Soft-paste porcelain (sometimes simply "soft paste", or "artificial porcelain") is a type of ceramic material in pottery, usually accepted as a type of porcelain. It is weaker than "true" hard-paste porcelain, and does not require either its high firing temperatures or special mineral ingredients. There are many types, using a range of materials. The material originated in the attempts by many European potters to replicate hard-paste Chinese export porcelain, especially in the 18th century, and the best versions match hard-paste in whiteness and translucency, but not in strength. But the look and feel of the material can be highly attractive, and it can take painted decoration very well.
The ingredients varied considerably, but always included clay, often ball clay, and often ground glass, bone ash, soapstone (steatite), flint, and quartz. They rarely included the key ingredients necessary for hard-paste, china clay including kaolin, or the English china stone, although some manufacturers included one or other of these, but failed to get their kilns up to a hard-paste firing temperature. They were called "soft paste" (after the French "pâte tendre") either because the material is softer in the kiln, and prone to "slump", or their firing temperatures are lower compared with hard-paste porcelain, or, more likely, because the finished products actually are far softer than hard-paste, and early versions were much easier to scratch or break, as well as being prone to shatter when hot liquid was suddenly poured into them.
The German Meissen porcelain had developed hard-paste porcelain by 1708, and later German factories usually managed to find the secret out from former Meissen employees, as did Austrian Vienna porcelain in 1718. The other European countries had much longer to wait, but most factories eventually switched from soft to hard-paste, having discovered both the secret and a source of kaolin. In France kaolin was only found in Limousin in 1768, and Sèvres produced both types from 1769, before finally dropping soft-paste in 1804. In England there was a movement in a different direction, as Spode's formula for bone china, developed in the 1790s, was adopted by most other factories by about 1820. By that point little soft-paste porcelain was being made anywhere, and little hard-paste in England, with Nantgarw (to 1820) and Swansea in Wales among the last factories making soft-paste.
Background
There were early attempts by European potters to replicate Chinese porcelain when its composition was little understood and its constituents were not widely available in the West. The earliest formulations were mixtures of clay and ground-up glass (frit). Soapstone (steatite) and lime are also known to have been included in some compositions. The first successful attempt was Medici porcelain, produced between 1575 and 1587. It was composed of white clay containing powdered feldspar, calcium phosphate and wollastonite (CaSiO3), with quartz. Other early European soft-paste porcelain, also a frit porcelain, was produced at the Rouen manufactory in 1673, which was known for this reason as "Porcelaine française". Again, these were developed in an effort to imitate high-valued Chinese hard-paste porcelain. As these early formulations slumped in the kiln at high temperatures, they were difficult and uneconomic to use. Later formulations used kaolin (china clay), quartz, feldspars, nepheline syenite and other feldspathic rocks. Soft-paste porcelain with these ingredients was technically superior to the traditional soft-paste and these formulations remain in production.
Characteristics
Soft-paste formulations containing little clay are not very plastic and shaping it on the potter's wheel is difficult. Pastes with more clay (now more commonly referred to as "bodies"), such as electrical porcelain, are extremely plastic and can be shaped by methods such as jolleying and turning. The feldspathic formulations are, however, more resilient and suffer less pyroplastic deformation. Soft-paste is fired at lower temperatures than hard-paste porcelain, typically around 1100 °C for the frit based compositions and 1200 to 1250 °C for those using feldspars or nepheline syenites as the primary flux. The lower firing temperature gives artists and manufacturers some benefits, including a wider palette of colours for decoration and reduced fuel consumption. The body of soft-paste is more granular than hard-paste porcelain, less glass being formed in the firing process.