Soft-paste porcelain was an early European attempt to imitate Chinese porcelain before the real hard-paste formula was known in Europe, made by adding glass or other fluxes to a clay body to reach translucency at a lower temperature rather than through true hard-paste chemistry.
Because true kaolin-and-feldspar hard-paste chemistry was unknown to European workshops, soft-paste formulas substituted ground glass, bone ash, soapstone, or other fluxing materials mixed into a clay body to imitate porcelain's whiteness and translucency at a fire range hard-paste's own ingredients could not reach that low. This let soft-paste reach a translucent, porcelain-like appearance without hard-paste's high peak temperature, but the result is generally weaker, more prone to warping in the kiln, and less durable in use than either true hard-paste or bone china. The gap is substantial: soft-paste typically matures around 1100 to 1250°C, well below hard-paste's own 1350 to 1450°C glost firing, and because that lower temperature was never enough to fully vitrify a true kaolin-based body in the first place, soft-paste never reaches hard-paste's degree of vitrification no matter how carefully it is fired.
Soft-paste porcelain developed in Europe during the period when Chinese porcelain was hugely prized but its actual manufacturing method remained a closely guarded secret (see porcelain's own history). Early French workshops, notably Saint-Cloud and later Vincennes (the factory that became Sèvres), produced soft-paste ware through much of the 17th and 18th centuries, prized in its own right for a soft, slightly milky glaze quality distinct from true hard-paste, even after Europe learned the real hard-paste formula following Meissen's 15 January 1708 breakthrough and its subsequent leak across the continent.
Soft-paste porcelain is mostly a historical category today rather than an active production choice: once hard-paste chemistry became widely known in Europe, most factories that could switch to it did, since it produced a stronger, more reliable body. Soft-paste pieces from workshops like Sèvres are collected and studied largely for their distinct surface quality and as artifacts of the period before Europe cracked the hard-paste formula.