ABOUT PORCELAIN

   The art of making ceramic (from the Greek word segamos - clay) dishes has its roots back centuries and even millennia. The ability of clay to easily take the desired shape and form containers of various sizes after firing, suitable both for serving daily needs and for long-term storage of water, wine and oil, led to the intensive development of pottery in pre-antique times and especially in the era of antiquity. Over the years, ceramics has ceased to be limited to purely utilitarian functions, but has become a form of arts and crafts and an object of collecting.

      Constant technological and artistic progress led to the emergence and improvement of new types of ceramics. At present, ceramic products can be divided into the following main groups, depending on the composition of the initial masses and manufacturing technology: pottery (clay), majolica, faience, stone products and porcelain. Of course, this classification is to a certain extent conditional and in the literature there may be such terms as “semi-faience”, “semi-porcelain”, “opaque”, “terracotta” and some others.

     Porcelain is the most valuable, noble and refined type of ceramics. It lends itself perfectly to the finest shaping and processing, perfectly perceives any color and gilding.

     The first European porcelain was obtained only at the beginning of the 18th century in Germany, although some more or less successful attempts were made earlier (Florence, Saint-Cloud).

     The basis of porcelain is white clay-kaolin. It is an earthy, very finely rubbed mass, with a slight matte sheen, sometimes with a slight yellowish or grayish tint. Kaolin is a decomposition product of rocks such as granite, porphyry, sandstone. The second component of the porcelain mass is feldspar, which acts as a flux during firing and ensures the creation of a strong shard due to vitrification. Various additives are also used to regulate the fusibility of the ceramic mass - first of all, quartz. The ratio of components varies for different industries within a fairly wide range, however, the most typical (for example, used in Sevres and Berlin) is the following composition:

• Clay - 55 - 65%,
• Quartz - 15 - 25%,
• Feldspar - 15 - 20%.

      A mixture of these components gives the initial mass, which, in the process of firing the product at temperatures of 1350-1450 ° C, forms the so-called hard porcelain (sometimes called real or genuine). Products made from this material have a characteristic luster, whiteness, transparency, high hardness, mechanical strength, lack of porosity, and resistance to rapid temperature changes.

     Along with hard porcelain, there is also soft porcelain, the main difference of which is a sharp decrease (up to a complete absence) of kaolin content. The basis of masses for the manufacture of soft porcelain is a glass-forming frit - a mixture of saltpeter, common salt, alum, gypsum and sand. Lime marl is usually used as the clay component of soft porcelain. The firing of such masses is carried out at a lower temperature - 1250-1300 ^° C. Soft porcelain is inferior to hard porcelain in strength and heat resistance, and has a characteristic milky-white shade of a shard. Sometimes frit soft porcelain is also called French.

     Another type of porcelain is English or bone, which is intermediate between hard and soft, and is sometimes classified as a separate type of porcelain. This type of porcelain got its name from the bone ash added to its composition. It is distinguished by exceptional transparency, is close to real porcelain in whiteness, but is inferior to it in hardness.

     In the last quarter of the 19th century, the Berliner A. Seger improved hard porcelain by changing the ratio of components in it as follows:

• Clay - 25%,
• Quartz - 45%
• Feldspar - 30%.

      This made it possible to reduce the firing temperature to 1250 - 1300 degrees, which, in turn, expanded the range of paints used. The shard of Seger or, as it is sometimes called, "new European" porcelain has an ivory or bluish-white color.

The process of manufacturing porcelain products consists of the following steps:

1. Cleaning, grinding and washing of raw materials.
2. Preparation of the mass by mixing the components.
3. Forming products by hand, modeling in molds and casting.
4. Mandrel and drying.
5. Firing in molds or tunnel kilns.
6. Underglaze decoration.
7. Glazing.
8. Re-fired.
9. Overglaze decoration.
10. Firing in muffle furnaces.

     Without dwelling on the first, purely technological stages of the process, we will briefly describe the operations associated with giving the product its final appearance and aesthetic merits.

     Primary firing, during which the formation of a shard, is carried out at 750-1050 ° C. Underglaze decoration is performed with mineral paints, usually containing metal oxides. The paint used must be resistant to high temperatures. Therefore, most underglaze paints are based on heat-resistant oxides of cobalt and, since the 19th century, chromium. The former create a blue color during firing, the latter green. There are also some impurities that allow you to still diversify the palette of underglaze paints. The technique of underglaze decoration also includes stamped reliefs or bas-reliefs, which are attached to the main product, and painting with engobe, a thin layer of mass that differs in color from the mass of the product. A drawing is sometimes scratched in the engobe layer (sgraffito technique). A few more specific decoration methods - lace trim, paie-sur-pate, "rice grains" are described in the "Terminological Dictionary". The drawing is applied manually - with a brush or swab, transfer printing, decal method (using transfer paper), silk-screen printing, spraying with a spray gun or airbrush - through a stencil.

     After decorating, the products are covered with glaze by dipping, brushing, pouring or spraying. During firing, the glaze melts and covers the shard with a shiny or matte, transparent or deaf layer. Glaze protects the surface of the product from external factors, hides individual defects in the shard and, finally, performs decorative functions. According to their chemical composition, glazes are divided into lead, tin-lead, boron, feldspar and salt. In appearance, glazes painted with metal oxides can be combined into the following groups: oxidizing, reducing, crystalline, dripping, matte and crackle glazes.

     Firing after decoration and glazing is carried out at a temperature of 1250-1450 C., depending on the type of porcelain.

     Overglaze decoration differs from underglaze decoration in a richer palette, since the low firing temperature of overglaze painting - 650-900 C - allows the use of a much larger number of colors. Another advantage of overglaze decoration is the possibility of correcting the painting both after its application and even after firing (re-painting and firing).

     In addition to conventional paints, overglaze decoration also uses:

1. Enamels or opaque deaf paints that lie on the product in a thick layer, which is why they are sometimes called "sublime enamels".

2. Chandeliers that cover the surface in the form of the thinnest layer and create opalescence effects (plays and reflections) due to the metals they contain. Chandeliers are colorless (bismuth, lead, zinc) and colored (iron, uranium, copper, cobalt, nickel, chromium). Chandeliers containing noble metals are especially beautiful: gold, giving a pink tint, silver - yellow, platinum-silver.

3. Gold | silver, which as overglaze paints create a variety of effects. This led to their widespread use by most European factories. Gold is applied from solutions of its compounds, from mechanical mixtures with other components, or by electroplating.