Glazing In Ceramics: All You Need To Know
There is more to pottery and ceramics than simply shaping the clay. One of the steps that is often overlooked by beginners is the glazing process.
Glazing might not sound as fun as throwing clay on a wheel at first. But once you start experimenting with different glazes, a whole new work opens up for you.
Whole books have been written on which glaze to use and how to apply them.
But for now, let’s first go over all the most important things!
What is glazing in ceramics?
A glaze is a suspension of ground minerals that can be applied to the surface of bisque-fired ceramic pieces. After drying, the coated pieces will be heated in a kiln to the appropriate temperatures. During this process, the ingredients in the glaze will melt and form a glassy surface.
When firing the glaze, it’s important to use the right temperatures. If the temperature is too low, the ingredients won’t melt. But if the temperature is too high, the glaze will melt completely and run off the surface.
Moreover, you want to make sure that the firing temperature for your glaze is close to the firing temperature of the clay. If the temperature difference is too big, you might end up with a very ceramic pieces.
What is the purpose of glazing?
A glaze can be used to color, decorate or waterproof ceramic items. Porous ceramics can be glazed to make them waterproof and food-safe. And applying one or more colored glazes can be used to decorate ceramic items.
For example, earthenware is a ceramic material that is porous even after firing. This means that water can leak through the walls and bacteria can easily grow in the pores. A ceramic glaze can be applied on earthenware to seal off the pores and make it safe to use as tableware.
Stoneware and porcelain on the other hand can be vitrified by firing at high temperatures. Meaning that the resulting ceramics aren’t porous. However, you can still opt to glaze these types of ceramics to make them more durable and chip-resistant.
Some glazes are also colorful and can be used to decorate ceramic pieces. There are many glazing techniques you can use to give your ceramics a unique look. If you are interested in these special effects, you can search for drip glaze or marble glaze.
Just keep in mind that most glazes look different after firing. So the color of the liquid glaze is not a good indication of the final appearance.
You can find some mugs with beautiful glazing in this article.
Do ceramics need to be glazed?
Not all ceramics need to be glazed. High fire stoneware and porcelain clays can fully vitrify at high firing temperatures and can therefore be used safely without a glaze. Low fire clays such as earthenware and mid-range stoneware often need to be glazed before they can be used as foodware.
Of course, ceramic pieces that are purely decorative and won’t be used for cooking, eating, or holding water don’t need to be glazed. For example, a vase that holds dried flowers doesn’t need to be glazed regardless of the clay used, because it won’t need to hold any water.
As mentioned before, a glaze can be functional or decorative. So even though you don’t have to glaze your ceramic to use them, you might still want to apply a glaze as decoration.
What are the main components of glazes?
A glaze consists out of five basic components: silica, alumina, flux, colorants, and modifiers. Although most glazes are made out of these ingredients, you can still find glazes in a large range of different colors and firing temperatures.
The silica sand is the main ingredient of the glaze. The silica will melt at high temperatures and form a glassy surface after cooling down. Different metal compounds such as copper oxide, cobalt oxide, and iron oxide can be added to change the color of the glaze.
The other ingredients are mainly there to change the properties of the glaze such as the melting temperature, opacity, how well the glaze sticks to the clay, etc.
Different types of glazes in ceramics
There are two ways to classify different types of ceramic glazes. The first one is to categorize them by their firing temperatures, and the second one is to organize them by their finish.
When you look at the first approach, there are three types of ceramic glazes: low-fire glazes, mid-range glazes, and high-fire glazes. The low fire glazes mature roughly between 1830 and 1950 degrees Fahrenheit, the mid-range glazes need to be fired around 2160 and 2260 degrees Fahrenheit, and the high-fire glazes need to be heated between 2280 and 2350 degrees Fahrenheit.
Most potters go with a mid-range glaze for functional ceramic pieces, because the mid-range glaze is more durable than the low-fire glazes and don’t require temperatures as high as the high-fire glazes. However, low-fire glazes are normally used for decorative pieces, because the low-fire glazes normally have more vibrant colors.
