The difference between underglaze and glaze

The Difference Between Underglaze and Glaze

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Underglaze and glaze do have some shared properties and functions.  However, there are significant differences.  After a bit of investigation, I discovered the following:

Underglaze and glaze can both be used to decorate a piece of pottery.  The difference is that underglaze is applied before a clear glaze.  It is easier to use underglaze for intricate designs.  However, a clear overglaze will seal the piece and make it non-porous. 

There are a number of differences between underglaze and glaze.  I’m going to take a closer look at these differences in the following article.  I will also explore, what, why, how and when there is a difference between an underglaze and a glaze.    

First, I will give you a broad overview of the different kinds of underglazes and glazes.  Then I will look at the specific differences more closely.

Underglazes can be split into three categories:

  • Colored slips that can be applied to wet clay.  These are a mixture of clay, water, and pigment.
  • Engobes which can be applied to bisque ware.  They contain silica and glass. 
  • Traditional underglazes that are applied to greenware at any stage before it is fired.  These contain colored metal oxides.

Glazes:

  • Glazes contain glass-forming ingredients that create a non-porous glass layer when fired.
  • They are also normally applied to bisque ware but are occasionally applied to greenware if they are single fire glazes. 

The Different Ingredients in Underglaze and Glaze

In this section, I will look at the differences and similarities in the ingredients of underglaze and glazes.

Clay in underglaze and glaze

An underglaze that is going to be applied to leather hard clay before a bisque fire needs to contain a significant amount of clay. The reason for this is that the pottery dries out and shrinks before it is fired.  Consequently, the underglaze needs to shrink at the same rate.

If the ware shrinks more than the underglaze it will flake off. Similarly, the ware will shrink when it is fired.  Therefore, the underglaze needs to shrink at the same rate to avoid flaking.

Unlike underglaze, overglaze does not contain clay particles as such. Glaze has ingredients, including the minerals alumina and silica that are derived from clay. However, they are not of themselves clay particles.

Alumina is used to stiffen glaze so that when it is in its melted state, it does not simply slide off the pottery being fired.  Silica is a glass-forming component in the glaze.

Gum

Underglazes must contain a small amount of gum.  The gum does a couple of things.  Firstly, it slows down the rate at which the underglaze dries.  This makes it easier to apply the glaze with a brush.  Hence why it is called a ‘brushing gum’.

Because the underglaze does not dry immediately on contact with the ware, it is more spreadable.  (A handy comparison, is thinking about how difficult it is to paint a newly plastered wall that has not been sealed). 

Secondly, the gum makes the underglaze sticky, which means that it adheres to the surface without slumping. 

Also, the gum content in underglaze is one of the reasons it can be applied to bisque ware.  Underglaze contains clay, and this suggests that it might shrink and flake off when applied to bisque.  However, the gum content of the underglaze enables it to stick well to the bisque, without cracking. 

Glaze does not, as a matter of course, contain ‘brushing gum’.  A potter can, however, add it if they want to have greater control over how they apply the glaze to a piece.  Adding gum to a glaze will thicken it a little and make its consistency more like that of an underglaze. 

Here is an interesting video by The Ceramic Arts Network, experimenting with gum in glaze.

Glass

Glass in Underglazes

Some underglazes contain more glass-forming properties than others.

Originally underglazes were simply colored clay slips. A colored clay slip is a mixture of clay, water, and pigment. Because slip is made of clay, it does contain silica, which is a glass-forming substance.

However, more recently, underglazes have been created that contain frit. Frit is a ceramic compound that contains silica. If an underglaze contains more frit than clay, it will melt in the same way that glazes melt when fired.

Image by Tony Takitani.  ‘Glazed/unfired‘ – Health Factory Tour  (CC BY-ND 2.0)

What kind of underglaze a potter decides to use depends on how they intend to apply it. For example, an underglaze is going to be applied by dipping needs contain less glass. This is because dipping covers the piece completely. If an overglaze is going to adhere to the ware, the water in the glaze needs to be able to pass through the underglaze and absorb into the clay body

When underglaze is very glassy, the water from the overglaze will not be able to pass through and the glaze will not adhere. Instead, the glaze will ball up and roll off, a bit like rain on a polished car.

