Stoneware craftsmanship is a detailed subject. That is why in our article we will dive deep into the following areas.

All ceramic ware is made from clay.  There are different types of mud that produce different types of ceramic dinnerware, ovenware and pottery. 

The three main types of mud used to make ceramics are as follows:

1. Stoneware
2. Earthenware containers
3. Porcelain

Each of these clay types has different qualities that distinguish them from each other.

Stoneware clays are hard and strong.  When ceramic ware is made, it goes through a firing process.  This process takes place in a large kiln that essentially reaches very high temperatures.  Stoneware ceramics are normally fired at temperatures above 2192F (1200C).

When firing ceramic ware, the clay undergoes many physical and chemical changes.  It is these changes that make the clay strong enough to be used as dinnerware. 

One of the changes that stoneware clay undergoes is a process called vitrification.  This word sounds complicated and off-putting, but it is a term you are probably already familiar with.  You may have heard ceramics referred to as ‘vitreous’ and ‘non-vitreous’. 

If you have not heard the terms ‘vitreous’ and ‘non-vitreous’ before, let us tell you what they are...

What does ‘vitreous’ and ‘non-vitreous’ mean?

When sludge is baked in an oven, the sludge particles undergo many changes.  One of the changes that occurs is that the particles move closer to each other.  In addition, the surface of the particles bonds together. 

This is known as ‘sintering’.  During the sintering process, the slurry becomes denser as the particles come closer together.  The bonded particles begin to form the basis of a matrix or structure within the clay that will harden as the kiln and pottery cools.

This structure is similar to the skeleton that keeps the ceramic ware hard after the clay is fired.  Although the clay particles are bonded together, there are small gaps or pores between them.  You can think of these as tiny pores in a hard sponge.

Liquid Glass

All muds contain a mineral called silica, also known as quartz, which is a component of glass. Some types of mud contain more silica than others.

When mud is baked in an oven, the silica melts and becomes liquid.  In its molten state, the silica flows into the small spaces between the mud particles.  The silica acts on the mud particles, causing them to flow (or melt) too.  Thus, the silica and clay particles form a mixed material. 

At this point, the pottery in the kiln is quite soft and pliable. The clay must be strong enough to support itself in this malleable state.  Stoneware clay is strong enough to be soft and malleable in the kiln without deforming, bending or collapsing in a puddle of molten water.

When the kiln and pottery cool down, the molten glass mixed together becomes solid again.  This creates a strong ceramic body.  In addition to being strong, the ceramic ware is non-porous because the spaces between the clay particles are filled with glass. This is what is meant by the term ‘vitreous’.

In some types of clay, more pores are filled with liquid glass than in other types.  The more the pores are filled, the less porous the final artefact will be. 

Stoneware usually has a porosity of about 1-2%.  This is low enough for the artefacts to be waterproof and non-absorbent.  The hard mixture of glass and clay mixed together also makes stoneware strong and resistant to chipping or breaking.

Stoneware is therefore a good choice as a kitchen and dining set because it does not absorb food and liquids, even when unglazed.

What is the Difference of Stoneware from Ceramic Mud?

Like stoneware, ceramic clay is fired in a kiln to become ceramic material.  However, unlike stoneware, ceramic clay is still highly porous after firing. 

Ceramic mud is sometimes defined as mud with a porosity of more than 5%, but it can be a much higher percentage than this. 

This means that ceramic products are not waterproof and will absorb liquid from food and drinks unless well glazed.  Because it is porous, ceramic products are also weaker than Stoneware and more vulnerable to chipping and breaking in everyday use.  

Ceramic muds are more porous than Stoneware because earthenware ceramic mud contains less of the glass-forming materials needed to be vitrified. 

When ceramic mud is fired, it reaches a high enough temperature to form some liquid glass.  However, there is not enough molten glass to fill all the spaces between the mud particles.

The point at which ceramic mud begins to warp, twist and deform in the kiln is much earlier than stoneware mud.  As a result, ceramic clay needs to be fired at lower temperatures than stoneware. 

Stoneware clay has a higher glass-forming content and is stronger.  Therefore, it can be fired at higher temperatures. As a result, stoneware mud becomes more ‘mature’ when fired and is more fully vitrified.

The other main type of ceramic ware mentioned above is porcelain. Similar to stoneware, porcelain artefacts are strong and non-porous. Porcelain artefacts usually have less than 1% porosity.  Some porcelain is completely vitrified and completely non-porous. 

Porcelain pottery can usually tolerate firing to the point where it has almost 0% porosity.  This ceramic ware is very strong and durable, so porcelain is used for all kinds of purposes other than pottery.  For example, some types of porcelain are used in dentistry and surgery for joint replacements.

