PORCELAIN
Porcelain  belongs to the large group of ceramics (from the greek chéramos = clay) that includes everything that is manufactured with clay or, better, with a raw material that contains clay. This group has a very large extension: from the so-called coarse ceramic, a typical example is the clay brick, up to the wide range of fine ceramic products ranging from the lowest step of baked clay up to culminate of porcelain. The basic components of porcelain are:
- kaolin, which is a very white friable sedimentary rock;
- quartz, which is a wide spread mineral;
- feldspar, which is found  (in its various compositions) in approximately 60% of minerals of earth's crust.

The porcelain is made up of about 50 parts of kaolin, 25 of quartz and 25 of feldspar, which are linked through appropriate processing (milling and mixing) and turned to a single body by cooking at high temperatures.

KAOLIN
Kaolin is white clay whose name has Chinese origin and means in Italian "high ridge" (with obvious derivation from the places of origin). It is the element that gives plasticity to porcelain, making possible the moulding, turning and pressing of single pieces.

QUARTZ
Quartz is a milk-white colour mineral, scientifically defined as oxide silicon (Si02), which is added to mixture in  ground or sand form. During cooking quartz, binding with feldspar,  forms the structure of the mass and prevents deformation;  furthermore it’s  resistant to heat and chemical agents.

FELDSPAR
Feldspar is a mineral in various colours: whitish or yellowish, greenish or reddish. Exist potassium and sodium feldspar. For manufacture of porcelain are mainly used potassium feldspars, which are finely minced and added to mixture. Feldspar serves as melting and forms vitreous mass that cement the magma of porcelain. This vitreous mass  becomes a bonding agent connecting all the  crystalline not dissolved and formed during the process, and thus maintains earthenware compact. Feldspar makes possible the sintering of earthenware and therefore it’s determinant for the  transparency of porcelain.

BONE CHINA
Porcelain from bones, also known with  the English name Bone China, so called for the bone  ashes  or phosphates introduced in considerable percentage  in the mixture, which includes kaolin and pegmatite (mineral made up of quartz and feldspar). Pegmatite (cornish stone) is mined in England in Cornwall. In this porcelain percentage of kaolin is considerably lower than one used for hard porcelain, and also cooking  temperature is significantly lower, as it’s typical for tender porcelain. Bone China results  shiny  also after first cooking (1240/1280°C). The enamelling with lead ceramic glaze is done at a temperature from 920 to 1125°C.

BISCUIT PORCELAIN 
This porcelain is cooked without enamel. The  finished piece is equally waterproof, but the surface is rough, which limits the use almost exclusively in general for little statues and figures.

VITREOUS CHINA
Under the word porcelain there is also a product called Vitreous China, but has substantial differences either for quality of raw materials and cooking temperature, which is lower than for porcelain. Generally is subject to a single cooking.

CELADON
It's called Celadon a pale-green coloured  porcelain, the name, given by French traders, comes from the protagonist of a French XVII century novel ( "Astrée" by Honore d'Urfé) who always dressed in green. The first Celadon porcelain production in China dates back to the time of the Sung’s dynasty. The Celadon is obtained by mixing to the paste or to ceramic glaze a chromium compound. Initially, however, instead of chromium was added small doses of iron oxide and then cooking at reduced flame.

BLACK PORCELAIN
It is real and true porcelain, black coloured, with all peculiar characteristics  of porcelain: composition of the mixture,  properties of clay, cooking temperature (about 1300°C). Only lacks of transparency:  this is caused by mixture of oxides that causes  the black colour. Was placed in world market in last two decades.

PINK PORCELAIN
The pink porcelain, also called "rosée", is obtained by adding manganese salts and gold compounds to the mixture  which is then treated at usual temperature for normal porcelain cooking.

IVORY PORCELAIN
It’s a  porcelain whose dye is obtained by colouring of mixture or enamel. A certain lack of uniformity in shades of colour is the specific characteristic of this porcelain and guarantees authenticity.

