Soft-paste porcelain is a type of a ceramic material, sometimes referred to simply as "soft paste". The term is used to describe soft porcelains such as bone china, Seger porcelain, vitreous porcelain, new Sèvres porcelain, Parian porcelain and soft feldspathic porcelain, and is also used more narrowly to describe clay bodies mixed with glass frit, used mainly in the production of decorative figures and domestic wares in 18th century Europe. The porcelain was termed "soft" because of their lower firing temperates compared with hard-paste porcelain.
The history of soft-paste porcelain dates back to early attempts by European potters to replicate Chinese porcelain at a time when its composition was little understood and its constituent materials were not widely available in the West. The earliest formulations were mixtures of clay and ground-up glass (frit). Soapstone and lime are also known to have been included in some compositions.
Medici porcelain was the first successful attempt in Europe to make imitations of Chinese porcelain. Produced between 1575 and 1587 the body is a type of soft-paste porcelain, 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 suffered from high pyroplastic deformation, or slumping in the kiln at raised temperatures, they were difficult and uneconomic to use in mass. Formulations were later developed based on 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.
Soft-paste made with little clay is not very plastic and shaping it on the potter's wheel is difficult. Those 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 1100oC for the frit based compositions and 1200 to 1250oC 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.
History of its manufacture
Chinese porcelain, which arrived in Europe before the 14th century, was much admired and expensive to purchase. Attempts were made to imitate it from the 15th century onwards but its composition was little understood. Its translucency suggested that glass might be an ingredient, so many experiments combined clay with powdered glass (frit), including the porcelain made in Florence in the late 16th century under the patronage of the Medicis. In Venice there were experiments supposedly using opaque glass alone.
Experiments at the Rouen manufactory produced the earliest soft-paste in France, when a 1673 patent was granted to Louis Poterat, but it seems that not much was made. An application for the renewal of the patent in 1694 stated, "the secret was very little used, the petitioners devoting themselves rather to faience-making". Rouen porcelain, which is blue painted, is rare and difficult to identify.
The first important French porcelain was made at the Saint-Cloud factory, which was an established maker of faience. In 1702, letters-patent were granted to the family of Pierre Chicaneau, who were said to have improved upon the process discovered by him, and since 1693 to have made porcelain as "perfect as the Chinese". The typical blue-painted Saint-Cloud porcelain, says Honey, "is one of the most distinct and attractive of porcelains, and not the least part of its charm lies in the quality of the material itself. It is rarely of a pure white, but the warm yellowish or ivory tone of the best wares of the period is sympathetic and by no means a shortcoming; and while actually very soft and glassy, it has a firm texture unlike any other. The glaze often shows a fine satin-like pitting of the surface that helps to distinguish it from the brilliant shiny glaze of Mennecy, which is otherwise similar. The heavy build of the pieces is also characteristic and is saved from clumsiness by a finer sense of mass, revealed in the subtly graduated thickness of wall and a delicate shaping of edges."
A soft-paste factory was opened at Mennecy by François Barbin in 1750. The Vincennes porcelain factory was established in 1740 under the supervision of Claude-Humbert Gérin, who had previously been employed at Chantilly. The factory moved to larger premises at Sèvres in 1756. A superior soft-paste was developed at Vincennes, whiter and freer of imperfections than any of its French rivals, which put Vincennes/Sèvres porcelain in the leading position in France and throughout the whole of Europe in the second half of the 18th century.
The use of frit in this paste lent it the names "Frittenporzellan" in Germany and "frita" in Spain. In France it was known as "pâte tendre" and in England "soft-paste", perhaps because it does not easily retain its shape in the wet state, or because it tends to slump in the kiln under high temperature, or because the body and the glaze can be easily scratched. (Scratching with a file is a crude way of finding out whether a piece is made of soft-paste or not.)
The first soft-paste in England was demonstrated by Thomas Briand to the Royal Society in 1742 and is believed to have been based on the Saint-Cloud formula. In 1749, Thomas Frye, a portrait painter, took out a patent on a porcelain containing bone ash. This was the first bone china, subsequently perfected by Josiah Spode.
Recipes were closely guarded, as illustrated by the story of Robert Brown, a founding partner in the Lowestoft factory, who is said to have hidden in a barrel in Bow to observe the mixing of their porcelain. A partner in Longton Hall referred to "the Art, Secret or Mystery" of porcelain.
In the fifteen years after Briand's demonstration, half a dozen factories were founded in England to make soft-paste table-wares and figures:
- Chelsea 1743 
- Bow 1745.
- St James's 1748 
- Bristol porcelain 1748
- Longton Hall 1750 
- Royal Worcester 1751
- Derby 1757 
- Lowestoft 1757 
Hard-paste porcelain was successfully produced at Meissen in 1708 by Ehrenfried Walther von Tschirnhaus, though Johann Friedrich Böttger who continued his work has often been credited with the discovery of this recipe. As the recipe was kept secret, experiments continued elsewhere, mixing glass materials (fused and ground into a frit) with clay or other substances to give whiteness and a degree of plasticity.
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