting mass is ground into a powder, which is called 'frit'. Glaze must have a coefficient of expansion equal to the coefficient of expansion of the biscuit, and must melt simultaneously with the flux (i.e., the substance added to the clay to cement it). The composition of this substance usually includes feldspar, chalk, dolomite, and some others. If the first condition is not met, the glaze will crack easily; if the second is not met, the glaze will not be firmly connected to the flux. The components of the glaze are of considerable importance for health. Bokhvar divides glazes into those containing lead and those not containing lead. Lead glazes are easily fusible and are divided into 1) transparent, consisting of PbO, clay, and feldspar; used for simple clayware; 2) transparent (sometimes colored), consisting of PbO, SiO₂ and B(OH)₃; used for higher grades of earthenware and faience; 3) opaque, lead-tin (opaque), often colored; used for tiles and other products. Sometimes PbO (litharge) is replaced by Pb₃O₄ (red lead). According to analyses by Khlopin, glazes used in Perm province contained lead salts in significant quantities: black-68.8% PbO, yellow-61.7% PbO, green, prepared with red lead-64.9% Pb₃O₄. Lead-free glazes are refractory and are divided into alkaline, which represent silicate-alkaline compounds, and alkaline-earth, consisting of clay, feldspar, and sand with impurities of alkaline-earth and alkaline oxides. For coloring glazes, iron oxide (yellow color), manganese oxide (black color), copper oxide (green color), cobalt oxide (blue color), etc. are added. For covering clay products, lead glaze is used almost exclusively due to its low melting point. With a properly formulated mixture and proper thermal treatment, the oxide compounds of lead in the glaze pass into silicates, which are difficult to dissolve in water, diluted acids and alkalis; however, with insufficient furnace temperature and poor melting or with excessive lead content in the mixture, a glaze is obtained that is easily dissolved even by very weak acids, passes into acidic foods and beverages during cooking or long storage (e.g., into sour milk, sauerkraut and pickled cabbage, jam, wine, etc.) and serves as a cause of chronic, and sometimes acute, lead poisoning (Khlopin, Lehmann). In most European countries, the quality of glaze in tableware is regulated. The German law of June 25, 1887 states: 'Vessels for food, beverages and jam, as well as for measuring liquids, must not be covered with enamel or glaze which, when boiled for half an hour with acetic acid (4 parts by weight of acetic acid per 100 parts by weight of the vessel), would release lead into the latter.' The French decree of April 15, 1912 prohibits the use of vessels coated inside with lead glaze that does not form a good glassy alloy for the manufacture and storage of food products. By circular of the People's Commissariat of Health of the RSFSR of December 22, 1927 (Bulletin of the People's Commissariat of Health, 1927, No. 24), enamel and glaze of tableware used for storing and cooking food must not release lead into a 4% acetic acid solution when boiled in the specified vessel. When determining the migration of lead from glaze by boiling with 4% acetic acid, discrepancies in analytical methods are often observed, which (according to Th. Stiden-dorf and Penndorf) greatly affects the test results. Therefore, the Kaiserl. Gesundheitsamt proposed a uniform method, according to which the vessel before testing is only rinsed several times with distilled water, but not boiled with it. Then proceed to boiling with 4% acetic acid, and to prevent rapid evaporation, and therefore a change in the concentration of acetic acid, it is recommended to place on the vessel being tested during boiling, if its shape allows, a round-bottomed flask of appropriate size filled with cold water. If the shape of the vessel does not allow this, the evaporating liquid is replenished with 4% acetic acid. After half an hour of boiling, the liquid is filtered into a glass vessel and the presence of lead is detected by precipitation with H₂S (black precipitate), potassium iodide (yellow precipitate, which dissolves on boiling and on cooling settles as yellow crystals), sulfuric acid (white precipitate), and potassium bichromate (yellow precipitate). Further, for the quantitative determination of Pb (according to Khlopin), the liquid is treated with H₂S. If a brown turbidity or precipitate is obtained, it is filtered off, dissolved in HNO₃, the resulting solution is diluted with water, filtered from insoluble salts, and in the filtrate, after adding sodium acetate, lead is precipitated as sulfate or lead bichromate. According to observations by Khlopin and Brickman, thick glaze, both smooth and rough, which absorbs water when wet and takes on a darker shade in the process, has the greatest ability to release Pb; while thin, shiny glaze, which forms a homogeneous, strong alloy with the vessel, releases the least amount of Pb. Well-fired pottery, when tapped, gives a clear, high sound; poorly fired pottery gives a lower, dull sound. With use, the amount of dissolving Pb decreases, but when the glaze is damaged and cracked, the amount of dissolving Pb may increase again, as new areas rich in soluble Pb are exposed. As a preventive measure for detoxifying glazed ware, it is recommended to boil new ware before use with acetic acid with the addition of NaCl. On glaze from a prophylactic hygiene point of view-see Ceramic production.