FIRE PORCELAIN
It’s a hard porcelain especially suitable for oven food cooking,  in gas or electric stove, composed of same raw materials as normal porcelain, but with other proportions. It has more resistance to heat-shock, but for direct flame it’s advisable the  use of  fire screen.
This porcelain is  cooked at a temperature of about 1400°C.  Its typical colouring (burnt), is obtained with a double spray painting and subsequent firing in flask (third cooking).

UNDER GLAZE DECORATION
As name suggests, this decoration is done before applying the glaze, and therefore directly after the first firing, after a careful cleaning (and dusting) of the piece. The decoration can be painted by hand or by masks, or with stamps (for example:  the brand-name).

OVER GLAZE DECORATION
The over glaze decoration, as name suggests,  takes place on the glaze already cooked  by high fire, and therefore melted. The colours used are composed by oxide metals that are mixed with the frit  (a mixture of minium,  boric acid, quartz flour, feldspar flour). Determinant for firt composition  is that melting point must be  inferior to softening point of glaze, that is at temperature of about 750/900°C.  In  case of hand painting over glaze  these colour mixtures ready for use are mixed with essential oils, such as oil of turpentine , of clove, lavender, balms, etc., and made suitable for  painting with spatulas on  palette. The pieces, once decorated, are subject to a further cooking called  third fire, whose temperatures range from 750 to 900°C, and change  according to colour. During this cooking the frit  melts, and in this way fixes the colour to underneath  glaze. Therefore in under glaze decoration  the colours  available are limited to only two;  with the in-glaze method the number of colours is much wider but still limited, while with over glaze decoration the assortment of colours  available  is practically unlimited, which allows any shade of tint.

DECAL DECORATION
The technique of decals  is a process which, thanks to lower cost respect to hand-painting, has almost completely eliminated this last  production especially in industrial production of household pottery. Even with this procedure the colours used are obtained by mixtures with frit metals, including noble-ones, bought  on the surface of  porcelain fired at large flame. Today exist  two decal’s  systems:
a) the direct decal;
b) the slide decal or flowing image.
In both cases, after transferring  decoration on the piece, this is subjected  to further firing  (third fire) until 750/900°C exactly as for hand decoration. Both systems are suitable for decoration on glaze and over glaze.

HOTEL PORCELAIN
For hygienic properties of porcelain, not matched by any other product, this was automatically preferred to other materials for larger supplies in the field of gastronomy. The need for resistance to all stresses, especially mechanical, much higher to those found in normal domestic use,  has led to develop a type of porcelain particularly suitable for hotel use. This development has affected  either  the external shape (special profiles for  edges, for  angles, for bases, for handles and others) and  on its intrinsic properties (glaze of higher hardness, special mass, improved use resistance). Also  shape has found its best achievement:  considered the needs represented by a rational customer service, a rapid and frequent washing, and also its aesthetic value. The special quality of porcelain have meant that hotel production has become a very important branch of sector, aimed at increasing development.

GEDECK AND COVERS
These are usual naming used in porcelain trade. The  "Gedeck" is composed of:   a cup of tea with saucer and a cake plate. The "Covers" is composed of:  a dinner  plate  with a  soup plate  and a dessert plate, so everything that a person uses, as kitchenware, to eat a normal meal.

GLASS
From physics point of view  under the name "glass" is a product of inorganic melting that solidifies without crystallization. This definition corresponds also to the common meaning attributed to glass, both in language and in trade.

THE GLASS IN NATURE
In nature glass is found in  fusion state made by fire of volcanoes. It’s the "obsidian", also called volcanic glass, whose composition is identical to manufactured  glass:  sand, sodium, calcium compounds.  Different types of obsidian are distinguished : basalt, trachyte,  andesitic, etc. . Since ancient times man knew these products and used them  to make tools, tips of arrows, knives, ornaments and gemstones. Another type of natural glass is represented by laparite  and  pumice;  this last one is well known to be used to clean and sharpen.

SODIUM GLASS
Sodium glass is the quality of glass more common, so called because among its components, for melting, there is sodium carbonate (soda).

