Replacing ceramic glazes containing hazardous substances with less harmful analogues

Some ceramic glazes contain lead and other hazardous elements. The authors suggest replacing them with less harmful analogues. The article describes various types of lead-free glazes, their compositions, properties and manufacturing methods. Particular attention is paid to decorative effects.

Glaze

Glaze is a glassy coating on the surface of a ceramic product, or a mixture of materials that, after application and firing, gives this coating. The basis of the glaze is quartz, feldspar, kaolin, marble or chalk, dolomite, clay. Color is achieved by introducing small amounts of metal oxides and salts. Cobalt oxide produces a light to dark blue color; chromium oxide – green, and in the presence of tin – pink, red; Copper oxide is used to produce emerald green, blue-green glazes, as well as copper-red glazes for reduction firing; compounds with manganese give brown, pink colors; iron oxide - from yellow and red to brown and black.

Types of glazes

Glazes can be shiny or matte, transparent or dull; they are also divided into colorless and colored (single-color and multi-color). Depending on the method of preparation, glazes are divided into raw and fritted. There are glazes that are refractory and fusible. The former are used for porcelain, fireclay, and hard earthenware. Their melting point is 1125 – 1360 °C. For majolica, low-melting glazes are used (900 – 1100 °C). When selecting a glaze based on its composition, pay attention to ensuring that the thermal expansion coefficients of the glaze coating and the product itself are close in value.

Environmental aspects

Ceramic products are considered convenient and harmless to use for many reasons:

• Ceramic tiles have low electrical conductivity.
• It differs from other finishing materials in its non-flammability.
• It is waterproof and relatively chemical resistant.
• Manufactured without the use of synthetic materials.

However, there are a number of negative properties of ceramics:

• When firing ceramic products, harmful gases may be released; therefore, kilns should be located in non-residential premises.
• Some glazes are made on the basis of lead oxides (hazard class 1) and other harmful substances (cadmium - class 2, chromium and zinc - 3). In this regard, it is necessary to work with them wearing gloves and a respirator.
• In everyday life, products coated with such glaze can be sources of pollution.

However, the extraction of these substances, the processing and production of glazes itself, as well as the pollution of atmospheric air, soil and water in the vicinity of these industries, are much more harmful. Currently, lead ranks first among the causes of industrial poisoning. Almost all glazes containing lead can be replaced with analogues that use less harmful elements (strontium, etc.).

Next, we will talk about glazes with various decorative effects achieved by changing the composition, firing modes and other parameters. Most of these glazes do not contain lead.

Muffled glazes (enamels)

Some components (for example, tin oxide, zirconium) are capable of creating opacity in the glaze; they are called opacifiers. Insoluble in the glaze melt, they are suspended in it or, partially dissolving, fall out during cooling, i.e. prevent the penetration of light into the shard. A slightly different type of suppressor is titanium compounds (8–10% in an oxidizing environment), and under some conditions, zinc (5–6%), which favor crystallization of the glaze. Silencers usually give a milky appearance. But antimony oxide (9–12%) at low temperatures in an oxidizing environment gives a yellowish-white tint, and tin oxide (7–12%) in the presence of chromium gives a pinkish tint. In recent years, zirconium compounds have been widely used for dulling glazes, which can be colored with dyes with the soft colorful tone inherent in these glazes. For example, oxides of cobalt, nickel, and chromium give quite definite colors when introduced in an amount of 0.5 to 1% during the grinding process (“to the mill”), or even better, during fusion. It has been experimentally established that dark colors in zircon enamels are not very pure.

Matte glazes

They are valuable because the surface of the products acquires a muted texture. You can achieve matte in several ways:

• lowering the firing temperature, which produces an undermelted glaze;
• the formation of CO2 bubbles as a result of the decomposition of carbonates;
• fine crystallization of a glaze of a special composition upon cooling;
• reducing the acidity of lead glazes.

The last two types are considered good matte glazes. To transform the glaze into a matte one, an increased amount of oxides such as A12O3, TiO2, CaO, Na2O, ZnO, MgO, BaO is often introduced into it, while simultaneously reducing the SiO2 content.