When looking at the texture, glazes used for ceramics can be categorized in 5 types: gloss, matte, semi-gloss, semi-matte, and satin matte. It’s fun to experiment with all these different glazes and try out new glazing techniques. But keep in mind that not all of these glazes as food-safe. Especially, glazes with cadmium or lead are normally might result in safety issues. Make sure to check the ingredients of your glaze before using them!
Choosing the right glaze
When choosing a glaze, it’s important to check if the firing temperature for the glaze and the maturing temperature of the underlying clay body are close enough. If they are too far apart, you might find some crazing in your final pieces.
For more information, you can read our article on the causes of crazing on pottery.
How do you glaze ceramics?
For simple pieces, the process of glazing ceramics will include the following steps:
- Mix all the components of your glaze
- Apply the glaze to the bisque-fired clay
- Let the glaze coating dry
- Fire the coated clay in a kiln at the appropriate temperature
Of course, this is a very simplified process. And it might be a lot more complex for more advanced glazing techniques.
It’s important to notice that you can only glaze bisque-ware. So if you want to glaze stoneware or porcelain, you should only bisque-fire them before glazing and don’t fire them to complete maturation. This is because these clays are no longer porous after vitrification and the glaze will just drop off the surface. You can fire the clay to maturation after applying the glaze.
Can you glaze ceramics without a kiln?
Beginners often ask if you can glaze ceramic without the use of a ceramic kiln. Unfortunately, this is nearly impossible. Even the low-fire glazes still need to be heated above 1800 degrees Fahrenheit. This is way above temperatures that you can reach in a regular kitchen oven, and is even difficult to reach with pit firing.
However, you might be able to use a local firing service, or you can contact a local pottery studio to ask if they have communal kilns available.
Glazing techniques used in ceramics
There are many ways to apply glaze to ceramics. The basic glazing techniques that any beginner can try are as follows:
Of course, simply dipping the bisque-ware into the glaze is the simplest and fastest way to glaze. But you can use a combination of the other techniques if you want a more interesting finish.
Besides these 8 basic techniques, there are also more advanced methods, such as using an underglaze to create a marble effect. But each of those advanced glazing techniques deserve a dedicated post of their own.
Do you need to bisque fire pottery before glazing?
It’s generally recommended to bisque fire pottery before glazing. Bisque firing is an important stage in pottery making that hardens and stabilizes the clay, making it more resilient and water-resistant. It also burns out any chemically bonded water molecules and organic gases that could cause problems during the glaze firing stage.
Without bisque firing, the clay may not be strong enough to withstand the glaze firing temperatures and may crack or fall apart. Also, the porous quality of bisque-fired pottery is ideal for glaze application, as the glaze is absorbed into the clay, helping it adhere better.
While it is technically possible to skip the bisque firing stage and glaze fire pottery directly, it can be risky and may result in ruined pieces. So, it’s best to bisque fire pottery before glazing, to ensure a successful final product.
Also read this article on bisque firing for more information.
Single fire glazing
Single fire glazing is a pottery technique where the bisque firing stage is skipped, and the pottery is glazed and fired in one step. In this method, the clay body is soft and porous, and the glaze is applied directly to it before firing. The pottery is then fired once to achieve the final product.
The main advantage of single fire glazing is that it can save time and energy because it eliminates the bisque firing step. This method can also create unique effects on the pottery, as the glaze may interact differently with the clay body when it’s fired in one step.
However, there are also some drawbacks to single fire glazing. One of the major concerns is that the clay body may not be strong enough to withstand the high glaze firing temperatures, which can result in cracking or breaking. Additionally, because the clay body is not bisque fired, any moisture or organic materials that are still present in the clay can cause issues during the firing process, such as bubbling, blistering, or pinholing in the glaze.
Overall, single fire glazing can be a useful technique for some potters, but it requires careful consideration of the specific clay body and glaze being used, as well as close attention to the firing process.
Therefore, I would only recommend single fire glazing to advanced potters that really know what they are doing.