GREEN GLASS
Green glass is a sodium glass that contains, further  sodium carbonate, the ferric oxide  that gives it the characteristic pale green colour. It’s used to manufacture bottles, flasks, demijohns and similar.

POTASSIUM GLASS
Potassium glass contains,  for melting, the potassium carbonate (potash). It’s  'also named crystal glass (sonorous  glass and superior sonorous glass)  because of its high brightness  that grants  a greater value.

CRYSTAL
Crystal is a quality of potassium glass, whose name derives from its similarity with crystal rock, and is characterized by an exceptional brightness and gives  high light refraction that makes more effective the effect of carving. This is achieved with pure raw materials, highly selected.
There are two distinct qualities of crystal:
- potash crystal, commonly called crystal glass;
- lead crystal, so named because in its components there is a percentage of oxide lead (PbO).

The name crystal glass is often improperly used  in mirror field, but in this case is simply chosen with care  a glass for its optical qualities, but it has nothing in common with lead crystal.

LEAD CRISTAL
The main characteristics of lead crystal are as follows:
- Its specific  weight is much more than other glass because of high content of lead  in the form of oxide;
- It has an extraordinary brightness even in carved and grinded parts;
- The adding  of oxide lead gives a light purple shade that gives an exceptional brilliance;
- The hollow objects (glasses, goblets,  cups) in crystal resound at touch in a  particular pleasant way.

GLASS SCRAPS
Glass scraps are carefully selected,  according to their quality and their colour, depending on the work to be performed. These scraps serve as melting for mixture, since at  temperature of 1200°C these are already reduced in a smooth and homogeneous paste.

COMPOUND
The word compound   (in German: Glassatz) indicates the mixture made according to the recipe that sets quantities of different raw materials needed to produce a certain type of glass. It’s obvious that each glass, depending on the purpose is destined, requires special recipe and therefore a special compound.

COLOURED GLASS
While the production is pursuing the search of glass always more pure, colourless and  of maximum richness, likewise the desire of ornaments and friezes that shows  in taste’s changing of consumers also requires production of coloured glass. The colouring is obtained introducing oxide metals in the mixture and also partly of rare earths.

OPAQUE GLASS
Opaque is  synonym for "not transparent" and "impenetrable to light." This glass is obtained by introducing in mixture muddy  substances , so it looses transparency but  keeps unaltered  the other peculiar properties of glass. The  opaque glass may also be coloured.

PRESSED GLASS
Pressed glass is  an English inventions and dates from the early nineteenth century. The pressing of viscous  mass in a form is the reason for this  name. The glassy  mass taken out from the oven with the so-called "fork" is placed in the form, whose top is removed; into  opening is introduced with pressure a piston that distributes the vitreous mass in the form and compresses  to internal walls  shaping the model. When cooling is done, the form is opened and the piece is put in the cooling oven. In  pressed glass  mechanization production is very advanced, and sometimes reaches the total automatism.  It’s a mass manufacture that is especially different  regarding quality of finishing and design. Pressed glass can be a simple glass for domestic purposes, such as beer mugs, cups, flower holders, vases and similar objects, that seem decorated with  carving operation while instead are only obtained with stamp.  It’s  called pressed crystal, or pressed lead crystal according to how much lead is contained in glass.

FIRE RESISTANT GLASS
Fire resistant glass has respect to common  glass a more high resistance to heat shock, due to its low coefficient of expansion. This property is obtained by adding to  mixture special substances and in particular  boric acid as melting. The fire resistance could also be  obtained  with quartz, using for fusion pure quartz;  however the high temperature needed for melting will bring  costs so  high not to allow a commercial price in the field of pottery for kitchen. The lower melting point can be obtained,  for normal glass, with the help of a melting, for example soda or potash, but these  are not suitable for manufacture of fire-resistant material because these will significantly raise the coefficient of thermal expansion of glass, so that in case of temperature change will generate dangerous tensions. The boric acid causes a decrease of quartz melting temperature from 1800°C to 1500°C and also 1200°C without significantly altering the coefficient of thermal expansion. Therefore the boric silicate glass is particularly resistant to temperature change and at consequent  concentration in glass because the internal tensions remain limited. Fire resistant glass was discovered only in the late nineteenth century for demands of scientific research in field of chemical and industrial production. The first step towards the domestic glass tableware resistant to fire started  in the early twenties  of this century with manufacture of baby’s bottles.