Glass producing ingredients in an underglaze helps it to stick to the clay body. However, if they contain too much glass, this will cause the colors in the underglaze to bleed when they are heated. One of the valuable qualities of underglaze is that it can be used for intricate design. It is important that this is not compromised by adding too much glass to the mix.

Glass in Glazes

Glazes are essentially a glass layer on the pottery ware. A glaze consists of ground-up materials suspended in water, which is applied to the piece. When it is fired, the ingredients melt together to from glass.

The clay body and the underglaze contain glass-forming ingredients. However, when the glaze is fired, all the particles in the glaze melt to form glass. None of the ingredients in the glaze keep their original crystal composition.

 

Color

Because underglaze is used for decoration it invariably contains color. Over glazes can be colored or clear. The coloring ingredients in underglaze and a glaze are usually metal oxides.

Both underglaze and glaze can be used to add color and design to a piece of pottery.

The differences between a colored underglaze and a colored glaze are as follows:

Because underglaze has a much lower glass content than glaze, it can be used for much more detailed decoration. Fine details can be painted onto greenware or bisque ware and it will not run or bleed as much as a colored glaze.

Underglaze can lose its pigment if it is fired in the kiln at too high a temperature. Some colors are more resilient at higher temperatures than others. For example, cobalt blue can survive higher temperatures than other colors, which can turn black if they are overheated.

The difference between underglaze and glaze

World Imaging / CC BY-SA

Interestingly, this is one of the reasons why a lot of design on porcelain has historically been blue. Porcelain is fired at higher temperatures and blue was one of the colors that could survive the process.

By contrast, glazes can retain very vivid colors when fired at higher temperatures. In particular, glazes that are fired in the mid-temperature range, through a process of oxidation, create very bright colors.

The Different Reasons Why We Use Underglaze and Glaze

It is too simple to say that underglaze is used for decoration because of course, overglaze can be extremely beautiful too.

However, here are some features that distinguish underglaze from overglaze:

Why We Use Underglaze

As stated above, underglaze can be easier to apply with a brush than overglaze and is better at times for intricate and detailed design. Furthermore, the look of the final product is more controllable with underglaze. This is because it does not bleed or feather as much as glaze when it is fired.

Also, the color of wet, unfired underglaze is similar to its color once it has been glazed. This is another reason why underglaze is popular for decoration. Potters have a better idea of how the fired piece will look.

By contrast, overglaze often looks very different before it has been fired than afterward. It can be hard to predict what a glaze will look like.  Therefore, it is often recommended that you practice with glazes to get the kind of effect you are going for.

In addition to this, sometimes after the bisque fire, the underglaze has imperfections that need to be touched up. Applying underglaze to greenware gives the potter a chance to finesse their design before the glaze fire.

Also, a second coat of underglaze can be applied after the bisque fire. This can be useful if the colors have burnt off a bit in the first fire.

Finally, using an underglaze for decoration with a clear overglaze can be cheaper. One way of getting an intricate design using overglaze is to use a technique called overglaze decoration.

In overglaze decoration, colored decoration is painted onto a glazed surface. The pottery is then fired a second time to fix the painted decoration.  However, because this method involves two glaze firings, it is more costly and time-consuming.

Why We Use Glaze

On the other hand, glaze has important properties that underglaze does not have. These are:

Because glaze seals the surface of the pottery completely, it creates a barrier. This barrier can help with something called ‘food safety’.

The term ‘food safety’ is used in a couple of different ways. In one usage the glaze is seen as being a barrier that protects the pottery. For example, preventing the pottery from being stained by food and drink.

Unlike underglaze glaze makes pottery food safeHowever, ‘food safe’ also refers to whether the glaze itself is safe to eat and drink off.
It is important that glazes are responsibly produced and reliably tested. There has been more awareness recently around toxins leaching from ceramics into food1

That being said, assuming that a glaze has been responsibly produced, it does provide a glass-like barrier. This barrier enables the piece to be used for more purposes than an underglaze alone would afford.

