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THE QUEST FOR PORCELAIN
BASICS
When first examining a piece of porcelain or pottery having a general idea of the materials and techniques used can help you identify its origin, date, and how much it might be worth. There are three factors to access: material, glaze and decoration.
Porcelain is made from fine white clay known as “kaolin”, and other fine materials such as feldspar known as “china stone or petuntse”. This can be fired to a higher temperature than traditional earthenware or stoneware. Meaning that the minute particles of clay fuse together forming a hard vitreous body only distinguished by it translucency. Porcelain is tougher than pottery and can be crafted into more delicate forms. This will allow it to reach a degree of fragility.
HARD-PASTE PORCELAIN
Hard-paste porcelain is made from kaolin (china clay) and petuntse (china stone). The object is fired at a very high temperature first, then dipped in glaze, then refined. To get the translucency the china stone bonds to the particles of clay together. The finished object appears to have the consistency of glass. All Chinese and much of the Continental porcelain is hard-paste.
SOFT-PASTE PORCELAIN
Soft-paste porcelain on the other hand is more vulnerable to scratching than hard-paste. There are several types of soft-paste porcelain allowing different degrees of translucency. It is identifiable by a surface glaze and feels warmer and softer to the touch, looking less glittery in appearance than hard-paste. You’ll notice the chips in soft-paste look floury where as hard-paste look glassy. Soft-paste porcelain was produced in the 16th Century in Italy.
BONE CHINA
This type of English Porcelain was first produced in c.1794 with the large portion use of bone ash added to the hard-paste ingredients. This body was generally used by distinguished English factories such as Rockingham, Flight & Barr, Coalport & Minton, and Spode.
GLAZE
There are a three main types of glazes used to make a porous body watertight as well as decorative.
- Lead Glaze - generally used on soft-paste porcelain, earthenware’s like creamware
- Tin Glaze - this is a glaze to which tin oxide was added to give an opaque white finish
- Salt Glaze - when adding common salt into the kiln during firing, and temperatures reaching
1800°F (1000°C).
DECORATIVE TECHNIQUES
Decoration is a technique that is added before or after glazing. Underglaze decorating means the colours have been added before glazing.
- Underglaze blue - this is a blue pigment known as cobalt blue. Normally used on Chinese blue and white porcelain, soft-paste porcelain and European delftware.
- Overglaze enamels - this is a procedure where by adding metallic oxide to molten glass by reducing the cooled mixture, which is then combined with a oily medium, could be painted over the glaze and fused by firing it. The ranges of colour was much larger than with underglaze colours.
HOW TO DATE 19TH AND 20TH CENTURY PORCELAIN
- "Royal"
in trademark after 1850
- "Limited or LTD"
after name 1860
- "Trademark"
after c.1870
- "England"
in trademark after 1890
- "Bone China"
20th century
- "Made in England"
20th century
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