CROWN AND FLINT GLASS
These are two optical glasses, used in combination to make achromatic the lenses. The Crown contains sodium oxides and calcium, the Flint contains a significant percentage of lead oxide.

TEMPERATE GLASS
Temperate  glass, also called shatterproof  glass,  is a low melting glass which is found in state of superficial tension due to a sudden cooling  caused by a cold air jet blown  after it has been  heated to the softening point. It is mechanically very strong even at  shocks (six times more than normal glass). If the hardened coat is scratched,  the glass will not  break but will disintegrates in tiny pieces that are not dangerous.

GABLONZ GLASS
It  mean buttons, chandelier’s pendants, drops, pearls, stones, flints, strass in different shapes and glass colours. The name derives from the place of origin, because until 1945 these items came from Gablonz, in the Sudeten region, and were obtained from faceted glass bars, pressed and cut.

GLASS PEARLS
Glass pearls exist from almost six thousand years and belong to the oldest objects produced with glass. Once these objects, reduced to a filiform state through a lamination and drawing process, were obtained from a pasty mass of opaque glass, in which was inserted a thin metal rod to practice holes for following threading.  Today proceed in accordance to ancient systems pressing the bars in suitable  shapes; then with special equipment tubes are  cut and then placed in a mill containing fine sand and high temperature water so to reduce them into small spheres.

DIATRETI GLASSES
The denomination  is of uncertain  derivation from greek-latin. The expression was used for first time by the famous archaeologist   l.l. Winkelmann. Typical for all these glasses is a very light net attached to the cup through thin bars, length 5 to 10 mm, which form a kind of stem. This stem is often decorated with inscriptions, with deeply cut figures,with semicircular collars and even representation of animals deeply engraved. It’s supposed  they were produced with against  drawing  of two glass bells, mono  or double coloured, forming a double wall  container.  On the outer wall were applied  decors of fluid glass pressed until touched  the inner wall, while on the inner wall  were carved,  with fine grindstone, grids, texts and figures.

RÓMER
The name Ròmer means originally a  pot-bellied cups slightly restricted upward, equipped with a foot made  by  a large twisted wire of glass  (that during manufacture was rolled around a wooden form). It was born with this form in Rhineland in the seventeenth century and became the most beautiful glass of the Rhine Renaissance, is still  much appreciated today although it has undergone to significant variations. Today the cup is often produced by applying the technique of coating  with rich carvings. Even the foot and the stem have undergone  to many changes: these  are much thicker, colourless and carved in many various ways. The name has nothing to do with the ancient Romans, as  often believed, but developed in the use of language from "Rümeh" meaning a  container for drinking to someone's health, and became "Romer" in Denmark, "Remmare”  in Sweden," Rumme” in England, "Rome” in France," Roemer" in Holland. In Italy  today, with the name "Rómer of Treviri”  is usually indicated  a single glass to be used for vintage wine. It is also sold in  series of six different colours variously cut.

PORTLAND VASE
This vase is considered the most beautiful, well known,  and certainly the most famous object  of ancient  Alexandria’s glass manufacturing and Roman’s carving glass art of the first century D.C.. It consists in a dark cobalt blue glass that, in its figure’s  decor,  shows the representations of the mystical wedding  of Peleo and  Tetide with carving from white coating. This vase was found in a marble sarcophagus in a room of a Roman tomb and from the family of princes Barberini (so once was called vase Barberini) it came up until the Duchess of Portland (hence the name); finally  was donated to the British Museum,  where currently is. Once it was presented at  a Christie’s  auction and  had an offer corresponding to approximately 330 million at today's value;  however was withdrawn from auction and never been sold. Since 1790 Josiah Wedgwood produces  good copies of this strong ceramic vase.


Drawn from the site AssoCeramVetro: www.art-tavolaregalo.it