Crystallizing glazes

Under certain firing and cooling conditions, many glazes crystallize. Some produce decorative designs that resemble frosty patterns on glass. Conditions of receipt:

• Supersaturation with oxides capable of creating crystallization centers when cooled, or the formation in the alloy of two immiscible liquids of different viscosities, giving both glass and crystals.
• The presence of a slight viscosity in the cooling glaze, at which the crystal nuclei are able to increase in size.
• Holding at final firing temperature (1 – 2 hours).
• The rate of heating and cooling of the glazed product should be relatively low - about 50 degrees per hour.
• Increasing the thickness of the wet glaze layer to approximately 2 mm.

Crystalline glazes can be of two types:
Coarse-crystalline ones form group implantations of crystals in the glaze layer or on the surface (in the form of spherulites).
These are high temperature glazes and contain zinc (titanium). They are often baked into the underlying glaze. The addition of coloring oxides (from 0.5 to 2%) when grinding frits allows them to be painted in different colors. For example, the crystalline composition obtained by Yu. G. Steinberg (GIKI) gives a blue color when cooled quickly (water granulation), and green when cooled slowly. Aventurine glazes have a sparkling golden hue due to the formation of many small crystals of hematite (Fe2O3) or fayalite (2FeO-Si02). Such glazes can be either high-temperature (with iron oxide) or low-temperature (with ferrous iron). Iron glazes are quite dark; it is difficult to make them lighter, but it is possible by increasing the firing temperature. The aventurine effect is also created by chrome compounds.

Reconstituted glazes

Obtained by firing in a reducing environment. Their decorative effect resembles the play of colors in a film of gasoline on the surface of water. A reducing atmosphere is obtained by introducing rosin, naphthalene, etc., after which the furnace is closed and sealed. Reduction can be carried out in an oxidizing environment with silicon carbide (SiC). To enhance the decorative effect, you can periodically change the reducing and oxidizing atmospheres. Compounds of silver and gold are quite easily reduced even in an oxidizing atmosphere. Oxygen can be produced from some oxides, such as mercury (HgO), by simply heating without a reducing agent. A significant disadvantage of this composition is harmful mercury fumes.

Cracked glazes

Crackle (a network of cracks on the surface) is created due to the coefficient of thermal expansion of the glaze being too high compared to the shard. Enlargement of cracks is achieved by spraying hot products with cold water. To identify and highlight the mesh, various coloring substances are rubbed into it after firing, and after removing their excess, they are fired again lightly or heavily. In low-melting glazes, a good network is often formed by replacing lead oxides (PbO) with sodium oxide (Na ₂ O). In refractory glazes (for porcelain, stone goods), part of the calcareous compounds is replaced with feldspar. The ability to “control” the crack pattern noted in foreign literature is not justified in practice.

Salt glazes

The so-called salt glaze has been known since the 12th century. To prepare it, highly moistened table salt (NaCl), in the amount of 1 kg per 1 m3 of the oven, is thrown into the firebox of the oven that has begun to cool (1200 ° C). A film resembling an orange peel, consisting mainly of sodium aluminosilicate, forms on the surface of the sintered shard. The amount of iron oxides to create a yellow color should not exceed 5 - 6%. Copper compounds introduced into the mass in the form of soluble or partially soluble salts (for example, copper carbonate) form efflorescences on its surface, which, when fired, give the white mass a beautiful blue color. Such compositions are called blue Egyptian.

Porcelain glazes

The above glazes can be used for different types of ceramics, provided they are consistent with it. For porcelain masses, special glazes are used, the composition of which is close to the composition of the shard, which greatly facilitates fitting to it.

Faience glazes

More fusible than porcelain. They come with both lower and higher filling temperatures. The former are used to produce decorative items; they are fired at an average of 1000° C; can also be used for majolica. The second ones are used for hard household faience, in which underglaze painting is often used. Glaze firing takes place at temperatures up to 1160 – 1180 °C. Due to the relative fusibility of earthenware glazes, the palette of underglaze colors is very rich and bright. The use of strontium glazes has shown that they are not inferior to lead in many properties and contribute to the development of the underglaze palette.

Conclusion

From all of the above, we can conclude: it is possible to replace toxic glazes with less harmful, and often even more decorative, analogues. This helps improve working conditions in ceramic production and reduces the negative impact of recycled products on the environment.

The article was prepared based on information kindly provided by employees
of the Moscow State Mining University