A look into the ingredients used in porcelain clay and how the firing will transform it into porcelain, is needed to understand the differences.
In this blog-post I discuss porcelain and the characteristics of the different porcelains and follow it up in a next post with kilns suitable for porcelain.
Porcelain used in the ceramic arts and craft industry.
It was mainly made up of glass, clay and other melting agents such as lime and chalk that could not withstand high temperatures, but started to collapse at 1976ºF (1080ºC).
Formulations were later developed based on kaolin (china clay), silica, feldspar, low firing fluxes like Nepheline Syenite and frit.
Today Bone China is a very sophisticated type of porcelain, mostly used in industrial situations, but potters that succeed in hand forming of bone china, comes up with true jewels.
Bernard Leach, after the industrial revolution, was one of the first to make a fairly reliable porcelain clay for ceramic artists, but it was Lucy Rie who succeeded to fire porcelain in electric kilns for the first time.
Debates about global warming and cries to save the earth along with rising prices in fuel costs, sparked a new curiosity (and demand) to lower the temperatures at which clay must be fired. The result was the making of cone 6 stoneware clay bodies. To fire porcelain, traditionally seen as hard paste porcelain, is just another natural corollary to the whole movement of more efficient use of resources, but it raises more questions in the whole saga of porcelain.
At which point would hard paste and soft paste porcelain collide? Will it still be considered porcelain as we know it? Bone china used to be a casting body: Nowadays it is throw-able on the potter’s wheel. Soft paste porcelain used to be ornamental, but it becomes more and more user-friendly as the materials gets strong enough to withstand every day household activities.
Testing porcelain clay bodies for cone 6 firing
In the June/July/August 2016 article “Better Porcelain” in Ceramics Monthly is an article in which I documented the results of creating a cone 6 porcelain clay body, I wrote the following:
“In my recent testing of porcelain clay recipes, I wanted to stay as close as possible to the ingredients used in traditional porcelains. My intent was to lower the temperature at which it fires, without sacrificing the properties of vitrification, translucency, and durability”.
We know the following about ingredients in original porcelain body recipes:
Silica is a refractory glass maker and filler, available in different # sizes with a melting point roughly at 1700C (3092F). This is a much higher heat than what our average pottery kilns can provide.
Kaolin (china clay) is refractory aluminum silicate, with an average melting point around 1800C (3275F). Coming from different regions, it may have percentages of iron and titanium that will determine the whiteness and its potential to become translucent.
Fluxes (melters) comes in a broad range of varieties and is available almost everywhere in the world today; some purer than others. Melting at different temperatures over longer or shorter periods and staying viscose in a variety of ways it has the ability to affect and change the character of silica and kaolin as the melted lava infiltrate the pores in the clay, making it into a semi-glass mass, known as porcelain.
Throughout the explanation of the different porcelain recipes described in the above-mentioned article, the melting agent (Flux) made anywhere from 45-50% of the recipe. This is where I found my answer and succeeded in creating porcelain clay recipes at cone 6 with the following traditional qualities:
It worked like clay, but when I fired it, it rings like glass when tapped on, is translucent, hard, white and durable.
Pushing for lower firing porcelain
In an article “How Low Can You Go” in Ceramics Monthly (June/July/August 2014) he wrote the following:
“The clay body I have come up with is similar to the above definition of soft-paste porcelain, but its physical attributes are too far removed to be considered true soft paste. I am able to throw my clay body on the potter’s wheel with no more difficulty than my cone 10 porcelain body, and it warps no more than the cone 10 body. My clay body does not need to be glazed, as it is fully vitrified, and the surface cannot be easily scratched. It has no absorption of water, is impervious to staining, and is unaffected by rapid heating (submerging in boiling water)”