The Difference Between How We use Underglaze and Glaze

Both underglazes and clays can be applied to a piece using broadly similar techniques. For example, both can be painted to a piece of ware.

However, glaze and underglaze have different properties. Therefore, whilst you may be able to paint both, the experience and the effect will be different.

For instance, glaze that you can paint has quite a thick consistency. A little like heavy cream. As such they are not ideal for finer details.

By contrast, underglaze can be painted on in quite a watery state. This means that different layers of color can be built up. Also, underglaze can be painted on much like a watercolor. Used in this way, different colors can blend into one another creating a gradient look.

Because glaze that you can paint is thick, you have to be careful how many layers you apply. Applying too many layers of glaze can cause it to slump, drip and deform in the kiln. Whereas, you can apply up to six layers of underglaze without that worry.

Although single fire glazes can be used, normally it is applied to bisque ware.

Some of the ways that glazes are commonly applied:

  • Dipping the ware in the glaze
  • Painting with the glaze
  • Pouring the glaze on or around the ware
  • Spraying the glaze with an airbrush
  • Sponging on the glaze

Underglaze can also be applied using the above methods. In addition to this, there are other techniques that can add details to the pottery.

For example, underglaze can be drawn onto bisque ware. Underglaze pencils, crayons, and pens exist for this purpose and can be used to create very fine details on a design.

The difference between underglaze and glazeAlternatively, a potter may want to use a bulb syringe to apply underglaze to a piece. This technique is a little like slip trailing. Slip trailing with slip can create quite a defined texture on the piece. By contrast, using underglaze, in the same way, does not create textural patterns. However, a bulb syringe can be used to create lovely color designs.

Another advantage of using underglaze is that you can mix underglaze colors together. This can be done either as you paint your piece. For example, mixing watercolor style paints as they go onto the bisque.

Or you can mix the underglaze up prior to application, to create a color that is to your exact preference. This differs from regular glazes which generally cannot be combined together to create a third color.

The Difference Between When We Use Underglaze and Glaze

Although single fire glazing is an option, most of the time, glaze is applied to bisque ware. Part of the reason for this is that bisque ware is porous and absorbs the water from the glaze. The result is that the glaze adheres to the piece.

By contrast, many modern underglazes are designed to be applied either to greenware or to bisque ware. In fact, underglazes can be used on wet clay, leather hard clay, bone dry clay or bisque ware.

When you apply the underglaze depends on the kind of underglaze you are using. For example, if the underglaze is a colored slip, it can be applied to wet clay. Whereas an underglaze pencil needs to be applied to bisque ware. If a pencil was used on unfired clay it could damage the surface of the piece.

Another factor that impacts when an underglaze is used is the water content involved. For example, underglaze can be applied to greenware. However, the water in the underglaze can soften the clay in the ware and compromise its shape or design. As such, it can be easier to apply a wet underglaze to bisque ware.

Having said that, if you dip underglaze in clear overglaze, the colors in the underglaze can run. So, there is also an argument for applying the underglaze to greenware, firing it and then glazing it.

Final Thoughts

In this article, I have looked at the what, why, how and when underglazes are different from glazes. In short, glazes produce a glass-like layer, usually over the whole of the ceramic work, except for the base. This layer is protective, but it can also be decorative.

Underglazes are more versatile in their decorative capacity. However, although they can be vitreous, they don’t offer the same protective layer of a glaze. So, whilst an underglaze can enable detailed decoration, a glaze offers a glass-like layer of protection.

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Pottery Tips from the Pottery Wheel

Lesley

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Pottery Tips from the Pottery Wheel

I’m Lesley Milne, the creator of The Pottery Wheel.  Like many people, I used the potter’s wheel at school.  But then I began to focus on clay sculpture and I left the wheel behind.  However, more recently, I found myself being drawn back to pottery and the potters wheel.  And so, I have tried to pick up where I left off all those years ago at school. This blog is a chronicle of what I have learned as I got back into the potters saddle!

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