One of the reasons porcelain is so strong is that it is made up of particles of similar size.  In other words, the porcelain clay body is consistent and homogenous.  When fired, all parts of the body vitrify equally. 

In contrast, stoneware usually contains particles of different sizes, such as sand.  As a result, vitrification is not as consistent in stoneware.  At the microscopic level, some areas of the pottery are more glassy and stronger than others.

In fact, usually if you try to fire pottery clay so that it has less than 1% porosity, it will lose its strength and become more brittle and vulnerable to breakage.

As stoneware has a very low porosity, it is not necessary to glaze the pottery, provided it has been fired well and at the correct temperature.  Unglazed pottery can be used and washed.

However, although it is non-absorbent, the texture of the pottery is often slightly rough to the touch.  It can have a toothed feel. 
There are many different types of pottery and some are much harder than others.  However, even smooth stoneware has more teeth than unglazed porcelain, which can feel as smooth as glass.  Unglazed smooth stoneware can feel a bit like very fine sandpaper. 

Because stoneware has an organic tooth feel, it can discolour over time when used unglazed.

For this reason, pottery ceramics are often glazed, especially in areas in direct contact with food and drink. 

There are a wide variety of types of ceramic glaze.  They can be glossy, matt, translucent or opaque.  They can be designed to flow over the pottery as it is fired, creating beautiful drips and pools of colour.  Or they can be designed to remain fairly static on the glaze, creating bold patterns or blocks of colour. 

These are just a few of the different types of glaze used on stoneware pottery.  However, a common characteristic of pottery glazes in general is that their colours are quite organic and earthy in appearance. 

Greens, browns, reds and even blues have a natural, somewhat rustic look. This contrasts with the glazes used on earthenware.

Glazes on earthenware can be more brightly coloured and are usually high-gloss primary colours.  Although some earthenware glazes are designed to resemble stoneware glazes. 

This is partly due to the temperature at which stoneware and earthenware are fired.  Earthenware ceramics are low-fired.  This means that they were fired at relatively low temperatures, such as 1852-2109F (1011-1154C).  The bright colours in earthenware glazes can easily survive these temperatures. 

In contrast, stoneware pieces are medium to high fire ceramics and are fired in the range 2158 -2377 (1181-1303C).  Bright primary colours, especially primary red colours, do not work well in this temperature range.  Therefore, colours for stoneware glazes tend to be earthy and organic rather than primary colours.

Glazed Stoneware Vs Glazed Earthenware

When pottery is glazed, the point where the glaze meets the underlying ceramic body is called the clay-glaze interface. The glaze binds to the underlying clay body.  The strength of this bond depends on the type of clay used to make the ceramic.

As described above, earthenware clay is fired at lower temperatures and the clay itself does not become glassy or molten when fired. 

When earthenware clay is glazed, the glaze grips the rough texture on the surface of the pottery.  The bond between clay and glaze is formed by the glaze locking onto the rough texture of the clay.

Something different happens when stoneware ceramics are glazed.  As mentioned earlier, stoneware clay has a higher glass content, which melts during the firing process. 

When the glaze melts during the glaze firing, the molten glaze mixes with the liquid glass in the clay.  This intermixed layer creates a very strong bond between the clay and the glaze. 

Stoneware glazes become part of the body of the ceramic ware instead of remaining on the surface of the clay like earthenware glazes.  Therefore, stoneware glazes are strong and resistant to the bumps and knocks associated with daily use in the kitchen and dining room.

Glazed Stoneware and Glazed Porcelain

Having said that, it is worth noting that the strongest bond between clay and glaze is actually formed by porcelain fired with glaze.  When porcelain is fired with glaze, this mixture spreads deeper through the clay and glaze and the interface becomes wider and more complete. 

In contrast, when clay is fired in stoneware, some integration takes place between the clay and the glaze.  However, there is still some tension between the underlying clay body and the applied glaze.

Because of this possible tension, it is important that the glazes used for stoneware are compatible with the underlying clay.  Some glaze and clay combinations work well together.  Others, on the other hand, do not fit together.  This is known as clay-glaze ‘compatibility’. 

If a glaze is not compatible with a particular type of ceramic clay, defects will appear in the fired pottery.  A common defect is ‘cracking’, where the glaze forms very fine cracks after the pottery is removed from the kiln. 

You know this is happening if your pottery makes a ringing sound when it is taken out of the kiln.  Finding a glaze suitable for a particular stoneware clay is sometimes a matter of trial and error.