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.