3rd
Feb
2017

Posted in Kafevend Blog

What makes a cup? Hard and soft


Last week, we had a look at the history of porcelain, the material that has been used for thousands of years to make all sorts of things- including cups. Today we are going to tell you how it is actually made- so fix yourself a brew in your best bone china and buckle up!

Hard paste


There are three main catergories of porcelain, but all feature some amount of the clay kaolin in their make up. This clay provides the plasticity to the body (the unfired mixture of raw materials), allowing it to be moulded and sculpted into its myriad forms. Alongside kaolin, a variety of rocks can be found in porcelain such as feldspar, soapstone, quartz and alabaster. Glass and bone ash can also be found in some mixes- the latter being particularly obvious where it shows up.

The first type of porcelain made in ancient China was hard paste porcelain. This was made from a compound of kaolin and a feldspathic rock called petuntse. It was fired at a temperature typically around 1400ºC- for those of you not in the know, this is what we would consider really rather hot. Firing the mixture results in a host of complicated chemical reactions, chief amongst which is vitrification. This strengthens the bonds between the various chemicals and makes it impermeable- that is, it won't leak! Understandably given its popularity at home and abroad, those Chinese who knew how to make porcelain kept it a closely guarded secret. That didn't stop a number of Europeans from having a guess at how it was made though...

Soft paste


Most European attempts to replicate Chinese porcelain weren't that great, if we're being honest. The recipe wasn't known , and the various mixes that were invented weren't quite up to scratch. The earliest attempts used ground up glass with clay, along with soapstone and lime. The first (sort of) successful attempt was in the late 16th century in Italy with Medici porcelain, though it didn't take off. The main problem with these mixes, known as soft paste porcelain, was that they suffered from pyroplastic deformation- which is just a technical way of saying they went a bit droopy in the kiln.

Soft paste recipes became better over time, but they were never quite up to the same standard as the hard past porcelain made in China. Fired at a lower temperature of around 1100-1200°C, they weren't quite as strong and were more easily scratched. However, one advantage of the lower firing temperature was that a wider range of colours could be used when it came to decoration. Hard paste porcelain was finally successfully produced in Meissen, near Dresden, in 1708 by Ehrenfried Walther von Tschirnhaus in a victory for the European pottery industry.

Next week, we will discover the secrets behind bone china. This is the most popular form of porcelain these days and a good cause for national pride as it was invented here in Britain!

References:

Hard paste
Soft paste

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